Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S20: Active Matter in Complex Environments IIIRecordings Available
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Sponsoring Units: DSOFT DBIO GSNP DFD Chair: Dan Beller, UC Merced Room: McCormick Place W-185BC |
Thursday, March 17, 2022 8:00AM - 8:12AM |
S20.00001: Stimulus-responsive surfaces for controlled cell adhesion during mammalian cell culture Caroline McCue, Kripa Varanasi Cell culture is a critical laboratory task involved in all research that involves cells. However, trypsinization, the primary method used to passage cells, requires lots of time from researchers, creates liquid and solid waste, and can cause damage to sensitive cell types. The ability to culture cells without the need to use trypsin or similar chemicals to detach adherent cells from cell culture dishes could save time for researchers, streamline the process of cell culture, and improve the outcomes of cell line development. We have designed a fast, simple platform for on-demand cell detachment that uses transparent, biocompatible composite surfaces to trigger cell detachment in response to simple external stimuli without the need for trypsinization. We first demonstrate a simple, reproducible method for producing micromolded and infused polystyrene surfaces which cells can adhere to, and measure cell growth and proliferation of a range of cell types on these composite microtextured surfaces. We then show the ability of these active surfaces to detach live cells by applying a voltage, and measure detachment efficiency and cell removal forces, all within the complex environment of cell culture conditions. |
Thursday, March 17, 2022 8:12AM - 8:24AM |
S20.00002: Thermodynamics of Active Field Theories: Informatic vs Thermodynamic Entropy Production Tomer Markovich, Etienne Fodor, Elsen Tjhung, Michael E Cates The hallmark of active matter is the autonomous directed motion of its microscopic constituents driven by consumption of energy resources. This motion leads to the emergence of large-scale dynamics and structures without any equilibrium equivalent. Though active field theories offer a useful hydrodynamic description, it is unclear how to properly quantify the energetic cost of the dynamics from such a coarse-grained description. In this talk I will discuss a widely used measure of irreversibility we refer to as informatic entropy production and describe how it relates to the underlying energy dissipation or thermodynamic entropy production. Based on linear irreversible thermodynamics, we determine how active fields couple with the underlying reservoirs at the basis of nonequilibrium driving, which provide a thermodynamically consistent framework to identify the energy exchanges between active systems and their surrounding thermostat at the hydrodynamic level. This will be demonstrated in two popular active field theories: (i) the dynamics of a conserved density field reproducing active phase separation and (ii) the coupled dynamics of density and polarization describing motile deformable droplets. |
Thursday, March 17, 2022 8:24AM - 8:36AM |
S20.00003: Melting of a two dimensional crystal driven far from equilibrium Ankit Vyas, stefano sacanna, Andrew D Hollingsworth, Rodrigo E Guerra, Paul M Chaikin Particles confined to a 2D interface interacting with a repulsive dipolar interaction are known to form crystals that melt through 2 stages via the Kosterlitz-Thouless-Halperin-Nelson-Young mechanism of defect unbinding. Here we are interested in how the crystal melts when defects are created by activity. We explore this question by introducing a small fraction of magnetic spinners to our system of charged particles confined to a water-oil interface. The hydrophobic charged particles interact in the oil phase while the magnetic spinners sit directly below them on lattice sites in the aqueous phase. The spinning particles drive their neighbors orbitally and induce the creation of disclinations and dislocations which melt the lattice. We study and contrast this active melting process with thermodynamic melting. |
Thursday, March 17, 2022 8:36AM - 8:48AM |
S20.00004: Light-activated kinesin tune defect density and nematic speed Zahra Zarei, John P Berezney, Zvonimir Dogic, Seth Fraden Active nematics are intrinsically unstable and unconfined active nematics generate turbulent flows. To control the flow and suppress turbulence we developed a 2D active nematic system consisting of microtubule bundles driven by light-activated kinesin motor clusters. Here, we investigate how the intensity of uniformly applied light affects active nematic properties. We use particle image velocimetry to calculate the nematic speed and the nematic director field to extract spatial and temporal nematic characteristics, such as the defect density. We find that at low light intensities, the nematic speed and the defect density are proportional to the intensity of light. This system has the potential to be combined with control theory protocols to create designed spatiotemporal flows. |
Thursday, March 17, 2022 8:48AM - 9:00AM |
