Showing 1–50 of 109 results for author: Morozov, A

No need to stay positive: a practical approach to direct numerical simulations of elastic turbulence

Authors: Damiano Capocci, Moritz Linkmann, Alexander Morozov

Abstract: Successfully performing direct numerical simulations of polymeric flows remains a major challenge in computational fluid mechanics. In addition to the velocity field, such simulations must resolve polymeric degrees of freedom, often expressed via the conformation tensor, $\mathbf{c}$, which captures the local stretch of polymer molecules. A key difficulty here lies in maintaining the physical requ… ▽ More Successfully performing direct numerical simulations of polymeric flows remains a major challenge in computational fluid mechanics. In addition to the velocity field, such simulations must resolve polymeric degrees of freedom, often expressed via the conformation tensor, $\mathbf{c}$, which captures the local stretch of polymer molecules. A key difficulty here lies in maintaining the physical requirement $\mathrm{Tr}\, \mathbf{c}>3$, which is not explicitly enforced by the governing equations. Consequently, simulations initiated from physical conditions may silently drift into unphysical states with $\mathrm{Tr}\, \mathbf{c}<0$, indicating a loss of positive-definiteness of the conformation tensor. Existing numerical methods to prevent this are costly, making direct numerical simulations of chaotic polymer flows, such as elastic turbulence, heavily reliant on high-performance computing. Here, we ask whether simulations that violate $\mathrm{Tr}\, \mathbf{c}>3$ can still yield meaningful physical insight into the underlying dynamics. We simulate a model dilute polymer solution driven through a plane channel at low Reynolds number and observe the transition to elastic turbulence. Our simulations exhibit two threshold resolutions: below the first, they become numerically unstable and exhibit a finite-time blow-up; above the second, they maintain positive-definiteness. In between, simulations remain stable and chaotic despite local violations of $\mathrm{Tr}\, \mathbf{c}>3$. Surprisingly, these violations do not affect mid-plane statistics of velocity, its gradients, or polymer stretch, which match results from fully positive-definite simulations. This suggests that resolving flow structures or key flow statistics may not require the extreme resolutions needed to preserve positive-definiteness, potentially lowering computational barriers for studying elastic turbulence. △ Less

Submitted 8 June, 2026; originally announced June 2026.

Comments: 16 pages, 10 figures. Submitted to J. Fluid Mech

Experimental Scaling of Diffraction Efficiency in Laser-Induced Plasma Gratings

Authors: M. M. Wang, V. M. Perez-Ramirez, N. M. Fasano, K. Ou, S. Cao, V. Dewan, A. M. Giakas, A. Morozov, P. Michel, M. R. Edwards, J. M. Mikhailova

Abstract: We demonstrate efficient diffraction of intense ultrashort laser pulses using optical-field-ionization-induced plasma-neutral gratings formed by spatially structured ionization of a neutral molecular gas in the interference field of two femtosecond pump pulses. The transient refractive index modulation of the plasma structure persists for at least 10 picoseconds and is used to diffract intense fem… ▽ More We demonstrate efficient diffraction of intense ultrashort laser pulses using optical-field-ionization-induced plasma-neutral gratings formed by spatially structured ionization of a neutral molecular gas in the interference field of two femtosecond pump pulses. The transient refractive index modulation of the plasma structure persists for at least 10 picoseconds and is used to diffract intense femtosecond signal pulses into the 1st order of diffraction with an average efficiency of up to 35$\%$. Plasma gratings are shown to provide stable diffraction at signal laser intensities greater than $ 10^{14}\text{ W/cm}^2$, exceeding the damage thresholds of conventional solid-state optics by more than two orders of magnitude, continuously for hours at a 10-Hz repetition rate. The experimental diffraction efficiency scales with the grating aperture allowing for a larger millimeter-scale plasma optic, increases with the pump energy and electron density, and reaches a maximum at a specific grating length in agreement with the coupled-mode theory for periodic media. These results demonstrate the scalability, tunability, and high damage threshold of transmissive plasma-based photonic structures, opening new prospects for controlling multi-petawatt laser beams. △ Less

Submitted 30 April, 2026; v1 submitted 17 March, 2026; originally announced March 2026.

Coarse-grained Shannon entropy of random walks with shrinking steps

Authors: Alexander Feigel, Alexandre V. Morozov

Abstract: In one-dimensional diffusive processes with discrete steps characterized by geometrically decaying magnitudes, the usual Gaussian broadening familiar from Brownian motion is replaced by bounded probability distributions over particle positions that are characterized by multi-scale fractal structures. In this work, we study random walks with shrinking steps (known as Bernoulli convolutions), focusi… ▽ More In one-dimensional diffusive processes with discrete steps characterized by geometrically decaying magnitudes, the usual Gaussian broadening familiar from Brownian motion is replaced by bounded probability distributions over particle positions that are characterized by multi-scale fractal structures. In this work, we study random walks with shrinking steps (known as Bernoulli convolutions), focusing on their behavior in the vicinity of the dyadic contraction ratio 1/2. Our analytical and numerical results show that the coarse-grained Shannon entropy of particle distributions induced by Bernoulli convolutions exhibits a local maximum at the dyadic ratio, arising from the competition between diffusive spreading, which increases entropy, and emergent fine structure, which tends to decrease it. This entropy maximum is a general property of systems driven by non-Gaussian discrete noise, whose dynamics near stable fixed points can be viewed as an autoregressive process - an approximation that is mathematically equivalent to unbiased random walks with shrinking steps. We discuss potential implications of Bernoulli convolution dynamics for protocell self-replication and vesicle proliferation, establishing a link between our information-theoretic approach and biophysical models of cell division. △ Less

Submitted 27 February, 2026; originally announced March 2026.

Comments: 10 pages, 6 figures, 2 appendices

MSC Class: 60G50; 60G18 ACM Class: G.3

Tipping points in complex ecological systems

Authors: Alan Hastings, Sergei Petrovskii, Valerio Lucarini, Andrew Morozov

Abstract: Tipping points are one of the hot topics in modern physics of complex systems. But what is a tipping point? A generic definition declares it as ``a state of the system where a small change in its parameters can lead to a significant change in its properties''. Additional ingredients that often enter the definition of tipping process are the abruptness of the resulting change and its irreversibilit… ▽ More Tipping points are one of the hot topics in modern physics of complex systems. But what is a tipping point? A generic definition declares it as ``a state of the system where a small change in its parameters can lead to a significant change in its properties''. Additional ingredients that often enter the definition of tipping process are the abruptness of the resulting change and its irreversibility, i.e. it is impossible to recover the initial state if one reverses the protocol of change of the parameters. However, there exists a number of different mathematical structures that can show this behavior, the one that was originally suggested as a tipping point (nowadays usually referred to as bifurcation induced tipping) is just one of many. Different preconditions and/or different level of details included into the model, reflecting also different environmental forcing, can lead to a variety of tipping mechanisms. Furthermore, in a spatially extended system and/or a system with multiple scales, different parts can react to a change in environmental conditions differently or at a different time, interacting with each other to create a tipping cascade. In this paper, using ecosystems as a paradigm of complex nonlinear open systems, we provide a critical overview of the progress made in tipping point science over the last 15 years. We highlight the main findings, identify gaps in our knowledge, and outline a roadmap for further progress. △ Less

Submitted 24 February, 2026; originally announced February 2026.

One Rule to Bring Them All: Investigating Transport Connectivity in Public Transport Route Generation for Equitable Access

Authors: Aleksandr Morozov, Ruslan Kozliak, Georgii Kontsevik, Sergey Mityagin

Abstract: Designing a city-wide public transport network poses a dual challenge: achieving computational efficiency while ensuring spatial equity for different population groups. We investigate whether AI-based optimization hybrid neuroevolutionary methods combining graph neural networks with evolutionary algorithms - can scale Transit Network Design Problem (TNDP) solutions from synthetic tests to real urb… ▽ More Designing a city-wide public transport network poses a dual challenge: achieving computational efficiency while ensuring spatial equity for different population groups. We investigate whether AI-based optimization hybrid neuroevolutionary methods combining graph neural networks with evolutionary algorithms - can scale Transit Network Design Problem (TNDP) solutions from synthetic tests to real urban networks while preserving social fairness. Our contribution is to introduce a transport connectivity-aware accessibility metric that bases optimization on principles of equitable accessibility rather than traditional trade-offs between passenger and operator costs. The results show a noticeable improvement in network resilience by improving algebraic connectivity on synthetic datasets, and highlight the ambiguity of applying network generation to real data. △ Less

Submitted 11 February, 2026; originally announced February 2026.

To clean or not to clean: The free-rider problem in sequentially shared resources

Authors: Alexander Feigel, Alexandre V. Morozov

Abstract: Shared resources enhance productivity yet at the same time provide channels for biological and digital contamination, turning physical or digital hygiene into a cooperation dilemma prone to free-riding. Here we introduce a game of sequential sharing of common resources, an empirically parameterized evolutionary model of population dynamics in sequential-use settings such as gyms and shared workspa… ▽ More Shared resources enhance productivity yet at the same time provide channels for biological and digital contamination, turning physical or digital hygiene into a cooperation dilemma prone to free-riding. Here we introduce a game of sequential sharing of common resources, an empirically parameterized evolutionary model of population dynamics in sequential-use settings such as gyms and shared workspaces. The success of the strategies implemented in the model, such as cleaning equipment before or after use, are based on the trade-offs between cleaning costs, contamination risk, and social incentives to mitigate disease transmission. We find that cooperative hygiene can be achieved by lowering the effective costs of cleaning, strengthening pro-social incentives, and monitoring population-level noncompliance. Remarkably, stability of fully altruistic populations is primarily affected by the cleaning costs. In contrast, increasing effective infection costs, for example through punishment, appears less important in this case. The model's evolutionary dynamics exhibit multi-stability, hysteresis, and abrupt shifts in strategy composition, broadly consistent with empirical observations from shared-use facilities. Our framework offers testable predictions and is amenable to quantitative calibration with behavioral and environmental data. Our predictions can be used to inform the design of cost-effective public health and digital security policies. △ Less

Submitted 5 February, 2026; originally announced February 2026.

