Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M20: Active Matter and Liquid Crystals in Biological and Bio-Inspired Systems IIFocus Recordings Available
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Sponsoring Units: DSOFT DPOLY GSNP Chair: Kim Weirich, Clemson Room: McCormick Place W-185BC |
Wednesday, March 16, 2022 8:00AM - 8:36AM |
M20.00001: Do active nematic self-mixing dynamics offer evolutionary benefits to growing bacterial colonies? Invited Speaker: Daniel A Beller Recent works have shown that diverse types of cell packings, including eukaryotic cellular tissues and growing or swarming bacterial colonies, are well-described by hydrodynamic models of active nematic liquid crystals. A key property of volume-conserving active nematic dynamics is chaotic self-mixing characterized by motile topological defects. However, for active nematics driven by growth rather than motility, less is understood about mixing and defect motion. Mixing could provide evolutionary benefits to bacterial colonies by counteracting the tendency to spatially segregate into monoclonal sectors, which reduces the local genetic diversity. To study whether growth-driven active nematic physics could benefit bacteria evolutionarily, we conduct agent-based simulations of growing, dividing, and sterically repelling rod-like bacteria of various aspect ratios, and we analyze the results with tools from both active matter and population genetics. Our results demonstrate important differences in defect dynamics between growth-driven and motility-driven active nematics. We also show that at biologically relevant aspect ratios, self-mixing is more effective in growing active nematics of rod-like cells compared to growing isotropic colonies of round cells, predicting measurable differences in local genetic diversity. |
Wednesday, March 16, 2022 8:36AM - 8:48AM |
M20.00002: Dense topological defects in weak active nematics Ivan Maryshev, Timo Kruger, Erwin Frey So far, the emergence of half-integer topological defects has been considered a hallmark of dense, spatially uniform active nematics. Dilute or weakly interacting systems, on the other hand, exhibit qualitatively quite different ordering behavior and dynamics. One observes micro-phase separation into nematic bands with intriguing complex dynamics, but so far without the emergence of half-integer topological defects. Here we describe a new type of defects in weakly interacting active nematics that exhibit particle mass condensation and active particle currents near their center. They coexist with nematic lanes and are formed when three curved nematic lanes meet and condense into a topological defect with a high-density core region. We analyze this system combining two approaches: a previously developed agent-based model for weakly interacting self-propelled polymers and a phenomenological hydrodynamic theory of weak active nematics. We reveal the mechanisms underlying the new defects formation and identify the corresponding control parameters. In particular, we show that condensation of density around the defects cores is determined by the interplay between isotropic. Finally, we discuss the potential role of dense topological defects in the formation of active foams. |
Wednesday, March 16, 2022 8:48AM - 9:00AM |
M20.00003: Hierarchical self-organization in signaling polar active matter Alexander Ziepke, Ivan Maryshev, Igor S Aronson, Erwin Frey Self-organization in active matter plays an important role for various biological and artificial systems. In numerous cases, signaling is a key mechanism for the formation and localization of critical structures, such as the fruiting body in Dictyostelium discoideum or aggregation clusters in quorum-sensing bacteria. Despite its importance, the specific role of the signaling and its interplay with self-propulsion remains largely unexplored. |
Wednesday, March 16, 2022 9:00AM - 9:12AM |
M20.00004: Spatial control of topological defects through activity patterning Luca V Scharrer, Mark J Bowick, Cristina Marchetti, Suraj Shankar Active nematics are liquid crystals composed of elongated active particles that individually consume energy to exert stresses along their orientation, driving the fluid out of equilibrium. At high activity, topological defects in the nematic order parameter proliferate and drive self-sustained spatiotemporal chaotic flows. Given the intimate relation between active flows and defect textures, we ask how spatial variations in active stresses can be used to pattern active defects. By combining numerical simulations of active nematodynamic equations with a recently derived hydrodynamic description of defects as an interacting active Coulomb gas, we show how an active strip in a passive region can trap and sort defects. Spatial gradients in activity act like electric fields that can sort defects by topological charge. Remarkably, by tuning the sharpness of the activity gradient, we find a dramatic flip in the topological charge distribution along active-passive interfaces, that we explain via an interplay between defect self-propulsion and diffusion. Persistent kink-walls allow richer patterns with stable defect strings and active flows, paving the way for spatial control of defect patterns in active fluids. |
Wednesday, March 16, 2022 9:12AM - 9:24AM |
