Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M11: Active Colloids IRecordings Available
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Sponsoring Units: DFD Chair: Tom Solomon, Bucknell University Room: McCormick Place W-181B |
Wednesday, March 16, 2022 8:00AM - 8:12AM |
M11.00001: A touch of non-linearity: mesoscale swimmers and active matter in fluids Daphne Klotsa Living matter, such as biological tissue, can be seen as a nonequilibrium hierarchical assembly of assemblies of smaller and smaller active components, where energy is consumed at many scales. The functionality and versatility of such living or "active-matter" systems render it a promising candidate to study and to synthetically design. While many active-matter systems reside in fluids (solution, blood, ocean, air), so far, studies that include hydrodynamic interactions have focussed on microscopic scales in Stokes flows, where the active particles are <100μm and the Reynolds number, Re <<1. At those microscopic scales viscosity dominates and inertia can be neglected. However, what happens as swimmers slightly increase in size (say ~0.1mm-100cm) or as they form larger aggregates and swarms? The system then enters the intermediate Reynolds regime where both inertia and viscosity play a role, and where nonlinearities in the fluid are introduced. In this talk, I will present a simple model swimmer used to understand the transition from Stokes to intermediate Reynolds numbers, first for a single swimmer, then for pairwise interactions and finally for collective behavior. We show that, even for a simple model, inertia can induce hydrodynamic interactions that generate novel phase behavior, steady states and transitions. |
Wednesday, March 16, 2022 8:12AM - 8:24AM |
M11.00002: Unified analysis of Topological Defects in 2D systems of Active and Passive disks Pasquale Digregorio We have carried out a comprehensive quantitative analysis of localized and extended topological defects in the steady state of 2D passive and active repulsive Brownian disk systems. Both in and out-of-equilibrium, solid-hexatic melting is driven by the unbinding of dislocations, in quantitative agreement with the KTHNY singularity. Instead, although disclinations dissociate as soon as the liquid phase appears, they are not free, but rather always grouped together or with other more complex defect structures. Extended clusters of defects largely dominate below the solid-hexatic critical line. The latter percolate in the liquid phase very close to the hexatic-liquid transition, both for continuous and discontinuous transitions, in the homogeneous liquid regime. At critical percolation the clusters of defects are fractal with statistical and geometric properties that, within our numerical accuracy, are independent of the activity and compatible with the universality class of uncorrelated critical percolation. We show that the disclinations are not free, but rather always very near more complex defect structures. |
Wednesday, March 16, 2022 8:24AM - 8:36AM |
M11.00003: Tuning Nonequilibrium Phase Transitions with Inertia Ahmad K Omar, Katherine Klymko, Trevor K GrandPre, Phillip Geissler, John F Brady In striking contrast to equilibrium systems, inertia can profoundly alter the structure of active systems. Here, we demonstrate that driven systems can exhibit effective equilibrium statistics with increasing particle inertia, despite rigorously violating the fluctuation-dissipation theorem. Increasing inertia progressively eliminates motility-induced phase separation and restores equilibrium crystallization for active Brownian spheres. This effect appears to be general for a wide class of active systems, including those driven by deterministic time-dependent external fields, whose nonequilibrium patterns ultimately disappear with increasing inertia. The path to this effective equilibrium limit can be complex, with finite inertia sometimes acting to accentuate nonequilibrium transitions. The restoration of Boltzmann statistics can be understood through the conversion of active momentum sources to passive-like stresses, with the kinetic temperature serving as a now density-dependent effective temperature. Our results provide additional insight into the effective temperature ansatz while revealing a mechanism to tune nonequilibrium phase transitions. |
Wednesday, March 16, 2022 8:36AM - 8:48AM |
M11.00004: Transport of passive particles in active nematic films Louise C Head, Tyler N Shendruk, David P Rivas, Robert L Leheny, Daniel H Reich Suspended particles offer a means to measure transport and rheological properties in many turbulent flows in industrial, natural, and biological settings. A class of biologically inspired fluids, called active nematics, offer self-driven and spontaneous flowing properties, resulting in disorderly flows at low Reynolds numbers, called active turbulence. We study passive particles suspended in an active nematic film via a coarse-grained MPCD algorithm. We report how coupling to the nematic order and activity endows the passive particles with effective self-propulsion. To make connections to experimental systems, we consider the role played by coupling to the nematic field. By tracing their Lagrangian trajectories, we quantify the dynamical behaviours of the passive solutes, which we compare to the dynamics of particles suspended in traditional scale-invariant turbulence. These numerical results may shed light on the interplay between passive and active constituents in hybrid systems and offer insight into the design of autonomous micro devices composed of passive components powered by active environments. |