S20.00005: Mixing of active and passive fluids in microtubule-kinesin active fluid system Teagan Bate, Kun-Ta Wu Fluid mixing is driven by the passive process of diffusion and the active process of stretching and folding, which homogenize the system's constituents. Conventionally, the active process is applied via external shearing machines such as a kitchen stand mixer. However, applying external shearing becomes more challenging in mesoscopic fluid systems due to the increasing difficulty of controlling the injection of energy on the micron scale. To overcome this challenge, we introduced microtubule-kinesin active fluid to power the active mixing process. To demonstrate its mixing capability, we created a multi-fluid system where active fluid is adjacent to an inactivated, passive fluid and allowed the active fluid to blend with the passive fluid until the system reaches a homogeneous state. We found that the mixing dynamics of such active-passive fluid mixing was dominated by the passive process of diffusion, until the activity of active fluid was tuned to be sufficiently high and the active processes of active fluid began to dominate the mixing process. Our work will stimulate the development of utilizing active fluid to accomplish mesoscale mixing tasks in multi-fluid systems at the micron scale. |
Thursday, March 17, 2022 9:00AM - 9:12AM |
S20.00006: Motile particles without inertia form caustics in vortical flows Rahul Chajwa, Sriram R Ramaswamy, Rama Govindarajan When heavy particles in turbulent flow are centrifuged out of vortices they cluster to form singular features in the number-density called caustics [Physics of Fluids 27, 033305 (2015)]. We show that self-propelled particles (SPPs) with no inertia display similar behaviour thanks to the persistence of their motion in the direction of their intrinsic orientation. Using singular perturbation analysis and numerical studies we establish that SPPs form caustics at a critical distance from the origin of a point vortex. To capture the dynamics in a generic vortical flow we study SPPs suspended in turbulent flow and find pronounced caustics in straining regions, for intermediate values of a non-dimensional self-propulsion velocity. The occurrence of caustics limits the applicability of continuum descriptions in active suspensions, and opens the possibility that propagating singularities might be the building block of such dynamics. |
Thursday, March 17, 2022 9:12AM - 9:24AM |
S20.00007: Shape-Evolving Structured Liquids via Ferromagnetic Active Particles Paul Y Kim, Thomas P Russell The motion of active matter is the basic form of locomotion in biology, a vital ingredient in many functions of cells, and an essential design challenge in nanorobotics. Here, we integrated active matter into structured liquids to harness its motions to perform work on liquid interfaces. The structured liquids, produced by interfacial jamming of nanoparticle-surfactants (NPSs), are reconfigurable and therefore provide an ideal platform for generating active energy-consuming systems. The liquid shape will evolve and respond to external stimuli when the interfacial tension is sufficiently low, i.e., >0.05 mN/m in this study. In this study, we employed ferromagnetic active swimmers with relatively large momentum. Nickel particles with average diameters ranging from ~10 to ~60 microns were subjected to an external AC magnetic field to energize rolling motions on a solid substrate. When they are encapsulated in a droplet of structured liquids, the collisions of particles resulted in directional shape changes and translational motions of the droplet. This strategy would provide a route to a new class of biomimetic, reconfigurable, and responsive materials, delivering mechanical responses unlike those of conventional materials. |
Thursday, March 17, 2022 9:24AM - 9:36AM |
S20.00008: Collective motion of interacting swimmers in impure flows Yasser Almoteri, Enkeleida Lushi Micro-swimmers such as bacteria or spermatozoa navigate through viscous fluids interdispersed with much smaller fibers and other inert micro-particles. We look at the locomotion and interaction of individual micro-swimmers in such an impure viscous fluid. We utilize the Brinkman approximation to capture the effect of sparse and stationary obstacles that are represented via a single resistance parameter. We note the effect of the Brinkman resistance in the pair interaction of pusher and puller swimmers by comparing the trajectories, attraction, disturbance fluids-flows and effective speed-up or slow-down in the motion. We discuss the implications this has for collective behavior as well as motion near surfaces. Lastly, we present large-scale simulations of thousands of interacting microswimmers, pushers and pullers, in Brinkman flows and analyze the properties of the collective dynamics. |
Thursday, March 17, 2022 9:36AM - 9:48AM |
S20.00009: Interfacial flow fields generated by a pusher bacterium Jiayi Deng, Mehdi Molaei, Nicholas G Chisholm, Kathleen J Stebe Motile bacteria are model active colloids that navigate, interact, and self-organize, guided in part by hydrodynamic interactions (HI). While HI are well known to alter micro-swimmers’ behavior near solid surfaces, bacteria motion near fluid interfaces is far less understood, despite the importance of interfaces in nature and the complexity of these highly anisotropic domains. We study Pseudomonas aeruginosa PA01 in the pusher mode at aqueous-hexadecane interfaces. The bacteria become trapped with their bodies spanning the interface with pinned contact lines. Surfactant-associated interfacial incompressibility further constrains the flow. Analysis of correlated displacements of tracers and swimmers reveals flow fields generated by the bacteria motion with unexpected asymmetries. These fields can be decomposed into an expected force-doublet mode corresponding to propulsion and drag in an incompressible interface, and a second dipolar mode, associated with forces exerted by the flagellum in the aqueous phase. The balance of these modes depends on the bacteria’s trapped configurations. The implications of these flows on enhanced transport and swimmer pair interactions are explored. |
Thursday, March 17, 2022 9:48AM - 10:00AM |