Comments: 12 pages, 4 figures in the main text, 1 figure in the Supplement

MSC Class: 91A40

Poisson Log-Normal Process for Count Data Prediction

Authors: Anushka Saha, Abhijith Gandrakota, Alexandre V. Morozov

Abstract: Modeling count data is important in physics and other scientific disciplines, where measurements often involve discrete, non-negative quantities such as photon or neutrino detection events. Traditional parametric approaches can be trained to generate integer-count predictions but may struggle with capturing complex, non-linear dependencies often observed in the data. Gaussian process (GP) regressi… ▽ More Modeling count data is important in physics and other scientific disciplines, where measurements often involve discrete, non-negative quantities such as photon or neutrino detection events. Traditional parametric approaches can be trained to generate integer-count predictions but may struggle with capturing complex, non-linear dependencies often observed in the data. Gaussian process (GP) regression provides a robust non-parametric alternative to modeling continuous data; however, it cannot generate integer outputs. We propose the Poisson Log-Normal (PoLoN) process, a framework that employs GP to model Poisson log-rates. As in GP regression, our approach relies on the correlations between data points captured via GP kernel structure rather than explicit functional parameterizations. We demonstrate that the PoLoN predictive distribution is Poisson-LogNormal and provide an algorithm for optimizing kernel hyperparameters. Furthermore, we adapt the PoLoN approach to the problem of detecting weak localized signals superimposed on a smoothly varying background - a task of considerable interest in many areas of science and engineering. Our framework allows us to predict the strength, location and width of the detected signals. We evaluate PoLoN's performance using both synthetic and real-world datasets, including the open dataset from CERN which was used to detect the Higgs boson at the Large Hadron Collider. Our results indicate that the PoLoN process can be used as a non-parametric alternative for analyzing, predicting, and extracting signals from integer-valued data. △ Less

Submitted 5 February, 2026; originally announced February 2026.

Comments: 19 pages including the Supplement, 7 figures in the main text, 3 figures in the Supplement

MSC Class: 60G15

Multi-Diagnostic Characterization of Laser-Produced Tin Plasmas for EUV Lithography

Authors: Stanislav Musikhin, Anatoli Morozov, Alec Griffith, Shurik Yatom, Ahmed Diallo

Abstract: We present a comprehensive characterization of laser-produced tin (Sn) plasmas relevant to extreme ultraviolet (EUV) lithography using a multi-diagnostic suite integrated into the new experimental platform, "SparkLight". Tin plasmas are generated by irradiating a continuously moving tin-coated wire with laser pulses (1064 nm, 10 ns, up to $5.7\times10^{10}$ W/cm$^2$) and probed via coherent Thomso… ▽ More We present a comprehensive characterization of laser-produced tin (Sn) plasmas relevant to extreme ultraviolet (EUV) lithography using a multi-diagnostic suite integrated into the new experimental platform, "SparkLight". Tin plasmas are generated by irradiating a continuously moving tin-coated wire with laser pulses (1064 nm, 10 ns, up to $5.7\times10^{10}$ W/cm$^2$) and probed via coherent Thomson scattering, laser interferometry, and EUV emission spectroscopy. Thomson scattering measurements reveal electron temperatures and densities that decay with distance from the target. Densities derived from Thomson scattering are cross-validated against laser interferometry, showing excellent agreement. Correlating the results of these laser diagnostics with spatially resolved EUV spectroscopy suggests that the bulk of useful EUV emission originates within 150 $μ$m of the target and is generated under suboptimal plasma conditions. This work demonstrates a practical integrated approach for plasma characterization in EUV source development. △ Less

Submitted 3 February, 2026; originally announced February 2026.

Universal reconstructive polarimetry with graphene-metal infrared photodetectors

Authors: Valentin Semkin, Kirill Kapralov, Ilya Mazurenko, Mikhail Kashchenko, Alexander Morozov, Yakov Matyushkin, Dmitry Mylnikov, Denis Bandurin, Li Lin, Alexey Bocharov, Dmitry Svintsov

Abstract: Recent advent of smart photodetectors, where in-situ tuning of responsivity enables the reconstruction of light intensity, polarization and spectrum by a single device, has revolutionized the field of optoelectronics. So far, most such reconstructive detectors were realized with non-scalable technology of van der Waals stacking. Here, we demonstrate the infrared reconstructive polarimetry with pho… ▽ More Recent advent of smart photodetectors, where in-situ tuning of responsivity enables the reconstruction of light intensity, polarization and spectrum by a single device, has revolutionized the field of optoelectronics. So far, most such reconstructive detectors were realized with non-scalable technology of van der Waals stacking. Here, we demonstrate the infrared reconstructive polarimetry with photodetectors based on conventional gated graphene-metal junctions. The reconstruction exploits the gate tuning of polarization contrast, which enables the determination of both infrared power and polarization angle from photovoltage measurements at two different gate voltages. The physics enabling the polarimetry lies in polarization-dependent shift of the electron hot spot near the contact, and the gate tuning of photosensitive barrier width. We further show the universality of polarization reconstruction, i.e. its feasibility with different geometries of the junction, and with graphene of different quality, from boron-nitride encapsulated flakes to the scalable chemical vapor deposited films. △ Less

Submitted 23 March, 2026; v1 submitted 2 February, 2026; originally announced February 2026.

Enhanced thermal stability of inverted perovskite solar cells by bulky passivation with pyridine-functionalized triphenylamine

Authors: Ekaterina A. Ilicheva, Irina A. Chuyko, Lev O. Luchnikov, Polina K. Sukhorukova, Nikita S. Saratovsky, Anton A. Vasilev, Luiza Alexanyan, Anna A. Zarudnyaya, Dmitri Yu. Dorofeev, Sergey S. Kozlov, Andrey P. Morozov, Dmitry S. Muratov, Yuriy N. Luponosov, Danila S. Saranin

Abstract: Despite competitive efficiency compared to Si solar cells and relevant stability at near room temperatures the rapid degradation at elevated temperatures remains the critical obstacle for exploitation of perovskite photovoltaics. In this work, a 4-(pyridin-4-yl)triphenylamine (TPA-Py) with pyridine anchor group was employed for inter-grain bulk modification of double-cation CsCH(NH2)2PbI3 perovski… ▽ More Despite competitive efficiency compared to Si solar cells and relevant stability at near room temperatures the rapid degradation at elevated temperatures remains the critical obstacle for exploitation of perovskite photovoltaics. In this work, a 4-(pyridin-4-yl)triphenylamine (TPA-Py) with pyridine anchor group was employed for inter-grain bulk modification of double-cation CsCH(NH2)2PbI3 perovskite absorbers to enhance thermal stability. Through coordination and dipole-dipole interactions, nitrogen-containing fragments (diphenylamine and pyridine) of TPA-Py passivate uncoordinated cations and improve the phase resilience of perovskite films against segregation. This resulted in a power conversion efficiency of 21.3% with a high open-circuit voltage of 1.14 V. Notable impact of self-assembled monolayer incorporated into the bulk of perovskite film manifested in huge improvement of thermal stability at 85°C (ISOS-D-2). TPA-Py modification improved extended the T80 lifetime to ~600 h compared to only 200 h for the reference under harsh heating stress in ambient conditions. In-depth analysis using photoinduced voltage transients and admittance spectroscopy after different stress periods revealed the screening of ion migration (0.45 eV) for devices with TPA-Py. This work offers an important understanding of the bulk modification of microcrystalline perovskite absorbers and guide for robust design of bulk and buried interfaces in highly efficient perovskite solar cells. △ Less

Submitted 9 October, 2025; originally announced October 2025.

Narwhals and their blessings: exact coherent structures of elastic turbulence in channel flows

Authors: Alexander Morozov, Martin Lellep, Damiano Capocci, Moritz Linkmann

Abstract: Solutions of long, flexible polymer molecules are complex fluids that simultaneously exhibit fluid-like and solid-like behaviour. When subjected to external flows, dilute polymer solutions develop elastic turbulence - a unique chaotic flow state absent in Newtonian fluids such as water. Unlike turbulence in Newtonian fluids, elastic turbulence arises from polymer stretching and alignment in the fl… ▽ More Solutions of long, flexible polymer molecules are complex fluids that simultaneously exhibit fluid-like and solid-like behaviour. When subjected to external flows, dilute polymer solutions develop elastic turbulence - a unique chaotic flow state absent in Newtonian fluids such as water. Unlike turbulence in Newtonian fluids, elastic turbulence arises from polymer stretching and alignment in the flow, and can occur even at vanishing inertia. While experimental realisations of elastic turbulence are well documented, its underlying mechanism remains poorly understood. In this paper, we present a perspective on the transition to elastic turbulence in pressure-driven channel flows, drawing on recent computational work from our group. We outline our current understanding of the transition in both two and three spatial dimensions, centred on two key building blocks: (i) narwhals, exact coherent states of the flow, and (ii) blessings, spatio-temporal intermittent states made up of several localised narwhal solutions. This contribution is based on a talk given by one of us (A.M.) at the 2024 APS DFD meeting. △ Less

Submitted 3 September, 2025; originally announced September 2025.

Comments: 48 pages, 20 figures. Submitted to Phys. Rev. Fluids. Based on a talk given by one of us (A.M.) at the 2024 APS DFD meeting

Ants3 toolkit: front-end for Geant4 with interactive GUI and Python scripting

Authors: A. Morozov, L. M. S. Margato, G. Canezin, J. Gonzalez

Abstract: Ants3 is a toolkit that serves as a front-end for particle simulations in Geant4 and offers a custom simulator for optical photons. It features a fully interactive Graphical User Interface and an extensive scripting system based on general-purpose scripting languages (Python and JavaScript). Ants3 covers the entire detector simulation/optimization cycle, providing an intuitive approach for configu… ▽ More Ants3 is a toolkit that serves as a front-end for particle simulations in Geant4 and offers a custom simulator for optical photons. It features a fully interactive Graphical User Interface and an extensive scripting system based on general-purpose scripting languages (Python and JavaScript). Ants3 covers the entire detector simulation/optimization cycle, providing an intuitive approach for configuration of the geometry and simulation conditions, the possibility to automatically distribute workload over local and network resources, and giving a suite of versatile tools based on CERN ROOT for the analysis of the results. The intended application area is the development of new detectors and readout methods. The toolkit has been designed to be user-friendly for those with little experience in simulations and programming. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Comments: 32 pages, 11 figures

Interplay Between Structural Defects and Charge Transport Dynamics in MA and FA Modified CsSnI3 Thin Film Semiconductors

Authors: Gleb V. Segal, Anna A. Zarudnyaya, Anton A. Vasilev, Andrey P. Morozov, Alexandra S. Ivanova, Lev O. Luchnikov, Sergey Yu. Yurchuk, Pavel A. Gostishchev, Danila S. Saranin