Wednesday, March 16, 2022 8:48AM - 9:00AM |
M11.00005: Polar state memory in active fluids Bo Zhang, Hang Yuan, Andrey Sokolov, Monica Olvera De La Cruz, Alexey Snezhko Active liquids composed of densely packed spinning units represent a new class of active materials where both energy and angular momentum are injected into the system at the microscopic level. Spontaneous emergence of global polar states such as particle flocks and vortices are prime examples of remarkable collective dynamics and self-organization observed in active liquids. The formation of globally correlated polar states in geometrically confined systems proceeds through the emergence of a macroscopic steadily rotating vortex that spontaneously selects a clockwise or counterclockwise global chiral state. Here, we reveal that a global vortex of active rollers exhibits state memory. The instantaneous inter-particle positional order encodes the information about the chiral state. This information remains stored even if the energy injection is ceased and activity is terminated. When the system is re-energized, the subsequent formation of the collective states is not random. We demonstrate in experiments and simulations a controlled sequence of the emergent vortical states in an ensemble of Quincke rollers. Our work provides new fundamental insights into mechanisms of the spontaneous formation of the collective polar states in active systems. The particle local arrangement and inter-particle interactions can be exploited to systematically command the subsequent polar states of an active liquid through temporal control of the activity. With chirality of the emergent collective states controlled on-demand, active polar liquids offer new possibilities for flow manipulation, transport, and mixing at the microscale. |
Wednesday, March 16, 2022 9:00AM - 9:12AM |
M11.00006: Distinct regimes of dynamic clustering in mixtures of passive colloids and motile bacteria Shreyas Gokhale, Junang Li, Alexandre Solon, Jeffrey C Gore, Nikta Fakhri In a recent study, we have shown that passive colloids exhibit steady state dynamic clustering in dense suspensions of motile bacteria (Gokhale*, Li*, et al., arXiv:2110.02294, 2021). Here, by performing experiments over a wide range of bacterial densities, we uncover the existence of two dynamical regimes associated with dynamic clustering. In dilute bacterial suspensions, the mean cluster size decreases with increasing bacterial densities, whereas for dense suspensions, it increases with bacterial density. By analyzing colloid trajectories, we show that colloid displacements at short and intermediate times are strongly non-Gaussian in dilute suspensions, and Gaussian in dense ones. Further analysis reveals that the non-Gaussian displacements result from local spatial heterogeneity in bacterial flow fields, that is averaged out at high bacterial densities. Collectively, our experiments show that clustering at low bacterial densities is dominated by heterogeneous flow fields, whereas clustering at high densities is dominated by torque-induced effective attractions between colloids. |
Wednesday, March 16, 2022 9:12AM - 9:24AM |
M11.00007: How does the orientation of an active colloid influence the dynamics of clusters? Bipul Biswas, Manasa Kandula Synthetic active colloids exhibit interesting translational and rotational self-propulsion and collective behavior. The motion of individual synthetic active colloids is well-understood. However, there exist fewer studies focused on understanding the dynamical changes when a group of active particles are clustered. In this talk, I will present our efforts towards designing active particle assemblies with tunable translational and rotational dynamics. To this end, we assemble active colloid clusters with tailored orientations and shapes and study their dynamics. By extracting characteristic parameters like net force, the torques, and translational and rotational velocities we aim to find a generic relation between the cluster shape, particle distribution, and the resultant dynamical trajectories. We expect our work to provide the strategies for the designing and steering active entities with tailored dynamical trajectories. |
Wednesday, March 16, 2022 9:24AM - 9:36AM |