S20.00010: Collective motion of torque-dipolar micro-swimmers Enkeleida Lushi We present a model and simulations for micro-swimmers that take into account the counter-rotation of the body and flagella, as seen in motile bacteria or spermatozoa. The disturbance fluid flow of one such swimmer now contains a torque-dipole singularity in addition to the well-known force-dipolar singularity. The linear analysis of the coarse-grained model shows an instability for a range of parameters, which we summarise in a phase diagram. Lastly, we show large-scale simulations of torque-dipolar micro-swimmers and illustrate the collective behavior in the regions of parameter space indicated by the stability analysis. |
Thursday, March 17, 2022 10:00AM - 10:12AM |
S20.00011: Mixing with Acitivity: Transport of swimming bacteria in time-periodic flows Paulo Arratia, Ranjiangshang Ran, Quentin Brosseau, Boyang Qin Understanding mixing and transport of passive scalars in active fluids is important to many natural (e.g., algal blooms) and industrial (e.g., biofuel, vaccine production) processes. In this talk, I will discuss recent experiemnts on the mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli in experiments using a two-dimensional (2D) time-periodic flow. Results show that the presence of bacteria hinders large-scale transport and reduces overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye-concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, these results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport. |
Thursday, March 17, 2022 10:12AM - 10:24AM |
S20.00012: Activity-induced microswimmer interactions and cooperation in one-dimensional environment Stefania Ketzetzi, Melissa Rinaldin, Pim Dröge, Joost de Graaf, Daniela J Kraft Cooperative motion in biological microswimmers is crucial for their survival as it facilitates adhesion to surfaces, formation of hierarchical colonies, efficient motion, and enhanced access to nutrients. Here, we confine catalytic microswimmers along one-dimensional paths and demonstrate that they, too, show a variety of cooperative behaviours. We demonstrate that their speed increases with the number of swimmers, and find a preferred distance between swimmers. Using a minimal model, we ascribe this behaviour to an effective activity-induced potential that stems from a competition between chemical and hydrodynamic coupling. These interactions further induce active self-assembly into trains as well as compact chains that can elongate, break-up, become immobilized and remobilized. The cooperative behaviour of catalytic microswimmers opens the door to applications that need more efficient motion, with temporal and spatial control in complex environments. |
Thursday, March 17, 2022 10:24AM - 10:36AM |
S20.00013: Active particles in external fields Vaseem A Shaik, Gwynn J Elfring In external fields, active particles perform taxis by moving along or against the field. This directional migration can be taken advantage of in controlling the active matter using external fields. For example, in viscosity gradients, active squirmer type particles rotate to align against the gradients and swim in this steady state orientation with a speed different from that in the absence of the gradients. Here, we discuss how the boundary conditions on the particle can crucially alter the particle dynamics in the viscosity gradients. Unlike the viscosity gradients, more generally, an independent control of the particle speed and orientation is possible using two different fields. In the presence of a field that modulates the speed spatially, particles accumulate in the regions of low speed, whereas in the presence of a field that reorients the particles, they rotate to align along or against the field and accumulate in the downstream or upstream relative to the field. Here, we discuss this accumulation in the presence of walls and how reorienting fields can be used to clean off the accumulation at the walls. |
Thursday, March 17, 2022 10:36AM - 10:48AM |
S20.00014: Microfluidic SANS study of the lamellar-to-MLV transition in model surfactant system of varying membrane bending rigidity Liva Donina, Joao T Cabral The formation of multilamellar vesicles (MLVs) under flow can be achieved by tuning the shear field and by manipulating the lamellar membrane physical properties. Here we investigate the flow response of a model surfactant system, Sodium Dodecyl Sulfate (SDS)/octanol/brine, in the lamellar phase and, specifically, the mechanism and conditions required for the formation of MLVs. We employ microfluidics in continuous and oscillatory flow modes, to precisely control the shear magnitude, residence time, amplitude, and frequency. Small Angle Neutron Scattering (SANS) with a small beam footprint (illuminating down to 20 nL sample) was utilised to spatially and temporally resolve the lamellar to MLV transformation, in terms the structure factor and anisotropy of the scattering signal. Polarised optical microscopy provide complementary insight into the evolution of long range order accompanying the MLV transformation. In order to examine the effect of membrane bending rigidity on the conditions required for MLV formation, we have varied the membrane bending modulus by salt (NaCl) addition, and quantified its consequence to vesicle formation kinetics. |
Thursday, March 17, 2022 10:48AM - 11:00AM |
S20.00015: Flow Fluctuations in Confined Bacterial Suspension Cristian Villalobos Concha, Maria Luisa Cordero, Rodrigo B Soto Microswimmers move the fluid around their bodies, creating flows that influence the movement of other swimmers and particles in the medium. Once the number of microswimmers increases, these hydrodynamic interactions result in a fluctuating flow, which we aim to characterize. |
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