M20.00005: Two-temperature activity drives liquid-crystal and crystalline order in soft repulsive spherocylinders Jayeeta Chattopadhyay, Sindhana Pannir-Sivajothi, Kaarthik Varma, Sriram R Ramaswamy, Chandan Dasgupta, Prabal K Maiti We study the scalar activity induced phase separation and liquid crystal ordering in a system of Soft Repulsive Spherocylinders (SRS) of various aspect ratios (L/D). Activity was introduced by increasing the temperature of half of the SRS (labeled ‘hot’) while maintaining the temperature of the other half constant at a lower value (labeled ‘cold’). The difference between the two temperatures scaled by the lower temperature provides a measure of the activity. We find that activity drives the cold particles through a phase transition to a more ordered state and the hot particles to a state of less order compared to the initial equilibrium state. For L/D = 5, the cold components of a homogeneous isotropic (I) structure acquire nematic (N) and, at higher activity, crystalline (K) order. Similarly, the cold zone of a nematic initial state undergoes smectic (Sm) and crystal ordering while the hot component turns isotropic. Interestingly, we observe liquid crystal ordering for the spherocylinders having aspect ratio below Onsager’s limit. The hot particles occupy a larger volume and exert an extra kinetic pressure, confining, compressing, and provoking an ordering transition of the cold-particle domains. |
Wednesday, March 16, 2022 9:24AM - 9:36AM |
M20.00006: Buckling Instability in 3D Active Nematics Tobias Struebing, Amir Khosravanizadeh, Andrej Vilfan, Eberhard Bodenschatz, Ramin Golestanian, Isabella Guido Networks of biopolymers and motor proteins are useful model systems for the understanding of emergent behaviours of active matter. In this study we investigate how this active filamentous structures promote nonequilibrium processes induced by active stress at the microscale. By combining passive processes that produce entropic forces and extensile and contractile forces exerted by motors we show that the system exhibits a nematic organization characterised by long-range orientational order. The evolution of the system over time is particularly interesting and unique. We observe 3D to 2D transition by contracting into a sheet, expansion in the direction perpendicular to the contraction, 3D wrinkling pattern formation, and finally, explosion into a spatio-temporal disordered state. Finally, we examine the influence of external stimuli such as confinement, crowding agent and filament length on the properties of the different development phases of the system. |
Wednesday, March 16, 2022 9:36AM - 9:48AM |
M20.00007: Data-driven continuum modeling of active nematics via sparse identification of nonlinear dynamics Connor Robertson, Anand U Oza, Travis Askham Data-driven modeling methods have recently shown great potential in determining accurate continuum models for complex systems directly from experimental measurements. One such complex system is the active nematic liquid crystal system consisting of microtubule-motor protein assemblies immersed in a fluid. This system exhibits rich non-equilibrium behavior, including spontaneous creation and annihilation of topological defects. Although several models have been proposed for the system, the governing equations remain under debate. |
Wednesday, March 16, 2022 9:48AM - 10:00AM |
M20.00008: Interplay of Driven Flow and Active Stress in Tumbling Nematic Liquids Weiqiang WANG, Rui Zhang Active liquid crystals (LCs) are a paradigm of active matter in which topological defects can constantly nucleate and annihilate, accompanied by a spontaneous flow. It has invoked recent interests thanks to its potential applications in microfluidics and close connections to certain biological systems. One experimental system of active LCs is bacteria swimming in a nontoxic, lyotropic chromonic liquid crystal. Recent works have revealed its tumbling character and low twist modulus. However, how these unique features modify the fluid dynamics of an active nematic in the presence of an external flow is poorly understood, hindering its applications in microfluidics. Here, we use hydrodynamic simulations to address this question. We find that the direction of the spontaneous flow developed in a flow-tumbling active nematic is tunable by tuning the active stress. We further show that for a flow-tumbling nematic subject to a pressure-driven flow, activity can suppress the flow rate in a planar-anchoring cell but promote a director field transition in a homeotropic-anchoring cell. We also predict a pattern formation in an unsteady flow of a flow-tumbling active nematic. As such, our results pave the way towards microfluidic applications of tumbling active nematics. |
Wednesday, March 16, 2022 10:00AM - 10:12AM |
M20.00009: 3D-induced polar order and topological defects in growing bacterial populations Kazumasa A Takeuchi, Takuro Shimaya Colonies of rod-shaped bacteria, such as Escherichia coli, initially expand two-dimensionally but eventually show 3D growth. While it was recently reported for motile bacteria that the switch is promoted by influx of cells toward +1/2 topological defects, how cellular alignment plays a role in non-motile cases is largely unknown. Here, we study the role of cellular alignment and topological defects in colony formation of non-motile E. coli populations. We show that, while initially only +1/2 defects attract cells, cells gradually flow toward -1/2 defects as well, leading to vertical growth around both types of defects. This is in contrast to the current knowledge on 2D active nematics, which indicates repulsion of cells from -1/2 defects. We reveal that the key is in 3D tilting of cells around defects, leading to the formation of tilt-induced polar order and resulting polarity-induced force. We included this to the 2D active nematics theory and successfully accounted for the influx to -1/2 defects. We argue that 3D cell orientations may result in qualitative changes in properties of 2D active nematics, which may be characteristic of such non-motile but growing active matter. |