Abstract: Owing high conductivity in microcrystalline thin-films, CsSnI3 perovskite is a promising semiconductor for thermoelectrics and optoelectronics. Rapid oxidation of thin-film and intrinsic lattice strain hinders stabilization of the device performance. Cation engineering of perovskite molecule was considered as an effective strategy to tailor the structural properties and suppress the degradation pr… ▽ More Owing high conductivity in microcrystalline thin-films, CsSnI3 perovskite is a promising semiconductor for thermoelectrics and optoelectronics. Rapid oxidation of thin-film and intrinsic lattice strain hinders stabilization of the device performance. Cation engineering of perovskite molecule was considered as an effective strategy to tailor the structural properties and suppress the degradation processes. However, molecular engineering demands a thorough analysis of defect behavior, as it can influence ionic motion, recombination dynamics, and capacitive effects. The effective implementation of CsSnI3 in energy conversion devices requires careful consideration of the specific properties of thin films electrical conductivity, Seebeck coefficient, power factor, as well as electronic transients, and charge transport in the device structures. In this work, we performed a complex investigation for modified CsSnI3 through cation substitution with methyl ammonium (MA) and formamidinium (FA). Our findings highlight a complex interplay between electrical parameters of the bare thin films and stability of the devices (p-i-n diodes) after thermal stress. FA-CsSnI3 showed beneficial results for stabilization under elevated temperatures with improved non-ideality factor in diode structures, enhanced shunt properties and reduced trapping. The photo-induced voltage relaxation spectroscopy performed for MA-CsSnI3 showed relevant traps concentration of 1016 cm-3 with activation energy of 0.52 eV(210K) likely attributed to Sn atom defect. The obtained results are deeply analyzed and discussed. △ Less

Submitted 7 April, 2025; originally announced April 2025.

Pseudo-giant number fluctuations and nematic order in microswimmer suspensions

Authors: Ismail El Korde, Dóra Bárdfalvy, Jason M. Lewis, Alexander Morozov, Cesare Nardini, Joakim Stenhammar

Abstract: Giant number fluctuations (GNFs), whereby the standard deviation $ΔN$ in the local number of particles $\langle N \rangle$ grows faster than $\sqrt{\langle N \rangle}$, are a hallmark property of dry active matter systems with orientational order, such as a collection of granular particles on a vibrated plate. This contrasts with momentum-conserving ("wet") active matter systems, such as suspensio… ▽ More Giant number fluctuations (GNFs), whereby the standard deviation $ΔN$ in the local number of particles $\langle N \rangle$ grows faster than $\sqrt{\langle N \rangle}$, are a hallmark property of dry active matter systems with orientational order, such as a collection of granular particles on a vibrated plate. This contrasts with momentum-conserving ("wet") active matter systems, such as suspensions of swimming bacteria, where no theoretical prediction of GNFs exist, although numerous experimental observations of such enhanced fluctuations have been reported. In this Letter, we numerically confirm the emergence of super-Gaussian number fluctuations in a 3-dimensional suspension of pusher microswimmers undergoing a transition to collective motion. These fluctuations emerge sharply above the transition, but only for sufficiently large values of the bacterial persistence length $\ell_p = v_s / λ$, where $v_s$ is the bacterial swimming speed and $λ$ the tumbling rate. Crucially, these "pseudo-GNFs" differ from true GNFs, as they only occur on length scales shorter than the typical size $ξ$ of nematic patches in the collective motion state, which is in turn proportional to the single-swimmer persistence length $\ell_p$. Our results thus suggest that observations of enhanced density fluctuations in biological active matter systems actually represent transient effects that decay away beyond mesoscopic length scales, and raises the question to what extent "true" GNFs with universal properties can exist in the presence of fluid flows. △ Less

Submitted 6 August, 2025; v1 submitted 23 March, 2025; originally announced March 2025.

Comments: Accepted for publication in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 135, 098302 (2025)

Microfacet projected area-based correction for unified model of Geant4 for rough surfaces

Authors: A. Morozov

Abstract: A modification of the optical model for rough surfaces, implemented in Geant4 as a part of the unified model, is suggested. The modified model takes into account the variation of the interaction probability of the photon with the microfacet based on the relative orientation of the photon and the sampled microfacet's normal. The implementation is using a rejection algorithm and assumes the interact… ▽ More A modification of the optical model for rough surfaces, implemented in Geant4 as a part of the unified model, is suggested. The modified model takes into account the variation of the interaction probability of the photon with the microfacet based on the relative orientation of the photon and the sampled microfacet's normal. The implementation is using a rejection algorithm and assumes the interaction probability to be proportional to the projection of the microfacet area on the plane perpendicular to the photon direction. A comparison of the results obtained with the original and the modified models, as well as obtained in direct Monte Carlo simulations are presented for several test surfaces constructed using a pattern of elementary geometrical shapes. △ Less

Submitted 14 March, 2025; originally announced March 2025.

Comments: 11 pages, 7 figures

VO2 films grown on TiO2 sub-layer: influence of thickness on structural, electrical and optical properties

Authors: M. E. Kutepov, V. E. Kaydashev, D. V. Stryukov, A. S. Konstantinov, A. V. Nikolskiy, A. T. Kozakov, A. D. Morozov, I. K. Domaratskiy, S. S. Zhukov, E. M. Kaidashev

Abstract: Vanadium dioxide with metal-to-insulator transition (MIT) that is triggered by heat, current or light is a promising material for modern active THz/mid-IR metasurfaces and all-optical big data processing systems. Multilayer VO2-based active metasurfaces are urgently needed however several important issues related to VO2 properties in VO2/TiO2/Al2O3 films should be thoroughly examined first. We stu… ▽ More Vanadium dioxide with metal-to-insulator transition (MIT) that is triggered by heat, current or light is a promising material for modern active THz/mid-IR metasurfaces and all-optical big data processing systems. Multilayer VO2-based active metasurfaces are urgently needed however several important issues related to VO2 properties in VO2/TiO2/Al2O3 films should be thoroughly examined first. We study electrical, optical and structural properties of VO2 films as well as their composition and switching characteristics as function of the VO2 layer thickness in VO2/TiO2 composites. XRD analysis revealed an epitaxial growth of films with deformation of the monoclinic VO2 lattice to hexagonal symmetry. Reduced VO2 layer thickness from 170 nm to 20 nm results in increased phase transition temperature while the width of the resistance versus temperature hysteresis loop R(T) remains constant at ~6C for all VO2 thicknesses in the range of 20-170 nm. The resistance alteration ratio is reduced from 4.2e3 to 2.7e2 in thinner films. Raman spectra reveal a significant shift of VO2 lattice vibration modes for films thinner than 30 nm claiming a great structural strain whereas modes position for thicker VO2 layers are similar to those in bulk structure. Composition of VO2 films has revealed only a minor alteration of VO2/V2O5 phases ratio from 1.6 to 1.8 when the film thickness has been increased from 20 nm to 50 nm. Investigation of surface elemental composition and valence states of VO2 films revealed that VO2/V2O5 ratio remains practically unchanged with thickness reduction. The study of electrical MIT dynamics revealed the switching time of a 50 nm VO2 film to be as low as 800 ns. △ Less

Submitted 9 December, 2025; v1 submitted 22 November, 2024; originally announced November 2024.

Improvement of the perovskite photodiodes performance via advanced interface engineering with polymer dielectric

Authors: A. P. Morozov, L. O. Luchnikov, S. Yu. Yurchuk, A. R. Ishteev, P. A. Gostishchev, S. I. Didenko, N. S. Saratovsky, S. S. Kozlov, D. S. Muratov, Yu. N. Luponosov, D. S. Saranin

Abstract: Halide perovskite-based photodiodes are promising for efficient detection across a broad spectral range. Perovskite absorber thin-films have a microcrystalline morphology, characterized by a high density of surface states and defects at inter-grain interfaces. In this work, we used dielectric-ferroelectric poly(vinylidene-fluoride-trifluoroethylene-P(VDF-TrFE) to modify the bulk interfaces and ele… ▽ More Halide perovskite-based photodiodes are promising for efficient detection across a broad spectral range. Perovskite absorber thin-films have a microcrystalline morphology, characterized by a high density of surface states and defects at inter-grain interfaces. In this work, we used dielectric-ferroelectric poly(vinylidene-fluoride-trifluoroethylene-P(VDF-TrFE) to modify the bulk interfaces and electron transport junction in p-i-n perovskite photodiodes. Our complex work demonstrates that interface engineering with P(VDF-TrFE) induces significant Fermi level pinning, reducing from 4.85 eV for intrinsic perovskite to 4.28 eV for the configuration with dielectric interlayers. The integration of P(VDF-TrFE) into the perovskite film did not affect the morphology and crystal structure, but significantly changed the charge transport and device performance. IV curve analysis and 2-diode model calculations showed enhanced shunt properties, a decreased non-ideality factor, and reduced saturation dark current. We have shown that the complex introduction of P(VDF-TrFE) into the absorbers bulk and on its surface is essential to reduce the impact of the trapping processes. For P(VDF-TrFE) containing devices, we increased the specific detectivity from 10^11 to 10^12 Jones, expanded the linear dynamic range up to 100 dB, and reduced the equivalent noise power to 10^-13 W*Hz^-0.5. Reducing non-radiative recombination contributions significantly enhanced device performance, improving rise/fall times from 6.3/10.9 us to 4.6/6.5 us. The cut-off frequency (3dB) increased from 64.8 kHz to 74.8 kHz following the introduction of the dielectric. These results provide new insights into the use of organic dielectrics and an improved understanding of trap-states and ion defect compensation for detectors based on perovskite heterostructures. △ Less

Submitted 3 July, 2024; originally announced July 2024.

Greater than five-order-of-magnitude post-compression temporal contrast improvement with an ionization plasma grating

Authors: Matthew R. Edwards, Nicholas M. Fasano, Andreas M. Giakas, Michelle M. Wang, Jesse Griff-McMahon, Anatoli Morozov, Victor M. Perez-Ramirez, Nuno Lemos, Pierre Michel, Julia M. Mikhailova

Abstract: High-intensity lasers require suppression of prepulses and other non-ideal temporal structure to avoid target disruption before the arrival of the main pulse. To address this, we demonstrate that ionization gratings act as a controllable optical switch for high-power light with a temporal contrast improvement of at least $3\times10^5$ and a switching time less than 500 fs. We also show that a grat… ▽ More High-intensity lasers require suppression of prepulses and other non-ideal temporal structure to avoid target disruption before the arrival of the main pulse. To address this, we demonstrate that ionization gratings act as a controllable optical switch for high-power light with a temporal contrast improvement of at least $3\times10^5$ and a switching time less than 500 fs. We also show that a grating system can run for hours at 10 Hz without degradation. The contrast improvement from an ionization grating compares favorably to that achievable with plasma mirrors. △ Less

Submitted 17 March, 2024; originally announced March 2024.

