Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session T01: Collective Behavior and Active Matter III |
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Chair: Giuseppe Negro, University of Edinburgh Room: Ballroom A |
Monday, November 25, 2024 4:45PM - 4:58PM |
T01.00001: Arrested development of active suspensions in liquid crystals: traveling waves Jingyi Li, Laurel Ohm, Saverio Eric Spagnolie Many microorganisms must navigate complex biological fluid environments. To study such challenging systems, liquid crystals (LCs) have emerged as a model anisotropic, viscoelastic fluid. Recent experiments using bacteria in LCs have shown an alluring interplay between bulk fluid elasticity and flows generated by active stresses. We will present analytical and numerical results on bifurcations and arrested states in a model active suspension in a nematic LC. In addition, we identify weakly and strongly deformed traveling wave solutions in motile suspensions, with numerics and theoretical results showing close agreement. |
Monday, November 25, 2024 4:58PM - 5:11PM |
T01.00002: Coherent Structures and Transitional Behavior of Confined Polar Active Suspensions Nima Mirzaeian, Tong Gao Confined suspensions of self-propelled Brownian particles exhibit distinct behaviors, such as density fluctuations and polarization instabilities. This study explores the three-dimensional transport dynamics of dilute active fluids composed of oblong swimmers, extending earlier work on two-dimensional confined flows. We numerically investigate a thin fluid film confined between no-slip walls with periodic boundaries, where confinement critically shapes the dynamics of active polar suspensions. The Doi-Onsager kinetic theory in a mean-field framework provides moment equations for particle concentration, polarization, and nematic ordering. Two key parameters, the swimming Peclet number (governing confinement) and the propulsion parameter (characterizing activity), define the system’s behavior. Numerical simulations map the swimmer activity and confinement parameter space, revealing equilibrium profiles for concentration, polarization, and nematic ordering. Notably, boundary layers form within the film, resembling one-dimensional traveling waves with finite spatial wavelengths, a coherent feature of the system. Critical transitions from steady unidirectional flow at low propulsion to spatiotemporal chaos, or active turbulence at higher propulsion, are identified. Intermediate propulsion strengths reveal a bifurcation, where coherent boundary layers signal a shift from steady-state to oscillatory behavior. Stability analysis further supports these numerical findings, predicting the onset of finite-wavenumber instabilities. Confinement redistributes concentration, polarization, and nematic ordering across the film thickness, significantly impacting local dynamics. The combined insights from numerical simulations and stability analysis elucidate the structuring and transitions in three-dimensional polar active suspensions, particularly emphasizing the emergence of finite-wavelength boundary layers. |
Monday, November 25, 2024 5:11PM - 5:24PM |
T01.00003: Understanding particle-wall interactions through bulk mobility measurements of electric field-driven active colloids Sandeep Ramteke, Jordan Dehmel, Joshua Burget, Jarrod Edward Schiffbauer, Alicia Boymelgreen Active colloid experiments face significant challenges: theoretical models typically apply to isolated particles in bulk, such as metallodielectric Janus spheres, but sedimentation of the metallic coating leads to measurements near the substrate. This interation affects mobility through localized gradients, electrohydrodynamic flow, and Stokes drag. This paper addresses difficulties of sending electrokinetically driven active colloids in the international space station (ISS), where microgravity aids bulk mobility measurements. Key issues include mitigating bubble formation from dissolved gas and preventing particle adhesion to chamber walls that must remain stable for up to 10 days. |
Monday, November 25, 2024 5:24PM - 5:37PM |
T01.00004: Chaos and dynamic trapping of swimming microbes in a vortex chain flow Thomas H Solomon, Nghia Tuan Le We present experiments of the motion of swimming Tetraselmis algae microbes in a two-dimensional, vortex chain flow. For microbes swimming with small speed v0, simulated trajectories of idealized swimmers can be either chaotic or ordered, depending on the location in the flow. In the experiments, the slow-swimming microbes follow trajectories that appear to be chaotic, but which become dynamically trapped temporarily to ghosts of the ordered region in the flow. The long-range transport in this case is subdiffusive with variance <x2> ~ tγ with γ < 1. For larger swimming speeds, the simulated island of ordered trajectories disappears, resulting in long-range transport that is diffusive (γ = 1).We calculate Lagrangian-averaged trajectories (LATs) from the experimental data and use the LATs to measure trapping time probability distributions P(t). We find regimes with P(t) ~ t-η with η < 2 for small v0, consistent with the measured subdiffusion. |
Monday, November 25, 2024 5:37PM - 5:50PM |
