Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F20: Active Matter and Liquid Crystals in Biological and Bio-Inspired Systems IRecordings Available
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Sponsoring Units: DSOFT DPOLY GSNP Chair: Mehdi Molaei, U Chicago Room: McCormick Place W-185BC |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F20.00001: Distribution and pressure of active Levy swimmers under confinement (Edmond) Tingtao Zhou, Mamikon Gulian, Zhiwei Peng, John F Brady Many active matter systems are known to perform Levy walks during migration |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F20.00002: An Analytic Form of the Integrated Lennard-Jones Potential for Thin Rods Junwen Wang, Gary D Seidel, Shengfeng Cheng An analytic form has been derived using Ostrogradsky's integration method for the integrated interaction between two rigid and thin rods of finite lengths and in arbitrary relative configurations in three-dimensional space, each treated as a line of material points interacting through the Lennard-Jones 12-6 potential. Simplified analytic forms for coplanar or parallel rods are also derived. Exact expressions for the force and torque between the rods are obtained. The analytic forms have been fully verified via comparison to the results of direct numerical integration. These forms can be widely used for analytical descriptions of systems such as liquid crystals and colloids consisting of rod-like molecules and particles, as well as filamentous materials including carbon nanotubes, biological filaments, and their bundles. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F20.00003: Mechanical properties of adipose tissue mediates breast cancer invasion Dong Wang, John D Treado, Garrett Beeghly, Francesco Arceri, Michael P Murrell, Claudia Fischbach, Mark D Shattuck, Corey S O'Hern We carry out computational studies of breast cancer cell migration into white adipose tissue (WAT) to better understand tumor growth and invasion. We model the adipocytes and cancer cells as deformable particles that can explicitly change their shape. By varying the cancer cell deformability, adhesion, and activity, and the pressure from the invading canceer cells, we identify three invasion "phenotypes": 1) no invasion; 2) "single-cell" invasion where highly active cancer cells invade one-by-one; and 3) "collective" invasion where groups of cells penetrate the adipose tissue together. We also vary the stiffness and rate of lypolysis of adipocytes to determine how high fat diet and changes in WAT mechanical properties affect cancer cell invasion. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F20.00004: Active mixing of phase separating mixtures Fernando Caballero, Cristina Marchetti Activity is known to be a generic pathway to phase separation through mechanisms like persistent motility or quorum sensing, but its role in suppressing phase separation is far less explored. Here we examine a model for a mixture of two immiscible liquid crystals and show that endowing one of the components with activity suppresses phase separation, driving the mixture into micro phase separated or uniform states. This suppression arises from the active mixing due to local shear flows that continuously stir the fluid. The self-induced shear generated by active stresses creates an instability of the interface at early times, breaking bulk phase separation. At later times, strong enough active flows keep the system from phase separating again and strongly suppress the coexistence boundary. We relate this result to previous studies of suppressed phase separation in externally sheared mixtures. Our work is directly relevant to recent experiments on DNA-based membraneless condensates immersed in an active liquid crystal. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F20.00005: Kinetic theory modeling of bacteria diffusion and chemotaxis Rodrigo B Soto, Andrea Villa The standard description for bacterial motion is the run-and-tumble model. At a given rate μ, bacteria stop their run mode to rapidly change direction to a new one, chosen at random from an angular distribution. This Poisson process gives rise, for times longer than μ-1, to a diffusive motion. Chemotaxis is usually modeled by saying that the tumbling rate changes if the bacterium swims along or against the chemotactic signal, resulting in a biased random walk. Microscopically, the tumbling rate is controlled by the concentration of a protein whose production depends on the local values of the chemoattractant, responding with a characteristic time T. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F20.00006: From nematic defects to polar order: polar active flow of cytoskeletal filaments shaped by nematic defects. Alfredo Sciortino, Lukas Neumann, Chiao-Peng Hsu, Alice De La Trobe Yu, Andreas R Bausch Macroscopic materials can be affected by the presence of microscopic imperfections, called defects. For instance, passive nematic materials, composed of aligned, elongated building blocks, can contain defects in the local order (nematic defects) that induce large-scale distortions. Here we show that the presence of such defects can also play a role in the formation of active patterns in a system of active filaments. By using an experimental setup in which cytoskeletal filaments glide on a lipid membrane we enforce strong steric interactions between them. We then show that the alignment between individual filaments in these conditions leads to the formation of collective polar structures, despite a microscopic nematic symmetry. We also find that the transient formation of +1/2 defects plays a pivotal role in sorting the filaments’ polarity on the mesoscale. We then extend the system to filaments gliding inside a giant vesicle and find that spherical confinement enhances the global polarity, leading the formation of highly polar vortices. However, here, topological constraints on the alignment of filaments at high density hinder their ability to assemble effectively and lead to a “nematically jammed” state. We finally study the influence of passive defects by having filaments glide inside a nematic material. Again, we observe the formation of polar patterns whose emergence is now a direct effect of passive defects. All these results suggest strategies to tune active patterns by controlling defects. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F20.00007: Nematic vs. hexatic order in epithelial tissues: which is the right one? Livio Nicola Carenza, Josep-Maria Armengol, Julia Eckert, Dimitrios Krommydas, Luca Giomi Epithelial tissues are fundamental in a number of biological processes, such as morphogenesis and cancer development. A fundamental understanding of their dynamics, however, is limited by the current lack of knowledge of the symmetries underlying cells' collective motion. An important progress in this respect, was recently achieved by Saw et. al., who suggested that epithelial tissues could in fact behave as active nematic liquid crystals. In this work, we use a combination of in vitro experiments, numerical simulations and analytical work to identify the emergent order of epithelial tissues. Upon generalising the standard shape tensor to arbitrary ranks, we find that both nematic and hexatic order is in fact present in epithelial layers, with the former being relevant at the large scales and the latter at the short scales. This separation of length scales affects both the topological and dynamical properties of the system. Importantly, neglecting hexatic order leads to a misidentification of topological defects and the appearance of unphysical disclination lines. Finally, we discuss how such an emergent hexanematic order crucially affects the hydrodynamic feedback at different lengthscales. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F20.00008: Active hexanematics: a paradigm for epithelial tissues Josep-Maria Armengol, Livio N Carenza, Luca Giomi Tissue dynamics rules many biological processes such as embryogenesis, wound healing, or cancer metastasis. Even so, a framework that captures the order and symmetry of these phenomena has remained unclear. Active nematics has been typically used to model collective effects in cell monolayers, but recent research also shows that the six-fold anisotropy given by the polygonal shape of cells is relevant to understand the epithelial to mesenchymal transition, suggesting that hexatics plays a crucial role to describe the nature of these processes. Using cell-resolved numerical simulations and fluctuating hydrodynamics, we show that both nematic and hexatic order affect the collective behaviour of confluent epithelial layers, albeit at different length scales. While nematic order governs the system's dynamics at the large scale, thereby producing giant number-density fluctuations, hexatic order is relevant at smaller scales, where it suppresses density fluctuations and dictates the dynamics of topological defects. Motivated by these results, we propose a new paradigm for modelling epithelial layers based on the interplay between mechanical activity, nematic and hexatic order. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F20.00009: Autonomous microfluidics and active flow networks in active nematics Claire Doré, Justine Laurent, Jérôme Hardoüin, Jordi Ignés-Mullol, Francesc Sagués, Teresa Lopez-Leon Active systems are particularly promising in the design of autonomous machines because they are highly responsive to their environment and exhibit spontaneous flows. Developing techniques to control active flows is a current challenge. Here, we study experimentally how lateral confinement can control the dynamics of a 2-dimensional active nematic, made of microtubule bundles sheared by kinesin dimers. In long, straight microfluidic channels, we observe directional flows along one direction, selected by spontaneous symmetry breaking. Interestingly, in certain cases, the system can reverse the flow direction and transiently undergo an episode of shear flow. We show that the flow amplitude and switching frequency can be controlled through geometry. For instance, we are able to effectively enforce either shear or directed flow state by shaping the confining wall with a ratchet pattern. Geometrical patterning induces spatial and orientational ordering of motile topological defects. Finally, we build an active flow network by connecting straight and ratchet channels together. We show that, with the appropriate design, the circuit performs the AND and OR logical operations. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F20.00010: Dynamics of active defects under non-uniform Gaussian curvature Teresa Lopez-Leon, Martina Clairand, Ali Mozaffari, Jérôme Hardouin, Rui Zhang, Jordi Ignés-Mullol, Francesc Sagues, Juan De Pablo Living organisms are built from cells displaying a variety of shapes, morphologies and textures, which encode specific functions and physical behaviors. In this work, biomimmetic structural units are created by coating ellipsoidal droplets of a smectic liquid crystal with a protein-based active cytoskeletal gel, thus obtaining core-shell structures. We exploit the patterned texture and anisotropic shape of the core to mold the complex nematodynamics of the interfacial active material. New time-dependent states are identified in which topological defects periodically oscillate between a rotational and a translational regime. Nemato-hydrodynamic simulations of active nematics reveal that, beyond topology and activity, the dynamics of the active material is profoundly influenced by the local curvature and texture of the droplet, as well as by external hydrodynamic forces, which induce a solid-body rotation of the droplet. Our results illustrate how the incorporation of these constraints into active nematic shells orchestrates remarkable spatiotemporal motifs, providing new elements for the understanding of biological systems and fascinating perspectives for the design of bio-inspired micro-machines. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F20.00011: Light-induced 2D micro-swimmers at the nematic-isotropic interface of a thermotropic liquid crystal Antonio Tavera-Vazquez, Danai Montalvan, Noe d Atzin, Gustavo R Perez Lemus, Vinothan N Manoharan, Juan De Pablo Much of the inspiration in developing synthetic active materials comes from biological systems, such as bacteria species that move in water in the presence of light. It would be interesting to study the motile behavior of these micro-structures within structured liquids as thermotropic liquid crystals (LC). Therefore, we designed a novel LC-compatible system inspired by the structure of Paramecia. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F20.00012: Long-ranged velocity correlations in dense systems of self-propelled particles Elijah J Flenner, Grzegorz Szamel Experiments demonstrated that epithelial cell monolayers exhibit equal-time velocity correlations that extend over several cell sizes. Equal-time velocity correlations have also been observed in models of active matter systems. These correlations originate from the activity and are absent in equivalent equilibrium systems. Previous work that examined amorphous solid-like active systems rationalized the origin of the velocity correlations in terms of the elasticity of the solid, and found that the velocity correlations grow with the square-root of the persistence time. Here we use computer simulations to show that velocity correlations also exist in active fluids and also grow with the square root of the persistence time. In active fluids these correlations develop due to the combination of the activity and the virial bulk modulus that originates from the repulsive interactions between fluid's constituents. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F20.00013: Rheology of Entangled Active Polymer-Like T. Tubifex Worm Antoine Deblais We propose a new 'active particle' system in which the particles are in fact polymer-like. We experimentally study the rheology of long, slender, and entangled living worms (Tubifex Tubifex, or 'sludge worms'). Performing classical rheology experiments on this entangled polymer-like system, we find that the rheology is qualitatively similar to that of usual polymers, but, quantitatively, (i) shear thinning is reduced by activity, (ii) the characteristic shear rate for the onset of shear-thinning is given by the time scale of the activity, and (iii) the low shear viscosity as a function of concentration shows a very different scaling from that of regular polymers. The level of activity can be controlled by changing the temperature but also by adding small amounts of alcohol to make the worms temporarily inactive. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F20.00014: Thermodynamic equilibrium controls the growth rate of out-of-equilibrium myelin figures sepideh khodaparast, William Sharratt, Joao T Cabral Biological myelin sheaths are soft tubular substances formed of lipid-rich concentric layers that protect the neuron’s axon. Synthetic mimics of such structures are typically obtained by bringing a concentrated lamellar phase of surfactants or lipids into contact with a dilute aqueous solution. These observations are, however, impacted by the geometric, hydrodynamic, and physicochemical complexities arising from the interaction and entanglement of closely packed lamellar tubes. We investigate the growth of isolated myelin figures from multilamellar vesicles (MLVs) of an anionic surfactant, sodium linear alkylbenzene sulfonate (NaLAS), formed spontaneously through a thermally induced phase transformation. Using time-resolved small angle neutron scattering (SANS) and optical microscopy, we determine that the molecular mechanism underpinning the growth of both MLVs and (non-equilibrium) myelin figures can be explained by a population balance at thermodynamic equilibrium. Although similar at nanoscale, the growth of MLVs and myelin figures at microscale is influenced by dimensionality and scales differently with time: MLV diameter grows with t1/2, while myelins grow linearly in time, with a fixed diameter. |
Tuesday, March 15, 2022 10:48AM - 11:00AM |
F20.00015: Deformation Dynamics of Active Shells: Polar v Nematic Ludwig A Hoffmann, Livio N Carenza, Luca Giomi Recent observations suggest that topology enters biology in subtle ways that are only now beginning to be appreciated. In the last few years the framework of active liquid crystals has been used to gain insight into tissue dynamics. While much of this work was focused on flat geometries, more recently the focus has also moved to non-flat geometries, where the complex interaction of topological defects, hydrodynamics, and geometry is studied, trying to understand, e.g., morphogenesis better. |
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