Comments: 6 pages, 4 figures

In-beam test results of an RPC-based module for position-sensitive neutron detectors with timing readout

Authors: G. Canezin, L. M. S. Margato, A. Morozov, A. Blanco, J. Saraiva, L. Lopes, P. Fonte, Chung Chuan Lai, Per-Olof Svensson, G. Markaj, Florian M. Piegsa

Abstract: Recently we have proposed a new concept of a thermal neutron detector based on resistive plate chambers and 10B4C solid neutron converters, enabling to readout with high resolution in both the 3D position of neutron capture and the neutron time of flight (ToF). In this paper, we report the results of the first beam tests conducted with a new neutron RPC detection module, coupled to the position re… ▽ More Recently we have proposed a new concept of a thermal neutron detector based on resistive plate chambers and 10B4C solid neutron converters, enabling to readout with high resolution in both the 3D position of neutron capture and the neutron time of flight (ToF). In this paper, we report the results of the first beam tests conducted with a new neutron RPC detection module, coupled to the position readout units of a new design. The main focus is on the measurements of the neutron ToF and identification of the converter layer where the neutron is captured, giving the position along the beam direction. △ Less

Submitted 23 February, 2024; originally announced February 2024.

Topological spectra and entropy of chromatin loop networks

Authors: Andrea Bonato, Dom Corbett, Sergey Kitaev, Davide Marenduzzo, Alexander Morozov, Enzo Orlandini

Abstract: The 3D folding of a mammalian gene can be studied by a polymer model, where the chromatin fibre is represented by a semiflexible polymer which interacts with multivalent proteins, representing complexes of DNA-binding transcription factors and RNA polymerases. This physical model leads to the natural emergence of clusters of proteins and binding sites, accompanied by the folding of chromatin into… ▽ More The 3D folding of a mammalian gene can be studied by a polymer model, where the chromatin fibre is represented by a semiflexible polymer which interacts with multivalent proteins, representing complexes of DNA-binding transcription factors and RNA polymerases. This physical model leads to the natural emergence of clusters of proteins and binding sites, accompanied by the folding of chromatin into a set of topologies, each associated with a different network of loops. Here we combine numerics and analytics to first classify these networks and then find their relative importance or statistical weight, when the properties of the underlying polymer are those relevant to chromatin. Unlike polymer networks previously studied, our chromatin networks have finite average distances between successive binding sites, and this leads to giant differences between the weights of topologies with the same number of edges and nodes but different wiring. These weights strongly favour rosette-like structures with a local cloud of loops with respect to more complicated non-local topologies. Our results suggest that genes should overwhelmingly fold into a small fraction of all possible 3D topologies, which can be robustly characterised by the framework we propose here. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Comments: 8 pages, 2 figures

Combinatorics and topological weights of chromatin loop networks

Authors: Andrea Bonato, Dom Corbett, Sergey Kitaev, Davide Marenduzzo, Alexander Morozov, Enzo Orlandini

Abstract: Polymer physics models suggest that chromatin spontaneously folds into loop networks with transcription units (TUs), such as enhancers and promoters, as anchors. Here we use combinatoric arguments to enumerate the emergent chromatin loop networks, both in the case where TUs are labelled and where they are unlabelled. We then combine these mathematical results with those of computer simulations aim… ▽ More Polymer physics models suggest that chromatin spontaneously folds into loop networks with transcription units (TUs), such as enhancers and promoters, as anchors. Here we use combinatoric arguments to enumerate the emergent chromatin loop networks, both in the case where TUs are labelled and where they are unlabelled. We then combine these mathematical results with those of computer simulations aimed at finding the inter-TU energy required to form a target loop network. We show that different topologies are vastly different in terms of both their combinatorial weight and energy of formation. We explain the latter result qualitatively by computing the topological weight of a given network -- i.e., its partition function in statistical mechanics language -- in the approximation where excluded volume interactions are neglected. Our results show that networks featuring local loops are statistically more likely with respect to networks including more non-local contacts. We suggest our classification of loop networks, together with our estimate of the combinatorial and topological weight of each network, will be relevant to catalogue 3D structures of chromatin fibres around eukaryotic genes, and to estimate their relative frequency in both simulations and experiments. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Comments: 19 pages, 11 figures

Purely elastic turbulence in pressure-driven channel flows

Authors: Martin Lellep, Moritz Linkmann, Alexander Morozov

Abstract: Solutions of long, flexible polymer molecules are complex fluids that simultaneously exhibit fluid-like and solid-like behaviour. When subjected to an external flow, dilute polymer solutions exhibit elastic turbulence - a unique, chaotic flow state absent in Newtonian fluids, like water. Unlike its Newtonian counterpart, elastic turbulence is caused by polymer molecules stretching and aligning in… ▽ More Solutions of long, flexible polymer molecules are complex fluids that simultaneously exhibit fluid-like and solid-like behaviour. When subjected to an external flow, dilute polymer solutions exhibit elastic turbulence - a unique, chaotic flow state absent in Newtonian fluids, like water. Unlike its Newtonian counterpart, elastic turbulence is caused by polymer molecules stretching and aligning in the flow, and can occur at vanishing inertia. While experimental realisations of elastic turbulence are well-documented, there is currently no understanding of its mechanism. Here, we present large-scale direct numerical simulations of elastic turbulence in pressure-driven flows through straight channels. We demonstrate that the transition to elastic turbulence is sub-critical, giving rise to spot-like flow structures that, further away from the transition, eventually spread throughout the domain. We provide evidence that elastic turbulence is organised around unstable coherent states that are localised close to the channel midplane. △ Less

Submitted 6 February, 2024; v1 submitted 13 December, 2023; originally announced December 2023.

Comments: 7 pages, 3 figures, SI; accepted to PNAS

Micro pixelated halide perovskite photodiodes fabricated with ultraviolet laser scribing

Authors: A. P. Morozov, P. A. Gostishchev, A. Zharkova, A. A. Vasilev, A. E. Aleksandrov, A. R. Tameev, A. R. Ishteev, S. I. Didenko, D. S. Saranin

Abstract: In this study, we present a complex investigation for miniaturizing of perovskite photodiodes (PPDs) in various geometries with use of ultraviolet laser scribing (UV-LS). Employing a 355 nm (3.5 eV) pulsed laser at 30 kHz, we successfully manufactured PPDs with pixel configurations of 70x130 um2, 520x580 um2, and 2000x2000 um2. The utilization of UV-LS has a proven efficiency in achieving relevant… ▽ More In this study, we present a complex investigation for miniaturizing of perovskite photodiodes (PPDs) in various geometries with use of ultraviolet laser scribing (UV-LS). Employing a 355 nm (3.5 eV) pulsed laser at 30 kHz, we successfully manufactured PPDs with pixel configurations of 70x130 um2, 520x580 um2, and 2000x2000 um2. The utilization of UV-LS has a proven efficiency in achieving relevant diode characteristics, such as low dark currents and high shunt resistance, as well as ultrafast response. The multi-step scribing cycle provided precise patterning of perovskite photodiodes (PPDs) in a string design. The dark current densities demonstrated exceptional uniformity, ranging from 10-10 A/cm2 for 2000x2000 um2 pixelated PPDs to 10-9 A/cm2 for the 70x130 um2 configuration. The string PPDs, consisting of 10 pixels per string, displayed homogenous dark current values, ensuring effective isolation between devices. Under green light illumination (540 nm), all PPD types exhibited a broad Linear Dynamic Range (LDR). Specifically, LDR values reached 110 dB, 117 dB, and 136 dB for 70x130, 520x580, and 2000x2000 devices, respectively, spanning an illumination intensity range from 2*10-3 mW/cm2 to 2 mW/cm2. High responsivity values up to 0.38 A/W, depending on the PPDs geometry, highlight the potential of laser scribing devices for sensing in the visible range. The calculated specific detectivity performance (from 1011 to 1013 Jones) surpasses commercial analogs, while the sub-microsecond response of 70x130 um2 and 520x580 um2 miniaturized devices underscores their suitability for precise time resolution detection systems. △ Less

Submitted 9 December, 2023; originally announced December 2023.

Collective motion in a sheet of microswimmers

Authors: Dóra Bárdfalvy, Viktor Škultéty, Cesare Nardini, Alexander Morozov, Joakim Stenhammar

Abstract: Self-propelled micron-size particles suspended in a fluid, like bacteria or synthetic microswimmers, are strongly non-equilibrium systems where particle motility breaks the microscopic detailed balance, often resulting in large-scale collective motion. Previous theoretical work has identified long-range hydrodynamic interactions as the main driver of collective motion in unbounded dilute suspensio… ▽ More Self-propelled micron-size particles suspended in a fluid, like bacteria or synthetic microswimmers, are strongly non-equilibrium systems where particle motility breaks the microscopic detailed balance, often resulting in large-scale collective motion. Previous theoretical work has identified long-range hydrodynamic interactions as the main driver of collective motion in unbounded dilute suspension of rear-actuated ("pusher") microswimmers. In contrast, most experimental studies of collective motion in microswimmer suspensions have been carried out in quasi-2-dimensional geometries such as in thin films or near solid or fluid interfaces, where both the swimmers' motion and their long-range flow fields become altered due to the proximity of a boundary. Here, we study numerically a minimal model of microswimmers in such a restricted geometry, where the particles move in the midplane between two no-slip walls. For pushers, we demonstrate collective motion with only short-ranged order, in contrast with the long-ranged flows observed in unbounded systems. For front-actuated ("puller") microswimmers, we discover a long-wavelength density instability resulting in the formation of dense microswimmer clusters. Both types of collective motion are fundamentally different from their previously studied counterparts in unbounded domains. Our results illustrate that hydrodynamic screening due to the presence of a wall is subdominant in determining the collective state of the suspension, which is instead dictated by the geometrical restriction of the swimmers' motion. △ Less

Submitted 5 March, 2024; v1 submitted 9 October, 2023; originally announced October 2023.