T01.00005: Boundary-sensing mechanism in branched microtubule networks Meisam Zaferani, Howard A Stone, Sabine Petry, Ned S Wingreen, Ryungeun Song The self-organization of cytoskeletal networks in confined geometries requires sensing and responding to mechanical cues for dynamic adaptation. Here, we show that branching of microtubules (MTs) via branching MT nucleation combined with dynamic instability constitutes a boundary-sensing mechanism within confined spaces. Using a nanotechnology platform, we observe the self-organization of a branched MT network in a channel with a narrow junction and a closed end. Our observations show that branching MT nucleation occurs in the post-narrowing region only if it exceeds a certain length before terminating at the channel's closed end. We further show that this length dependency is tunable and forms the basis for mechanical feedback that adapts MT networks to local geometry, enabling tunable MT network formation in response to physical boundaries in confined spaces. After experimental characterization of boundary-sensing feedback, we propose a minimal model and conduct numerical simulations. We investigate how this feedback, wherein growing MTs dynamically sense their environment and provide nucleation sites for new MTs, sets a length/time scale that steers the architecture of MT networks in confined spaces. This "search-and-branch" mechanism has implications for the formation of MT networks during neuronal morphogenesis, including axonal growth and the formation of highly branched dendritic networks, as well as for plant development, MT-driven guidance in fungi, and engineering nanotechnologies. |
Monday, November 25, 2024 5:50PM - 6:03PM |
T01.00006: Abstract Withdrawn
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Monday, November 25, 2024 6:03PM - 6:16PM |
T01.00007: Phase behaviour and dynamics of three-dimensional active dumbbell systems Antonio Suma, Claudio Caporusso, Giuseppe Negro, Pasquale Digregorio, Livio Nicola Carenza, Giuseppe Gonnella, Leticia Cugliandolo We present a comprehensive numerical study of the phase behavior and dynamics of a three-dimensional active dumbbell system with attractive interactions. We demonstrate that attraction is essential for the system to exhibit non trivial phases. We construct a detailed phase diagram by exploring the effects of the system's activity, density, and attraction strength. We identify several distinct phases, including a disordered, a gel, and a completely phase-separated phase. Additionally, we discover a novel dynamical phase, that we name percolating network, which is characterized by the presence of a spanning network of connected dumbbells. In the phase-separated phase we characterize numerically and describe analytically the helical motion of the dense cluster. |
Monday, November 25, 2024 6:16PM - 6:29PM |
T01.00008: Self-organization of clusters of spheroidal squirmers William E Uspal, Anson Thambi The “squirmer model” is a classical hydrodynamic model for the motion of interfacially-driven microswimmers, such as self-phoretic active colloids or green algae. Recently (Poehnl and Uspal, Phys. Rev. Fluids, 2023), we found that stable bound pairs can occur for identical squirmers with oblate shape and non-axisymmetric interfacial actuation, as well for shape-heterogeneous squirmers (e.g., a prolate squirmer and an oblate one) with axisymmetric actuation. Here, using analytical theory and numerical calculations, we consider self-organization of small clusters of particles. For instance, we show that oblate squirmers can form an immotile polygonal cluster. In this type of cluster, the centers of the particles are located on the vertices of a polygon, and the particle axes are oriented towards the center of the polygon. Using coarse-grained simulations, we consider how this tendency to cluster affects the collective behavior of many swimmers moving in a two-dimensional layer. |
Monday, November 25, 2024 6:29PM - 6:42PM |
T01.00009: Microphase separation in confined swimmers. (Edmond) Tingtao Zhou, John F Brady Hydrodynamic interactions (HI) strongly influence the collective behavior of microswimmers, such as motility-induced phase separation (MIPS). We systematically study the collective dynamics of so-called squirmers using Active Fast Stokesian Dynamics simulations. We focus on ``stealth'' swimmers with 2D orientations but with fully 3D flow fields. We show that the collective behaviors depend not only on the squirming modes but also strongly on the boundary conditions for the flow in the system, as well as their hydrodynamic ensembles. Specifically, we show that ``stealth'' swimmers at fixed swim velocity exhibit micro-phase separations when the flow is confined by either no-slip walls or stress-free liquid-liquid interfaces. Under strong confinement, the difference between ``pushers" and ``pullers'' vanishes and they also show similar micro-phase separations. These results help to reconcile previous theoretical and numerical investigations. From the simulations, we assemble a comprehensive phase diagram of active squirmers. We present a simple theory to rationalize the HI-induced micro-phase separation behavior. |
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