Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T20: Active matter in Complex Environments IVRecordings Available
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Sponsoring Units: DSOFT DBIO GSNP DFD Chair: Enkeleda Lushi, NJIT Room: McCormick Place W-185BC |
Thursday, March 17, 2022 11:30AM - 11:42AM |
T20.00001: Emergent behavior of growing bacterial communities in polymeric environments Sebastian Gonzalez La Corte, Sujit Datta, Ned S Wingreen, Tapomoy Bhattacharjee Many bacteria live in polymeric environments, such as mucus in the body, exopolymers in the ocean, and cell-secreted extracellular polymeric substances (EPS) that encapsulate biofilms. However, most studies of bacteria focus on cells in polymer-free fluids. How do interactions with polymers influence the behavior of bacterial communities? To address this question, we experimentally probe the growth of non-motile Escherichia coli in solutions of inert polymers. We find that, when the polymer is sufficiently concentrated, the cells grow in striking “cable-like” morphologies—in stark contrast to the compact morphologies that arise in the conventionally-studied polymer-free case. Agent-based simulations suggest that these unusual community morphologies arise from an interplay between polymer-induced entropic attraction between pairs of cells and their hindered ability to diffusely separate from each other in a viscous solution. These results suggest a pivotal role of polymers in regulating microbe-host interactions, by promoting bacterial exposure to external biochemical groups that protect the host against pathogens. More broadly, this work helps to uncover quantitative principles governing the morphogenesis of diverse forms of growing active matter in polymeric environments. |
Thursday, March 17, 2022 11:42AM - 11:54AM |
T20.00002: Mammalian sperm navigation in response to biochemical factors Meisam Zaferani, Susan Suarez, Alireza Abbaspourrad Mammalian sperm migration within the female reproductive tract requires multiple navigational mechanisms to ensure that sperm maintain the correct swimming behavior as it proceeds toward the oocyte and the fertilization site. These navigational mechanisms are known to rely upon exogenous biophysical clues present in the tract including external fluid flow as well as the tract’s architecture. While sperm chemotaxis in marine invertebrates is well documented, the contribution of exogenous biochemical factors to the navigation of mammalian sperm is poorly understood. Here, using microfluidic experimentation as well as theoretical and computational modeling, we explore bull sperm motion in response to pharmacological agonists to identify whether biochemical factors modulate sperm navigation. |
Thursday, March 17, 2022 11:54AM - 12:06PM |
T20.00003: Evaporating droplet dynamics enhances bacterial pathogenesis on fomites Saptarshi Basu
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Thursday, March 17, 2022 12:06PM - 12:18PM |
T20.00004: Geometry-driven trapping and escaping of bacteria in arrays of micropillars Pooja Chopra, David A Quint, Ajay Gopinathan, Bin Liu Interactions between bacteria and solid structures often involve rich classes of forces, including hydrodynamic, electrostatic, and steric forces. Here, we investigate a potential geometry-based abstraction of such interactions by considering bacteria of varying sizes in pillar arrays with given geometries. Using the smooth-swimming Escherichia coli strain as an archetype, we observed that bacteria tended to orbit a single pillar when their sizes were sufficiently short while they crossed the lattice through those gaps when otherwise. We argue that such demarcated “trapping” and “escaping” states can be explained by the geometric constraints of the finite gaps between adjacent pillars, which prevent those long cells from orbiting any pillars. To validate this geometry-based argument, we performed the same measurements with enlarged pillar gaps. We show that those previously escaping long cells switch to a trapping state, consistent with the vanishing geometric constraints at large pillar gaps. We also explore these geometric effects on wild-type E. coli that are free to tumble, as advanced complicacies to the bacteria-structure interactions. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T20.00005: Controlling speed of bacterial active droplets in a nematic environment Hend M Baza, Yuhan Wang, Holly Matthews, Oleg D Lavrentovich Water-based droplets loaded with swimming bacteria Bacillus subtilis propel themselves unidirectionally when placed in a thermotropic nematic. We investigate the effect of the viscosity of the droplet medium on the propulsion speed of the droplets. The speed of droplets first increases and then decreases as the viscosity of the medium is increased by adding nontoxic disodium cromoglycate (DSCG) and polymer carboxymethyl cellulose (CMC). In the case of DSCG, the maximum velocity is observed at 13wt% concentration of the additive. On the other hand, individual bacteria show the fastest speed in a more dilute medium with 8wt% of DSCG. The results imply that the increase in the droplet's media viscosity causes a better momentum transfer at the droplet-nematic interface, which speeds up the droplets. |
Thursday, March 17, 2022 12:30PM - 12:42PM Withdrawn |
T20.00006: Active particle transport in disordered porous media Akhil Varma, David Saintillan Fluid suspensions of motile microswimmers such as bacteria and active colloids are often observed in porous environments, but the complexity of the medium geometry and its effect on particle dynamics makes the study of their transport properties challenging. We provide insight into this problem by performing Brownian dynamics simulations in disordered environments at the scale of the constituent microscopic solid inclusions. The particles are modeled as point-sized active Brownian particles swimming through the interstices of a fluid-saturated porous medium composed of randomly distributed polydisperse solid inclusions. Our simulations reveal how the swimmer activity, together with geometrical properties of the surrounding medium, affect statistics of swimmer distributions, orientations and long-time transport. We also study the effects of an imposed pressure-driven Stokes flow through the porous matrix, where we elucidate the role of advection and local shear-induced reorientations on microswimmer dynamics. |
Thursday, March 17, 2022 12:42PM - 12:54PM |
T20.00007: Locally Induced Analog Casimir Force from a Self-propelling Vibrating Robot Boat Steven Tarr, Enes Aydin, Daniel I Goldman Active agents on fluid surfaces can perturb their surroundings by creating waves that reciprocally affect the agent. Inspired by the wave-mediated dynamics of surface-bouncing droplets, we study the motion and wave-field dynamics of a 5.9 cm radius, 8.8 cm tall, eccentric motor-driven vibrating robot boat on the surface of a 4 cm deep pool of water. The boat's vibration creates outwardly propagating surface waves with frequency range 10-63 Hz; a Schlieren method enables surface wave visualization with submillimeter resolution. Far from boundaries, the boat generates circular waves for frequencies below ~40 Hz with maximum amplitude 0.6 mm; above ~40 Hz, the waves gain subharmonic components. When held near a planar boundary by a thin 1.4 m wire, the wave field becomes complex, and the boat experiences a force analogous to the Casimir effect. Below the subharmonic threshold, the boat moves less than 0.5 mm toward/away from the wall; above the threshold, displacements grow to 0.5-1.5 mm. For all frequencies, a lower/higher initial distance skews the force toward attraction/repulsion. The effect is stable to slight perturbation: for repulsive cases, a boat perturbed toward the wall returns to a stable position; for attractive cases, a boat perturbed away accelerates toward the wall. |
Thursday, March 17, 2022 12:54PM - 1:06PM |
T20.00008: Active nematic defect control via variations in oil sublayer thickness Dimitrius A Khaladj, Linda S Hirst Actively driven bundled microtubule networks are highly adaptive systems that are sensititve to changes to it’s microevironment. We study the behavior of a two-tier active nematic microtubule/kinesin system confined to elevated microfabricated pillars. Flow dynamics in these systems are influenced by a circular confining geometry where active material at the top of pillar is referred to as a “second tier” configuration. In this work, we develop a method to curve the surface of an isotropic oil layer via the wetting of cylindrical microfabricated pillars. Not unlike the active material confined by hard boundaries, the active nematic material in our elevated system exhibits similar defect dynamics where bundled microtubules braid and align at the boundary. The oil layer in the 2nd tier region is thinner than outside the boundary and consequently, becomes an area of high friction. Within the pillar region, active length scales and velocities are reduced due to larger viscosity thus we aim to calculate defect distributions, densities and velocity fields. |