Journal ref: Communications Physics 7, 93 (2024)

An adaptive Bayesian approach to gradient-free global optimization

Authors: Jianneng Yu, Alexandre V. Morozov

Abstract: Many problems in science and technology require finding global minima or maxima of various objective functions. The functions are typically high-dimensional; each function evaluation may entail a significant computational cost. The importance of global optimization has inspired development of numerous heuristic algorithms based on analogies with physical, chemical or biological systems. Here we pr… ▽ More Many problems in science and technology require finding global minima or maxima of various objective functions. The functions are typically high-dimensional; each function evaluation may entail a significant computational cost. The importance of global optimization has inspired development of numerous heuristic algorithms based on analogies with physical, chemical or biological systems. Here we present a novel algorithm, SmartRunner, which employs a Bayesian probabilistic model informed by the history of accepted and rejected moves to make a decision about the next random trial. Thus, SmartRunner intelligently adapts its search strategy to a given objective function and moveset, with the goal of maximizing fitness gain (or energy loss) per function evaluation. Our approach can be viewed as adding a simple adaptive penalty to the original objective function, with SmartRunner performing hill ascent or descent on the modified landscape. This penalty can be added to many other global optimization algorithms. We explored SmartRunner's performance on a standard set of test functions, finding that it compares favorably against several widely-used alternatives: simulated annealing, stochastic hill climbing, evolutionary algorithm, and taboo search. Interestingly, adding the adaptive penalty to the first three of these algorithms considerably enhances their performance. We have also employed SmartRunner to study the Sherrington-Kirkpatrick (SK) spin glass model and Kauffman's NK fitness model - two NP-hard problems characterized by numerous local optima. In systems with quenched disorder, SmartRunner performs well compared to the other global optimizers. Moreover, in finite SK systems it finds close-to-optimal ground-state energies averaged over disorder. △ Less

Submitted 8 September, 2023; originally announced September 2023.

Comments: 25 pages, 7 figures, 2 tables in the main text; 22 pages, 18 figures in the supplement

MSC Class: 65K10 ACM Class: G.3

Timing resistive plate chambers for thermal neutron detection with 3D position sensitivity

Authors: L. M. S. Margato, G. Canezin, A. Morozov, A. Blanco, J. Saraiva, L. Lopes, P. Fonte

Abstract: An optimized design of a neutron detector based on timing RPCs (Resistive Plate Chambers) with boron-10 neutron converters is presented. The detector is composed of a stack of ten double gap RPCs with aluminium cathode plates coated on both sides with $^{10}B_{4}C$. This design enables simultaneous determination with high accuracy of both the neutron time-of-flight (down to ns resolution) and the… ▽ More An optimized design of a neutron detector based on timing RPCs (Resistive Plate Chambers) with boron-10 neutron converters is presented. The detector is composed of a stack of ten double gap RPCs with aluminium cathode plates coated on both sides with $^{10}B_{4}C$. This design enables simultaneous determination with high accuracy of both the neutron time-of-flight (down to ns resolution) and the interaction position in 3D (down to 0.25 mm resolution across and ~1 mm along the beam). It is shown that the detection efficiency can approach 60% for neutrons with $λ$ = 4.7 Ȧ. A new geometry with less material budget is introduced for the signal pick-up strip arrays. The results of simulation-based optimization of the design are reported considering the trade-off between the detection efficiency, the count rate capability and the amount of elastic scattering on the detector components. △ Less

Submitted 6 March, 2023; originally announced March 2023.

Hydrodynamic instabilities in a 2-D sheet of microswimmers embedded in a 3-D fluid

Authors: Viktor Škultéty, Dóra Bárdfalvy, Joakim Stenhammar, Cesare Nardini, Alexander Morozov

Abstract: A collection of microswimmers immersed in an incompressible fluid is characterised by strong interactions due to the long-range nature of the hydrodynamic fields generated by individual organisms. As a result, suspensions of rear-actuated `pusher' swimmers such as bacteria exhibit a collective motion state often referred to as `bacterial turbulence', characterised by large-scale chaotic flows. The… ▽ More A collection of microswimmers immersed in an incompressible fluid is characterised by strong interactions due to the long-range nature of the hydrodynamic fields generated by individual organisms. As a result, suspensions of rear-actuated `pusher' swimmers such as bacteria exhibit a collective motion state often referred to as `bacterial turbulence', characterised by large-scale chaotic flows. The onset of collective motion in pusher suspensions is classically understood within the framework of mean-field kinetic theories for dipolar swimmers. In bulk 2-D and 3-D, the theory predicts that the instability leading to bacterial turbulence is due to mutual swimmer reorientation and sets in at the largest length scale available to the suspension. Here, we construct a similar kinetic theory for the case of a dipolar microswimmer suspension restricted to a two-dimensional plane embedded in a three-dimensional incompressible fluid. This setting qualitatively mimics the effect of swimming close to a two-dimensional interface. We show that the in-plane flow fields are effectively compressible in spite of the incompressibility of the 3-D bulk fluid, and that microswimmers on average act as sources (pushers) or sinks (pullers). We analyse stability of the homogeneous and isotropic state, and find two types of instability that are qualitatively different from the bulk, three-dimensional case: First, we show that the analogue of the orientational pusher instability leading to bacterial turbulence in bulk systems instead occurs at the smallest length-scale available to the system. Second, an instability associated with density variations arises in puller suspensions as a generic consequence of the effective in-plane compressibility. We conclude that confinement can have a crucial role in determining the collective behaviour of microswimmer suspensions. △ Less

Submitted 16 December, 2023; v1 submitted 27 February, 2023; originally announced February 2023.

Comments: 23 pages, 6 figures; accepted for publication in J. Fluid Mech

Interplay between Brownian and hydrodynamic tracer diffusion in suspensions of swimming microorganisms

Authors: Henrik Nordanger, Alexander Morozov, Joakim Stenhammar

Abstract: The general problem of tracer diffusion in non-equilibrium baths is important in a wide range of systems, from the cellular level to geographical lengthscales. In this paper, we revisit the archetypical example of such a system: a collection of small passive particles immersed in a dilute suspension of non-interacting dipolar microswimmers, representing bacteria or algae. In particular, we conside… ▽ More The general problem of tracer diffusion in non-equilibrium baths is important in a wide range of systems, from the cellular level to geographical lengthscales. In this paper, we revisit the archetypical example of such a system: a collection of small passive particles immersed in a dilute suspension of non-interacting dipolar microswimmers, representing bacteria or algae. In particular, we consider the interplay between thermal (Brownian) diffusion and hydrodynamic (active) diffusion due to the persistent advection of tracers by microswimmer flow fields. Previously, it has been argued that even a moderate amount of Brownian diffusion is sufficient to significantly reduce the persistence time of tracer advection, leading to a significantly reduced value of the effective active diffusion coefficient $D_A$ compared to the non-Brownian case. Here, we show by large-scale simulations and kinetic theory that this effect is in fact only practically relevant for microswimmers that effectively remain stationary while still stirring up the surrounding fluid, so-called \emph{shakers}. In contrast, for moderate and high values of the swimming speed $v_s$, relevant for biological microswimmer suspensions, the effect of Brownian motion on $D_A$ is negligible, leading to the effects of advection by microswimmers and Brownian motion being additive. This conclusion contrasts with previous results from the literature, and encourages a reinterpretation of recent experimental measurements of $D_A$ for tracer particles of varying size in bacterial suspensions. △ Less

Submitted 8 October, 2023; v1 submitted 27 February, 2023; originally announced February 2023.

Comments: Accepted in Journal of Fluid Mechanics

Journal ref: Journal of Fluid Mechanics, 974, A25 (2023)

Linear stability analysis of purely elastic travelling wave solutions in pressure driven channel flows

Authors: Martin Lellep, Moritz Linkmann, Alexander Morozov

Abstract: Recent studies of pressure-driven flows of dilute polymer solutions in straight channels demonstrated the existence of two-dimensional coherent structures that are disconnected from the laminar state and appear through a sub-critical bifurcation from infinity. These travelling-wave solutions were suggested to organise the phase-space dynamics of purely elastic and elasto-inertial chaotic channel f… ▽ More Recent studies of pressure-driven flows of dilute polymer solutions in straight channels demonstrated the existence of two-dimensional coherent structures that are disconnected from the laminar state and appear through a sub-critical bifurcation from infinity. These travelling-wave solutions were suggested to organise the phase-space dynamics of purely elastic and elasto-inertial chaotic channel flows. Here, we consider a wide range of parameters, covering the purely-elastic and elasto-inertial cases, and demonstrate that the two-dimensional travelling-wave solutions are unstable when embedded in sufficiently wide three-dimensional domains. Our work demonstrates that studies of purely elastic and elasto-inertial turbulence in straight channels require three-dimensional simulations, and no reliable conclusions can be drawn from studying strictly two-dimensional channel flows. △ Less

Submitted 16 November, 2022; originally announced November 2022.

Comments: 10 pages, 5 pages

Active turbulence and spontaneous phase separation in inhomogeneous extensile active gels

Authors: Renato Assante, Dom Corbett, Davide Marenduzzo, Alexander Morozov

Abstract: We report numerical results for the hydrodynamics of inhomogeneous lyotropic and extensile active nematic gels. By simulating the coupled Cahn-Hilliard, Navier-Stokes, and Beris-Edwards equation for the evolution of the composition, flow and orientational order of an active nematic, we ask whether composition variations are important to determine its emergent physics. As in active gels of uniform… ▽ More We report numerical results for the hydrodynamics of inhomogeneous lyotropic and extensile active nematic gels. By simulating the coupled Cahn-Hilliard, Navier-Stokes, and Beris-Edwards equation for the evolution of the composition, flow and orientational order of an active nematic, we ask whether composition variations are important to determine its emergent physics. As in active gels of uniform composition, we find that increasing either activity or nematic tendency (e.g., overall active matter concentration) triggers a transition between an isotropic passive phase and an active nematic one. We show that composition inhomogeneities are important in the latter phase, where we find three types of possible dynamical regimes. First, we observe regular patterns with defects and vortices: these exist close to the passive-active transition. Second, for larger activity, or deeper in the nematic phase, we find active turbulence, as in active gels of uniform composition, but with exceedingly large composition variation. In the third regime, which is uniquely associated with inhomogeneity and occurs for large nematic tendency and low activity, we observe spontaneous microphase separation into active and passive domains. The microphase separated regime is notable in view of the absence of an explicit demixing term in the underlying free energy which we use, and we provide a theoretical analysis based on the common tangent construction which explains its existence. We hope this regime can be probed experimentally in the future. △ Less

Submitted 6 December, 2022; v1 submitted 5 September, 2022; originally announced September 2022.