Wednesday, March 16, 2022 10:12AM - 10:24AM |
M20.00010: Density-dependent collective behaviour in Chlamydomonas reinhardtii Sujeet K Choudhary, Aparna Baskaran, Prerna Sharma Phototaxis is migration of motile cells in response to a light stimulus, a behaviour shown by many protists. Besides survival and growth of single-cell, it plays a crucial role at the global scale of the aquatic ecosystem and has technological relevance in bioreactors, microbiopropellers and artificial microswimmers. Here we present a quantitative measurement of phototaxis of single-celled biflagellate microalgae Chlamydomonas reinhardtii using high spatiotemporal video microscopy. Our findings reveal that the phototactic efficiency as a function of cell concentration is non-monotonic and re-entrant in nature. We demonstrate that collective enhancement in the phototactic efficiency originates from the density-dependent slowing down of the swim speed in the high-density regime. Finally, we show that the steady-state phototactic response is well captured by modelling it as active Brownian particles subject to density-dependent external torque. |
Wednesday, March 16, 2022 10:24AM - 10:36AM |
M20.00011: Universal scaling in the turbulence of active flow-aligning nematics Ido Lavi, Ricard Alert, Jean-François Joanny, Jaume Casademunt Active nematic fluids, such as bacterial monolayers and microtubules-based suspensions, have been shown to exhibit two-dimensional turbulent-like flows at vanishing Reynolds number. However, it remains unclear whether these types of systems manifest universal scaling laws, akin to classic inertial turbulence [1]. Very recently, a q-1 scaling of the kinetic energy spectrum has been established for long wavelengths in a minimal model of active liquid crystals (ALCs) [2]. This model takes a particularly simple form as it excludes i) topological defects and ii) the nonlinear flow alignment coupling ν. Here, we study an augmented nonlinear model of ALCs that adequately incorporates flow alignment [3]. We find that ν controls a number of important and non-trivial effects, including the energy transfer across scales. Our large-scale numerical simulations reveal, for the first time, that chaotic flows could emerge not only in the 'tumbling' (|ν|<1) limit but also in the 'flow-aligning' (|ν|>1) regime. Most importantly, the scaling exponent in the turbulence remains independent of ν—a result that serves as a crucial milestone towards establishing the existence of a broader 'active turbulence' universality class. |
Wednesday, March 16, 2022 10:36AM - 10:48AM |
M20.00012: Transitional turbulence in active nematic channel flow: phase space geometry and exact coherent structures Caleb Wagner, Michael M Norton, Jae Sung Park, Piyush Grover Confined active nematics exhibit rich dynamical behavior, including spontaneous flows, periodic defect dynamics, and chaotic `active turbulence'. Here, we study these phenomena using the framework of Exact Coherent Structures, which has been successful in characterizing the routes to high Reynolds number turbulence of passive fluids. Exact Coherent Structures are stationary, periodic, quasiperiodic, or traveling wave solutions of the hydrodynamic equations that, together with their invariant manifolds, serve as an organizing template of the dynamics. Here, we apply this framework to active nematic channel flows exhibiting transitional turbulence, obtaining a reduced order—but fully nonlinear—description of the turbulent dynamics. We then discuss how this description can motivate control strategies for suppressing or delaying the onset of turbulence in the transitional regime. |
Wednesday, March 16, 2022 10:48AM - 11:00AM |
M20.00013: "Impact of e-cigarette flavor compounds and additives on lipid membrane diffusion" Rayner Hernandez Perez, Jocelyn Ochoa, Linda S Hirst Electronic Nicotine Delivery Systems (ENDS) are often promoted as safer alternatives to traditional, combustible cigarettes. However, there is evidence suggesting that e-cigarettes cause significant harm. In addition to nicotine, flavorings and other additives are a largely unrecognized potential hazard. It is vital that fundamental research be performed to understand the mechanisms that underlie the pulmonary effects of ENDS additives. This study focuses on understanding how inhaled chemicals in ENDS devices can harm the respiratory system and aims to identify the potentially most harmful compounds. For that purpose, we investigate the impact of additives on membrane fluidity by measuring lateral diffusion in surfactant membranes using Fluorescence Recovery After Photobleaching (FRAP). We prepared supported lipid bilayers with compositions designed to mimic the lung surfactant composition and measured their diffusion coefficients. Quantitative FRAP experiments were conducted using the confocal microscope at the Imaging and Microscopy Facility (IMF), University of California, Merced. |
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