Thursday, March 17, 2022 1:06PM - 1:18PM |
T20.00009: Fluctuations in pedestrian dynamics routing choices Alessandro Gabbana, Federico Toschi, Alessandro Corbetta A quantitative understanding of pedestrians choice processes and their macroscale |
Thursday, March 17, 2022 1:18PM - 1:30PM |
T20.00010: C. elegans motility in self-healing gels Saheli Dey, Tapomoy Bhattacharjee Nematode behavior is extensively explored and understood by studying them on soft 2D surfaces or inside homogeneous liquid media. However, in their natural habitat, many nematodes navigate a much complex three-dimensional self-healing environment such as soft soil, rotten fruit, and plant stem. Maintaining and exploring nematodes directly in such self- healing environment is critical for understanding their behavior in most natural setting. Hence, to mimic the natural habitat of nematodes, we have designed a transparent, self-healing soil-like material from jammed granular microgel systems. The transparent nature of this platform enables direct visualization of nematodes whereas self-healing nature allows them to move in 3D. Using the canonical example of model C. elegans, we investigate how they navigate through the jammed microgel system. Further, we explore how the stiffness of their microenvironment affects their locomotion. Together, our results will present an approach to maintain and image nematodes in 3D and harvest them from this medium for further processing. Preliminary data will be presented. |
Thursday, March 17, 2022 1:30PM - 1:42PM |
T20.00011: Analysis of Steered Molecular Dynamics Study of Sickle Hemoglobin Protein in Water Jhulan - Powrel, Narayan P Adhikari Abstract |
Thursday, March 17, 2022 1:42PM - 1:54PM |
T20.00012: Understanding the thermodynamic driving forces for directed motion at the nanoscale Kathleen T Krist, William G Noid Chemotaxis refers to directed motion in response to concentration gradients. The possibility of designing active materials with myriad applications has generated significant interest in directed motion at the nanoscale. In particular, considerable controversy surrounds the experiments and proposed mechanisms concerning enzymatic chemotaxis. We have utilized theory and simulation to understand the influence of specific binding interactions and catalysis in driving molecular chemotaxis. Starting from McMilllan-Mayer theory and Schellman's treatment of macromolecular binding, we have derived a thermodynamic force that drives two interacting co-solutes to move towards each other. To simulate chemotaxis, we have numerically solved the appropriate one-dimensional Fokker-Planck equations, and our results show qualitative agreement with several prior experimental studies. We have also derived a modified reaction-diffusion equation that couples the translational motion of an enzyme to the chemical reaction it catalyzes. We investigate the role of this coupling in allowing the chemotaxis of the enzyme to be driven by the energy dissipated during the reaction. Our results may potentially challenge the traditional view that reactions are solely the result of random molecular collisions. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T20.00013: Microtubules’ Dynamic Apolar Lane Formation and Long-Range Order on a Supported Lipid Bilayer Fereshteh L Memarian, Niranjan Sarpangala, Fabian Jan Schwarzendahl, Joseph D Lopes, Madhuvanthi Athani, Daniel A Beller, Kinjal Dasbiswas, Ajay Gopinathan, Linda S Hirst Microtubules (MTs) are a vital component of the eukaryotic cells’ cytoskeleton, forming a network utilized in cell transport. Kinesin motor proteins “walk” on MTs by converting chemical energy into mechanical energy to transport cargo. In this project, we studied MT-based active matter on a supported lipid bilayer. In a microtubule gliding assay configuration, kinesin motor proteins are immobilized by anchoring to a glass substrate. Here, MT behavior in the gliding assay method is compared with an assay in which motors are diffusing on a lipid bilayer. We find that the lipid membrane acts to promote filament-filament alignment within the gliding layer in high MT concentration. As a result, we observed dynamic apolar lane formation and long-range active nematic alignment. The enhanced collective motion of MTs with orientational order on different length scales is sensitive to MT concentration. These experiments are supported with continuum model simulations, together revealing that motors restructure on a lipid bilayer. |
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