Comments: 9 pages, 6 figures & Supplemental Material

Journal ref: Soft Matter, 2023, 19, 189-198

Network community detection and clustering with random walks

Authors: Aditya Ballal, Willow B. Kion-Crosby, Alexandre V. Morozov

Abstract: We present a novel approach to partitioning network nodes into non-overlapping communities - a key step in revealing network modularity and hierarchical organization. Our methodology, applicable to networks with both weighted and unweighted symmetric edges, uses random walks to explore neighboring nodes in the same community. The walk-likelihood algorithm (WLA) produces an optimal partition of net… ▽ More We present a novel approach to partitioning network nodes into non-overlapping communities - a key step in revealing network modularity and hierarchical organization. Our methodology, applicable to networks with both weighted and unweighted symmetric edges, uses random walks to explore neighboring nodes in the same community. The walk-likelihood algorithm (WLA) produces an optimal partition of network nodes into a given number of communities. The walk-likelihood community finder (WLCF) employs WLA to predict both the optimal number of communities and the corresponding network partition. We have extensively benchmarked both algorithms, finding that they outperform or match other methods in terms of the modularity of predicted partitions and the number of links between communities. Making use of the computational efficiency of our approach, we investigated a large-scale map of roads and intersections in the state of Colorado. Our clustering yielded geographically sensible boundaries between neighboring communities. △ Less

Submitted 22 February, 2022; originally announced February 2022.

Comments: 39 pages, 1 Table and 3 Figures in the main text, 5 Figures and 4 Tables in the Supplement

Understanding viscoelastic flow instabilities: Oldroyd-B and beyond

Authors: Hugo A. Castillo Sanchez, Mihailo R. Jovanovic, Satish Kumar, Alexander Morozov, V. Shankar, Ganesh Subramanian, Helen J. Wilson

Abstract: The Oldroyd-B model has been used extensively to predict a host of instabilities in shearing flows of viscoelastic fluids, often realized experimentally using polymer solutions. The present review, written on the occasion of the birth centenary of James Oldroyd, provides an overview of instabilities found across major classes of shearing flows. These comprise (i) the canonical rectilinear shearing… ▽ More The Oldroyd-B model has been used extensively to predict a host of instabilities in shearing flows of viscoelastic fluids, often realized experimentally using polymer solutions. The present review, written on the occasion of the birth centenary of James Oldroyd, provides an overview of instabilities found across major classes of shearing flows. These comprise (i) the canonical rectilinear shearing flows including plane Couette, plane and pipe Poiseuille flows; (ii) viscometric shearing flows with curved streamlines such as those in the Taylor-Couette, cone-and-plate and parallel-plate geometries; (iii) non-viscometric shearing flows with an underlying extensional flow topology such as the flow in a cross-slot device; and (iv) multilayer shearing flows. While the underlying focus in all these cases is on results obtained using the Oldroyd-B model, we also discuss their relation to the actual instability, and as to how the shortcomings of the Oldroyd-B model may be overcome by the use of more realistic constitutive models. All the three commonly used tools of stability analysis, viz., modal linear stability, nonmodal stability, and weakly nonlinear stability analyses are discussed, with supporting evidence from experiments and numerical simulations as appropriate. Despite only accounting for a shear-rate-independent viscosity and first normal stress coefficient, the Oldroyd-B model is able to qualitatively predict the majority of instabilities in the aforementioned shearing flows. The review also highlights, where appropriate, open questions in the area of viscoelastic stability. △ Less

Submitted 16 February, 2022; originally announced February 2022.

Comments: Invited review article for the special issue of JNNFM commemorating the birth centenary of James Oldroyd

Journal ref: Journal of non-Newtonian Fluid Mechanics, Vol 302, 104742, 2022

Model selection and signal extraction using Gaussian Process regression

Authors: Abhijith Gandrakota, Amitabh Lath, Alexandre V. Morozov, Sindhu Murthy

Abstract: We present a novel computational approach for extracting weak signals, whose exact location and width may be unknown, from complex background distributions with an arbitrary functional form. We focus on datasets that can be naturally presented as binned integer counts, demonstrating our approach on the CERN open dataset from the ATLAS collaboration at the Large Hadron Collider, which contains the… ▽ More We present a novel computational approach for extracting weak signals, whose exact location and width may be unknown, from complex background distributions with an arbitrary functional form. We focus on datasets that can be naturally presented as binned integer counts, demonstrating our approach on the CERN open dataset from the ATLAS collaboration at the Large Hadron Collider, which contains the Higgs boson signature. Our approach is based on Gaussian Process (GP) regression - a powerful and flexible machine learning technique that allowed us to model the background without specifying its functional form explicitly, and to separate the background and signal contributions in a robust and reproducible manner. Unlike functional fits, our GP-regression-based approach does not need to be constantly updated as more data becomes available. We discuss how to select the GP kernel type, considering trade-offs between kernel complexity and its ability to capture the features of the background distribution. We show that our GP framework can be used to detect the Higgs boson resonance in the data with more statistical significance than a polynomial fit specifically tailored to the dataset. Finally, we use Markov Chain Monte Carlo (MCMC) sampling to confirm the statistical significance of the extracted Higgs signature. △ Less

Submitted 11 February, 2022; originally announced February 2022.

Report number: FERMILAB-PUB-22-073-CMS

Coherent structures in plane channel flow of dilute polymer solutions with vanishing inertia

Authors: Alexander Morozov

Abstract: When subjected to sufficiently strong velocity gradients, solutions of long, flexible polymers exhibit flow instabilities and chaotic motion, often referred to as elastic turbulence. Its mechanism differs from the familiar, inertia-driven turbulence in Newtonian fluids, and is poorly understood. Here, we demonstrate that the dynamics of purely elastic pressure-driven channel flows of dilute polyme… ▽ More When subjected to sufficiently strong velocity gradients, solutions of long, flexible polymers exhibit flow instabilities and chaotic motion, often referred to as elastic turbulence. Its mechanism differs from the familiar, inertia-driven turbulence in Newtonian fluids, and is poorly understood. Here, we demonstrate that the dynamics of purely elastic pressure-driven channel flows of dilute polymer solutions are organised by exact coherent structures that take the form of two-dimensional travelling waves. Our results demonstrate that no linear instability is required to sustain such travelling wave solutions, and that their origin is purely elastic in nature. We show that the associated stress profiles are characterised by thin, filament-like arrangements of polymer stretch, which is sustained by a solitary pair of vortices. We discuss the implications of the travelling wave solutions for the transition to elastic turbulence in straight channels, and propose ways for their detection in experiments. △ Less

Submitted 6 June, 2022; v1 submitted 4 January, 2022; originally announced January 2022.

Concept of a fast neutron detector based on 10B-RPCs

Authors: A. Morozov, L. M. S. Margato, A. Blanco, D. Galaviz

Abstract: We propose an alternative approach for the detection of fast neutrons in the energy range from 10-4 to 5 MeV based on 10B-RPCs (hybrid double-gap Resistive Plate Chambers with 10B4C neutron converters) surrounded by a polyethylene moderator. The detection efficiency as a function of the neutron energy is obtained in Monte Carlo simulations performed with the Geant4 toolkit for several detector con… ▽ More We propose an alternative approach for the detection of fast neutrons in the energy range from 10-4 to 5 MeV based on 10B-RPCs (hybrid double-gap Resistive Plate Chambers with 10B4C neutron converters) surrounded by a polyethylene moderator. The detection efficiency as a function of the neutron energy is obtained in Monte Carlo simulations performed with the Geant4 toolkit for several detector configurations. The thickness of the neutron converter is optimized for the maximum efficiency. The results show that for this type of detector it is possible to obtain an average detection efficiency larger than 50% with a weak dependence on the neutron energy. The dark count rate and gamma sensitivity are also discussed. △ Less

Submitted 23 December, 2021; originally announced December 2021.

Polymers in Swarming Bacterial Turbulence

Authors: Ranjiangshang Ran, David A. Gagnon, Alexander Morozov, Paulo E. Arratia

Abstract: We experimentally investigate the effects of polymer additives on the collective dynamics of swarming Serratia marcescens in quasi two-dimensional (2D) liquid films. We find that even minute amounts of polymers (< 20 ppm) can significantly enhance swimming speed and promote largescale coherent structures. Velocity statistics show that polymers suppress large velocity fluctuation, transforming the… ▽ More We experimentally investigate the effects of polymer additives on the collective dynamics of swarming Serratia marcescens in quasi two-dimensional (2D) liquid films. We find that even minute amounts of polymers (< 20 ppm) can significantly enhance swimming speed and promote largescale coherent structures. Velocity statistics show that polymers suppress large velocity fluctuation, transforming the velocity distributions from super-Gaussian to Gaussian. Spatial and temporal correlation functions suggest that polymers increase both the size and lifetime of flow structures. The energy spectra show an exponential decay at low wavenumbers, with a characteristic length scale increasing with polymer concentration. Overall, these result show polymers can mediate bacteria interaction and promote large-scale coherence in dense active suspensions. △ Less

Submitted 5 August, 2022; v1 submitted 29 October, 2021; originally announced November 2021.

Perspectives on viscoelastic flow instabilities and elastic turbulence

Authors: Sujit S. Datta, Arezoo M. Ardekani, Paulo E. Arratia, Antony N. Beris, Irmgard Bischofberger, Jens G. Eggers, J. Esteban López-Aguilar, Suzanne M. Fielding, Anna Frishman, Michael D. Graham, Jeffrey S. Guasto, Simon J. Haward, Sarah Hormozi, Gareth H. McKinley, Robert J. Poole, Alexander Morozov, V. Shankar, Eric S. G. Shaqfeh, Amy Q. Shen, Holger Stark, Victor Steinberg, Ganesh Subramanian, Howard A. Stone

Abstract: Viscoelastic fluids are a common subclass of rheologically complex materials that are encountered in diverse fields from biology to polymer processing. Often the flows of viscoelastic fluids are unstable in situations where ordinary Newtonian fluids are stable, owing to the nonlinear coupling of the elastic and viscous stresses. Perhaps more surprisingly, the instabilities produce flows with the h… ▽ More Viscoelastic fluids are a common subclass of rheologically complex materials that are encountered in diverse fields from biology to polymer processing. Often the flows of viscoelastic fluids are unstable in situations where ordinary Newtonian fluids are stable, owing to the nonlinear coupling of the elastic and viscous stresses. Perhaps more surprisingly, the instabilities produce flows with the hallmarks of turbulence -- even though the effective Reynolds numbers may be $O(1)$ or smaller. We provide perspectives on viscoelastic flow instabilities by integrating the input from speakers at a recent international workshop: historical remarks, characterization of fluids and flows, discussion of experimental and simulation tools, and modern questions and puzzles that motivate further studies of this fascinating subject. The materials here will be useful for researchers and educators alike, especially as the subject continues to evolve in both fundamental understanding and applications in engineering and the sciences. △ Less

Submitted 22 August, 2021; originally announced August 2021.

