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Irreversibility and symmetry breaking in the creation and annihilation of defects in active living matter
Authors:
Avraham Beer,
Efraim Dov Neimand,
Yuv Agarwal,
Dom Corbett,
Daniel J. G. Pearce,
Gil Ariel,
Victor Yashunsky
Abstract:
Active living matter continuously creates and annihilates topological defects in a process that remains poorly understood. Here, we investigate these dynamics in two distinct active living systems: swarming bacteria and human bronchial epithelial cells. Despite their entirely different evolutionary origins, biological functions, and physical scales, both systems exhibit half-integer defects, consi…
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Active living matter continuously creates and annihilates topological defects in a process that remains poorly understood. Here, we investigate these dynamics in two distinct active living systems: swarming bacteria and human bronchial epithelial cells. Despite their entirely different evolutionary origins, biological functions, and physical scales, both systems exhibit half-integer defects, consistent with the nematic phase. However, in contrast to active nematic theory, we find that defect creation and annihilation undergoes spatial symmetry breaking. We propose that these results stem from a fundamental dualism between nematic structural organization and generated polar forces, which are intrinsic to living systems. Furthermore, estimation of entropy production reveals that creation and annihilation are not reversed processes. Our findings challenge conventional nematic models and emphasize the role of defect-mediated dynamics in non-equilibrium biological systems as a major source of entropy production.
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Submitted 4 March, 2026; v1 submitted 21 August, 2025;
originally announced August 2025.
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Immobility of isolated swarmer cells due to local liquid depletion
Authors:
Ajesh Jose,
Benjamin Perez-Estay,
Shira Omer Bendori,
Avigdor Eldar,
Daniel B. Kearns,
Gil Ariel,
Avraham Beer
Abstract:
Bacterial swarming is a complex phenomenon in which thousands of self-propelled rod-shaped cells move coherently on surfaces, providing an excellent example of active matter. However, bacterial swarming is different from most studied examples of active systems because single isolated cells do not move, while clusters do. The biophysical aspects underlying this behavior are unclear. In this work we…
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Bacterial swarming is a complex phenomenon in which thousands of self-propelled rod-shaped cells move coherently on surfaces, providing an excellent example of active matter. However, bacterial swarming is different from most studied examples of active systems because single isolated cells do not move, while clusters do. The biophysical aspects underlying this behavior are unclear. In this work we explore the case of low local cell densities, where single cells become temporarily immobile. We show that immobility is related to local depletion of liquid. In addition, it is also associated with the state of the flagella. Specifically, the flagellar bundles at (temporarily) liquid depleted regions are completely spread-out. Our results suggest that dry models of self-propelled agents, which only consider steric alignments and neglect hydrodynamic effects, are oversimplified and are not sufficient to describe swarming bacteria.
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Submitted 26 November, 2024;
originally announced November 2024.
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Highly superlinear photogalvanic effects in (Bi$_{0.3}$Sb$_{0.7}$)$_2$(Te$_{0.1}$Se$_{0.9}$)$_3$: Probing 3D topological insulator surface states at room temperature
Authors:
Sergey N. Danilov,
Leonid E. Golub,
Thomas Mayer,
Andreas Beer,
Stefan Binder,
Erwin Mönch,
Jan Minar,
Matthias Kronseder,
Christian. H. Back,
Dominique Bougeard,
Sergey D. Ganichev
Abstract:
We report on the observation of complex nonlinear intensity dependence of the circular and linear photogalvanic currents induced by infrared radiation in compensated (Bi$_{0.3}$Sb$_{0.7}$)$_2$(Te$_{0.1}$Se$_{0.9}$)$_3$ 3D topological insulators. The photocurrents are induced by direct optical transitions between topological surface and bulk states. We show that an increase of the radiation intensi…
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We report on the observation of complex nonlinear intensity dependence of the circular and linear photogalvanic currents induced by infrared radiation in compensated (Bi$_{0.3}$Sb$_{0.7}$)$_2$(Te$_{0.1}$Se$_{0.9}$)$_3$ 3D topological insulators. The photocurrents are induced by direct optical transitions between topological surface and bulk states. We show that an increase of the radiation intensity results first in a highly superlinear raise of the amplitude of both types of photocurrents, whereas at higher intensities the photocurrent saturates. Our analysis of the observed nonlinearities shows that the superlinear behavior of the photocurrents is caused by a heating of the electron gas, while the saturation is induced by a slow relaxation of the photoexcited carriers resulting in absorbance bleaching. The observed nonlinearities give access to the Fermi level position with respect to the Dirac point and the energy relaxation times of Dirac fermions providing an experimental room temperature probe for topological surface states.
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Submitted 3 August, 2021;
originally announced August 2021.
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A phase diagram for bacterial swarming
Authors:
Avraham Be`er,
Bella Ilkanaiv,
Renan Gross,
Daniel B. Kearns,
Sebastian Heidenreich,
Markus Bär,
Gil Ariel
Abstract:
Bacterial swarming is a rapid mass-migration, in which thousands of cells spread collectively to colonize a surface. Physically, swarming is a natural example of active particles that use energy to generate motion. Accordingly, understanding the constraints physics imposes on the dynamics is essential to understand the mechanisms underlying the swarming phenomenon. We present new experiments of sw…
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Bacterial swarming is a rapid mass-migration, in which thousands of cells spread collectively to colonize a surface. Physically, swarming is a natural example of active particles that use energy to generate motion. Accordingly, understanding the constraints physics imposes on the dynamics is essential to understand the mechanisms underlying the swarming phenomenon. We present new experiments of swarming Bacillus subtilis mutants with different aspect ratios and densities. Analyzing the dynamics reveals a rich phase diagram of qualitatively distinct swarming regimes, describing how the shape and density of cells govern the global dynamical characteristics of the entire swarm. Moreover, we show that under standard conditions bacteria inhabit a region of phase space that is associated with rapid mixing and robust dynamics, with homogeneous density and no preferred direction of motion. This contrasts characteristic clustering behavior of self-propelled rods that is recovered only for very elongated mutant species. Thus, bacteria have adapted their physics to optimize the principle functions assumed for swarming.
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Submitted 13 November, 2019;
originally announced November 2019.
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Label-free, Spectroscopic Detection of Leaflet Composition Asymmetry in Vesicles
Authors:
Matthew L. Strader,
Hilton Barbosa de Aguiar,
Alex G. F. de Beer,
Sylvie Roke
Abstract:
Vibrational sum frequency scattering (SFS) has been used to study sub-micron, catanionic vesicles in solution. The vesicles were synthesized from a binary mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylsulfate (SDS) surfactants in deuterated water, which spontaneously assemble into thermodynamically stable vesicles. The stability of these vesicles is attributed to a surfactan…
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Vibrational sum frequency scattering (SFS) has been used to study sub-micron, catanionic vesicles in solution. The vesicles were synthesized from a binary mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylsulfate (SDS) surfactants in deuterated water, which spontaneously assemble into thermodynamically stable vesicles. The stability of these vesicles is attributed to a surfactant concentration asymmetry between the inner and outer bilayer leaflets. This concentration asymmetry should be observable by SFS due to local inversion symmetry-breaking. Signal corresponding to the symmetric sulfate stretch mode of the SDS head group is observed at 1044 cm, indicating that there is indeed asymmetry in the local structure of the leaflets. The results indicate that it should be possible to measure the interfacial structure of liposomes in aqueous solution and study in-situ processes like the binding of sugars and proteins that are important for many processes in biophysical chemistry.
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Submitted 30 November, 2010; v1 submitted 23 May, 2010;
originally announced May 2010.