Comments: A perspective article based on a virtual workshop of the Princeton Center for Theoretical Sciences that took place in January 2021

Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions

Authors: Henrik Nordanger, Alexander Morozov, Joakim Stenhammar

Abstract: Tracer particles immersed in suspensions of biological microswimmers such as E. coli or C. reinhardtii display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, 3-dimensional suspensions, these phenomena have typically been explained by the hydrodynamic advection of point… ▽ More Tracer particles immersed in suspensions of biological microswimmers such as E. coli or C. reinhardtii display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, 3-dimensional suspensions, these phenomena have typically been explained by the hydrodynamic advection of point tracers by isolated microswimmers, while, at higher concentrations, correlations between pusher microswimmers such as E. coli can increase the effective diffusivity even further. Anisotropic tracers in active suspensions can be expected to exhibit even more complex behaviour than spherical ones, due to the presence of a nontrivial translation-rotation coupling. Using large-scale lattice Boltzmann simulations of model microswimmers described by extended force dipoles, we study the motion of ellipsoidal point tracers immersed in 3-dimensional microswimmer suspensions. We find that the rotational diffusivity of tracers is much less affected by swimmer-swimmer correlations than the translational diffusivity. We furthermore study the anisotropic translational diffusion in the particle frame and find that, in pusher suspensions, the diffusivity along the ellipsoid major axis is higher than in the direction perpendicular to it, albeit with a smaller ratio than for Brownian diffusion. Thus, we find that far field hydrodynamics cannot account for the anomalous coupling between translation and rotation observed in experiments, as was recently proposed. Finally, we study the probability distributions (PDFs) of translational and rotational displacements. In accordance with experimental observations, for short observation times we observe strongly non-Gaussian PDFs that collapse when rescaled with their variance, which we attribute to the ballistic nature of tracer motion at short times. △ Less

Submitted 26 January, 2022; v1 submitted 8 August, 2021; originally announced August 2021.

Journal ref: Phys. Rev. Fluids 7, 013103 (2022)

Purely elastic linear instabilities in parallel shear flows with free-slip boundary conditions

Authors: Martin Lellep, Moritz Linkmann, Bruno Eckhardt, Alexander Morozov

Abstract: We perform a linear stability analysis of viscoelastic plane Couette and plane Poiseuille flows with free-slip boundary conditions. The fluid is described by the Oldroyd-B constitutive model, and the flows are driven by a suitable body force. We find that both types of flow become linearly unstable, and we characterise the spatial structure of the unstable modes. By performing a boundary condition… ▽ More We perform a linear stability analysis of viscoelastic plane Couette and plane Poiseuille flows with free-slip boundary conditions. The fluid is described by the Oldroyd-B constitutive model, and the flows are driven by a suitable body force. We find that both types of flow become linearly unstable, and we characterise the spatial structure of the unstable modes. By performing a boundary condition homotopy from the free-slip to no-slip boundaries, we demonstrate that the unstable modes are directly related to the least stable modes of the no-slip problem, destabilised under the free-slip situation. We discuss how our observations can be used to study recently discovered purely elastic turbulence in parallel shear flows. △ Less

Submitted 21 September, 2021; v1 submitted 6 August, 2021; originally announced August 2021.

Comments: 10 pages, 7 figures

Journal ref: Journal of Fluid Mechanics, 928, 10 December 2021, R3

Statistical position reconstruction for RPC-based thermal neutron detectors

Authors: A. Morozov, L. M. S. Margato, V. Solovov, A. Blanco, J. Saraiva, T. Wilpert, K. Zeitelhack, K. Roemer, C. Höglund, L. Robinson, R. Hall-Wilton

Abstract: Multilayer position-sensitive 10B-RPC thermal neutron detectors offer an attractive combination of sub-millimeter spatial resolution and high (>50%) detection efficiency. Here we describe a new position reconstruction method based on a statistical approach. Using experimental data, we compare the performance of this method with that of the centroid reconstruction. Both methods result in a similar… ▽ More Multilayer position-sensitive 10B-RPC thermal neutron detectors offer an attractive combination of sub-millimeter spatial resolution and high (>50%) detection efficiency. Here we describe a new position reconstruction method based on a statistical approach. Using experimental data, we compare the performance of this method with that of the centroid reconstruction. Both methods result in a similar image linearity/uniformity and spatial resolution. However, the statistical method allows to improve the image quality at the detector periphery, offers more flexible event filtering and allows to develop automatic quality monitoring procedures for early detection of situations when a change in the detector operation conditions starts to affect reconstruction quality. △ Less

Submitted 26 July, 2021; v1 submitted 28 May, 2021; originally announced May 2021.

Towards high-rate RPC-based thermal neutron detectors using low-resistivity electrodes

Authors: L. M. S. Margato, A. Morozov, A. Blanco, P. Fonte, L. Lopes, J. Saraiva, K. Zeitelhack, R. Hall-Wilton, C. Höglund, L. Robinson, P. Svensson, L. Naumann, K. Roemer, D. Stach, Th. Wilpert

Abstract: We present experimental results on the counting rate measurements for several single-gap $^{10}$B lined resistive plate chambers ($^{10}$B-RPCs) with anodes made from standard float glass, low resistivity glass and ceramic. The measurements were performed at the V17 monochromatic neutron beamline (3.35 Ȧ) at the Helmholtz-Zentrum Berlin. For the $^{10}$B-RPCs with 0.28 mm thick float glass a maxim… ▽ More We present experimental results on the counting rate measurements for several single-gap $^{10}$B lined resistive plate chambers ($^{10}$B-RPCs) with anodes made from standard float glass, low resistivity glass and ceramic. The measurements were performed at the V17 monochromatic neutron beamline (3.35 Ȧ) at the Helmholtz-Zentrum Berlin. For the $^{10}$B-RPCs with 0.28 mm thick float glass a maximum counting rate density of about $8\times 10^{3}$ $Hz/cm^{2}$ was obtained. In the case of low resistivity glass and ceramic, the counting rate density did not deviate from linear dependence on the neutron flux up to the maximum flux available at this beamline and exceeded a value of $3\times 10^{4}$ $Hz/cm^{2}$. △ Less

Submitted 1 April, 2021; originally announced April 2021.

Optical valleytronics of impurity states in two-dimensional Dirac materials

Authors: Dogyun Ko, A. V. Morozov, V. M. Kovalev, I. G. Savenko

Abstract: We analyze the valley selection rules for optical transitions from impurity states to the conduction band in two-dimensional Dirac materials, taking a monolayer of MoS2 as an example. We employ the analytical model of a shallow impurity potential which localizes electrons described by a spinor wave function, and, first, find the system eigenstates taking into account the presence of two valleys in… ▽ More We analyze the valley selection rules for optical transitions from impurity states to the conduction band in two-dimensional Dirac materials, taking a monolayer of MoS2 as an example. We employ the analytical model of a shallow impurity potential which localizes electrons described by a spinor wave function, and, first, find the system eigenstates taking into account the presence of two valleys in the Brillouin zone. Then, we find the spectrum of the absorbance and calculate the photon-drag electric current due to the impurity-band transitions, drawing the general conclusions regarding the valley optical selection rules for the impurity-band optical transitions in gapped Dirac materials. △ Less

Submitted 6 April, 2021; v1 submitted 26 October, 2020; originally announced October 2020.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 103, 161301 (2021)

Electrostatic inactivation of RNA viruses at air-water and liquid-liquid interfaces

Authors: C. A. Brackley, A. Lips, A. Morozov, W. C. K. Poon, D. Marenduzzo

Abstract: Understanding the interactions between viruses and surfaces or interfaces is important, as they provide the principles underpinning the cleaning and disinfection of contaminated surfaces. Yet, the physics of such interactions is currently poorly understood. For instance, there are longstanding experimental observations suggesting that the presence of air-water interfaces can generically inactivate… ▽ More Understanding the interactions between viruses and surfaces or interfaces is important, as they provide the principles underpinning the cleaning and disinfection of contaminated surfaces. Yet, the physics of such interactions is currently poorly understood. For instance, there are longstanding experimental observations suggesting that the presence of air-water interfaces can generically inactivate and kill viruses, yet the mechanism underlying this phenomenon remains unknown. Here we use theory and simulations to show that electrostatics provides one such mechanism, and that this is very general. Thus, we predict that the free energy of an RNA virus should increase by several thousands of $k_BT$ as the virion breaches an air-water interface. We also show that the fate of a virus approaching a generic liquid-liquid interface depends strongly on the detailed balance between interfacial and electrostatic forces, which can be tuned, for instance, by choosing different media to contact a virus-laden respiratory droplet. We propose that these results can be used to design effective strategies for surface disinfection. Intriguingly, tunability requires electrostatic and interfacial forces to scale similarly with viral size, which naturally occurs when charges are arranged in a double-shell distribution as in RNA viruses like influenza and all coronaviruses. △ Less

Submitted 15 December, 2020; v1 submitted 23 October, 2020; originally announced October 2020.

Comments: 10 pages, 5 figures; minor corrections to the Appendix

Extrusion without a motor: a new take on the loop extrusion model of genome organization

Authors: C. A. Brackley, J. Johnson, D. Michieletto, A. N. Morozov, M. Nicodemi, P. R. Cook, D. Marenduzzo

Abstract: Chromatin loop extrusion is a popular model for the formation of CTCF loops and topological domains. Recent HiC data have revealed a strong bias in favour of a particular arrangement of the CTCF binding motifs that stabilize loops, and extrusion is the only model to date which can explain this. However, the model requires a motor to generate the loops, and although cohesin is a strong candidate fo… ▽ More Chromatin loop extrusion is a popular model for the formation of CTCF loops and topological domains. Recent HiC data have revealed a strong bias in favour of a particular arrangement of the CTCF binding motifs that stabilize loops, and extrusion is the only model to date which can explain this. However, the model requires a motor to generate the loops, and although cohesin is a strong candidate for the extruding factor, a suitable motor protein (or a motor activity in cohesin itself) has yet to be found. Here we explore a new hypothesis: that there is no motor, and thermal motion within the nucleus drives extrusion. Using theoretical modelling and computer simulations we ask whether such diffusive extrusion could feasibly generate loops. Our simulations uncover an interesting ratchet effect (where an osmotic pressure promotes loop growth), and suggest, by comparison to recent in vitro and in vivo measurements, that diffusive extrusion can in principle generate loops of the size observed in the data. Extra View on : C. A. Brackley, J. Johnson, D. Michieletto, A. N. Morozov, M. Nicodemi, P. R. Cook, and D. Marenduzzo "Non-equilibrium chromosome looping via molecular slip-links", Physical Review Letters 119, 138101 (2017) △ Less

Submitted 6 October, 2020; originally announced October 2020.

Comments: 7 pages, 3 figures

Journal ref: Nucleus 9, 95 (2018)

Distal edge determination precision for a multi-slat promptgamma camera: a comprehensive simulation and optimization of the detection system

Authors: A. Morozov, H. Simões, P. Crespo

Abstract: Multi-slat prompt-gamma camera is a promising tool for range monitoring during proton therapy. We report the results of a comprehensive simulation study analyzing the precision which is possible to reach with this camera in determination of the position of the distal edge of the Bragg peak. For the first time we include simulation of optical photons. The proton beam (single pencil beam, 130 MeV, 1… ▽ More Multi-slat prompt-gamma camera is a promising tool for range monitoring during proton therapy. We report the results of a comprehensive simulation study analyzing the precision which is possible to reach with this camera in determination of the position of the distal edge of the Bragg peak. For the first time we include simulation of optical photons. The proton beam (single pencil beam, 130 MeV, 10 ns bunch period, total of 10^8 protons) is interacting with a polymethyl methacrylate (PMMA) phantom, which is a cylinder of 200 mm in diameter and length. The prompt gamma rays generated in the phantom are collimated with a multi-slat collimator and detected using a combination of yttrium aluminum perovskite (YAP) scintillators, installed in the collimator apertures, and light sensors. Two scintillator packing schemes, with one and with two scintillator plates per aperture, are considered. The collimator configuration (the septal thickness, aperture and height), resulting in the best precision, is determined using two methods of detector optimization. Precision of 2.1 mm (full width at half maximum) in the edge position determination is demonstrated. △ Less

Submitted 19 January, 2021; v1 submitted 29 September, 2020; originally announced September 2020.

Creeping thermocapillary motion of a Newtonian droplet suspended in a viscoelastic fluid

Authors: Paolo Capobianchi, Mahdi Davoodi, Robert J. Poole, Marcello Lappa, Alexander Morozov, Mónica S. N. Oliveira

Abstract: In this work we consider theoretically the problem of a Newtonian droplet moving in an otherwise quiescent infinite viscoelastic fluid under the influence of an externally applied temperature gradient. The outer fluid is modelled by the Oldroyd-B equation, and the problem is solved for small Weissenberg and Capillary numbers in terms of a double perturbation expansion. We assume microgravity con… ▽ More In this work we consider theoretically the problem of a Newtonian droplet moving in an otherwise quiescent infinite viscoelastic fluid under the influence of an externally applied temperature gradient. The outer fluid is modelled by the Oldroyd-B equation, and the problem is solved for small Weissenberg and Capillary numbers in terms of a double perturbation expansion. We assume microgravity conditions and neglect the convective transport of energy and momentum. We derive expressions for the droplet migration speed and its shape in terms of the properties of both fluids. In the absence of shape deformation, the droplet speed decreases monotonically for sufficiently viscous inner fluids, while for fluids with a smaller inner-to-outer viscosity ratio, the droplet speed first increases and then decreases as a function of the Weissenberg number. For small but finite values of the Capillary number, the droplet speed behaves monotonically as a function of the applied temperature gradient for a fixed ratio of the Capillary and Weissenberg numbers. We demonstrate that this behaviour is related to the polymeric stresses deforming the droplet in the direction of its migration, while the associated changes in its speed are Newtonian in nature, being related to a change in the droplet's hydrodynamic resistance and its internal temperature distribution. When compared to the results of numerical simulations, our theory exhibits a good predictive power for sufficiently small values of the Capillary and Weissenberg numbers. △ Less

Submitted 13 December, 2023; v1 submitted 11 August, 2020; originally announced August 2020.

Comments: 28 pages, 7 figures, accepted for publication in Journal of Non-Newtonian Fluid Mechanics

Error estimation in the method of quasi-optimal weights

Authors: A. D. Morozov, A. V. Lokhov, F. V. Tkachov

Abstract: We examine the problem of construction of confidence intervals within the basic single-parameter, single-iteration variation of the method of quasi-optimal weights. Two kinds of distortions of such intervals due to insufficiently large samples are examined, both allowing an analytical investigation. First, a criterion is developed for validity of the assumption of asymptotic normality together wit… ▽ More We examine the problem of construction of confidence intervals within the basic single-parameter, single-iteration variation of the method of quasi-optimal weights. Two kinds of distortions of such intervals due to insufficiently large samples are examined, both allowing an analytical investigation. First, a criterion is developed for validity of the assumption of asymptotic normality together with a recipe for the corresponding corrections. Second, a method is derived to take into account the systematic shift of the confidence interval due to the non-linearity of the theoretical mean of the weight as a function of the parameter to be estimated. A numerical example illustrates the two corrections. △ Less

Submitted 26 May, 2020; originally announced May 2020.

Comments: 16 pages

Report number: INR-TH-2020-007

Symmetric mixtures of pusher and puller microswimmers behave as noninteracting suspensions

Authors: Dóra Bárdfalvy, Shan Anjum, Cesare Nardini, Alexander Morozov, Joakim Stenhammar

Abstract: Suspensions of rear- and front-actuated microswimmers immersed in a fluid, known respectively as ``pushers'' and ``pullers'', display qualitatively different collective behaviours: beyond a characteristic density, pusher suspensions exhibit a hydrodynamic instability leading to collective motion known as active turbulence, a phenomenon which is absent for pullers. In this Letter, we describe the c… ▽ More Suspensions of rear- and front-actuated microswimmers immersed in a fluid, known respectively as ``pushers'' and ``pullers'', display qualitatively different collective behaviours: beyond a characteristic density, pusher suspensions exhibit a hydrodynamic instability leading to collective motion known as active turbulence, a phenomenon which is absent for pullers. In this Letter, we describe the collective dynamics of a binary pusher--puller mixture using kinetic theory and large-scale particle-resolved simulations. We derive and verify an instability criterion, showing that the critical density for active turbulence moves to higher values as the fraction $χ$ of pullers is increased and disappears for $χ\geq 0.5$. We then show analytically and numerically that the two-point hydrodynamic correlations of the 1:1 mixture are equal to those of a suspension of noninteracting swimmers. Strikingly, our numerical analysis furthermore shows that the full probability distribution of the fluid velocity fluctuations collapses onto the one of a noninteracting system at the same density, where swimmer--swimmer correlations are strictly absent. Our results thus indicate that the fluid velocity fluctuations in 1:1 pusher--puller mixtures are exactly equal to those of the corresponding noninteracting suspension at any density, a surprising cancellation with no counterpart in equilibrium long-range interacting systems. △ Less

Submitted 7 July, 2020; v1 submitted 15 April, 2020; originally announced April 2020.

Journal ref: Phys. Rev. Lett. 125, 018003 (2020)

Simulation-based optimization of a multilayer 10B-RPC thermal neutron detector

Authors: A. Morozov, L. M. S. Margato, I. Stefanescu

Abstract: A Monte Carlo simulation-based optimization of a multilayer 10B-RPC thermal neutron detector is performed targeting an increase in the counting rate capability while maintaining high (>50%) detection efficiency for thermal neutrons. The converter layer thicknesses of individual RPCs are optimized for several configurations of a detector containing a stack of 10 double gap RPCs. The results suggest… ▽ More A Monte Carlo simulation-based optimization of a multilayer 10B-RPC thermal neutron detector is performed targeting an increase in the counting rate capability while maintaining high (>50%) detection efficiency for thermal neutrons. The converter layer thicknesses of individual RPCs are optimized for several configurations of a detector containing a stack of 10 double gap RPCs. The results suggest that it is possible to reach a counting rate which is by a factor of eight higher in comparison to the rate of a detector with only one double-gap RPC. The effect of neutron scattering inside the detector contributing to the background is analyzed and design modifications of the first detector prototype, tested at neutron beam, are suggested. △ Less

Submitted 18 February, 2020; v1 submitted 6 February, 2020; originally announced February 2020.

Multilayer $^{10}B$-RPC neutron imaging detector

Authors: L. M. S. Margato, A. Morozov, A. Blanco, P. Fonte, L. Lopes, K. Zeitelhack, R. Hall-Wilton, C. Höglund, L. Robinson, S. Schmidt, P. Svensson

Abstract: Resistive plate chambers (RPC) lined with $^{10}B_{4}$C neutron converters is a promising cost effective technology for position-sensitive thermal neutron detection capable to outperform $^{3}$He-based detectors in terms of spatial resolution and timing. However, as for the other types of gaseous detectors with a single layer of $^{10}B_{4}$C at normal beam incidence, the detection efficiency to t… ▽ More Resistive plate chambers (RPC) lined with $^{10}B_{4}$C neutron converters is a promising cost effective technology for position-sensitive thermal neutron detection capable to outperform $^{3}$He-based detectors in terms of spatial resolution and timing. However, as for the other types of gaseous detectors with a single layer of $^{10}B_{4}$C at normal beam incidence, the detection efficiency to thermal neutrons of a single-gap $^{10}B$-RPC is only about 6%. Aiming to overcome this limitation, we introduce a multi-layer $^{10}B$-RPCs detector with a stack of ten double-gap hybrid RPCs. A description of the detector design and the results of its characterization performed at the TREFF neutron beamline at the FRM II neutron facility are presented. The results demonstrate that the detection efficiency exceeds 60% for neutrons with a wavelength of 4.7 Å and the spatial resolution (FWHM) is about 0.25 mm and 0.35 mm in the X and Y direction, respectively. △ Less

Submitted 26 May, 2020; v1 submitted 3 February, 2020; originally announced February 2020.