M11.00008: Passive Janus particles are self-propelled in active nematics Benjamin Loewe, Tyler N Shendruk The pursuit of systems capable of extracting work from active media has proven to be a particularly challenging task. In this work, we extend these efforts to the realm of active liquid crystal composites and present a design for passive particles that become effectively self-motile when embedded in an active fluid. We study a colloidal particle with Janus anchoring conditions immersed in an active nematic liquid crystal. The colloid surface enforces an effective +1/2 topological charge in the surrounding active fluid, which gives rise to an effective self-propulsion of the Janus particle. We analytically study this self-propulsion, linking its orientational dependence on the position of a companion -1/2 defect. We predict that the colloid/defect pair remains bounded at small activity, with the defect firmly orienting the colloid to propel parallel or perpendicular to the nematic. Conversely, if the activity is sufficiently high, we predict an unbinding of the colloid/defect pair. This work demonstrates how engineered colloids suspended in active liquid crystals may present a path to functionality. |
Wednesday, March 16, 2022 9:36AM - 9:48AM |
M11.00009: Surface tension of soft active Brownian particles Nicholas J Lauersdorf, Thomas M Kolb, Moslem Moradi, Ehssan Nazockdast, Daphne Klotsa Active-matter systems consist of components that locally consume energy to move, exert forces or perform chemical reactions, thus being inherently out of equilibrium. Simple models have been developed to capture their emergent behavior, including the active Brownian particle (ABP) where each colloid is self-propelled by an active force. Even in the absence of any attractive potential, at sufficient activity, the system undergoes a non-equilibrium phase separation (liquid/gas). One question that we aimed to resolve was how should we define stress in active systems and its balance at steady state? Unique to active systems, active forces align at the interface. Extending equilibrium statistical mechanics to our non-equilibrium systems by using a volume-averaged swim pressure results in unrealistic surface tensions. We derived a continuum theory to investigate the relationship between the interparticle pressure, swim pressure, and macroscopic pressure in the momentum equation. We found that formulating the point-wise macroscopic pressure as the interparticle pressure and modeling the particle activity through a spatially variant body force-as opposed to a volume-averaged swim pressure-results in a surface tension that is negligible and intrinsic to all ABP steady states. |
Wednesday, March 16, 2022 9:48AM - 10:00AM |
M11.00010: Drag on a colloid straddling a fluid interface Petia Vlahovska, ZHI ZHOU, Michael J Miksis We analyze the dynamics of an active colloid moving along the interface between two immiscible fluids with similar viscosity. Under the assumption of a constant three-phase contact angle, we analytically obtain the interfacial deformation around a single particle and numerically the two-particle deformation. Applying the Lorentz reciprocal theorem to the zeroth-order approximation for spherical particles at a flat interface and to the first correction in Capillary number allows us to obtain explicit analytical expressions for the hydrodynamic drag in terms of the zeroth-order approximations and the correction deformations. The drag coefficients are computed as a function of the three-phase contact angle, the viscosity ratio of the two fluids, the Bond number, and the separation distance between the particles. |
Wednesday, March 16, 2022 10:00AM - 10:12AM Withdrawn |
M11.00011: Motion of discoidal catalytic Janus particles near a planar wall Amir Nourhani, Mohammad Nabil, William E Uspal Catalytic Janus particles self-propel by decomposition of chemical “fuel” available in liquid solution. The resulting self-generated hydrodynamic and chemical fields extend into, and are modified by, the surrounding environment, coupling back to the motion of the particle. Previous experimental and theoretical work has shown that spherical Janus particles near a hard planar wall can exhibit surface-bound “sliding” and “hovering” states of motion. More recently, the effect of non-spherical particle shape has come into focus, driven by developments in particle fabrication. Here, we consider the dynamics of a discoidal Janus particle near a planar wall. Via numerical calculations, we find that changing the particle’s aspect ratio can qualitatively change the particle’s dynamical behavior, e.g., by inducing two co-existing fixed points for the height and orientation. Using far-field analytical expressions for the interaction between the particle and the wall, we show how these bifurcations emerge from the interplay of hydrodynamics, phoresis, and particle shape. |
Wednesday, March 16, 2022 10:12AM - 10:24AM |
M11.00012: Constructing micro-chains: a study of self-phoretic torus-sphere interactions Ruben Poehnl, William E Uspal Self-phoretic particles are capable of propulsion in liquid solution by catalyzing the decomposition of chemical "fuel." Recent studies have focused on the self-organization of self-phoretic particles into larger assemblies , i.e., so-called "machines from machines." For instance, Nasouri et al. (JFM 2020) have shown that two spheres can form stable bound pairs for specific choices of the particle design parameters. Particle shape and topology may hold the key for achieving greater control over pairwise interactions. In particular, increasing the topological genus of one of the particles could increase pair stability and even allow for a "lock-and-key" assembly mechanism. Here, we study the pairwise interaction between a self-phoretic torus and a self-phoretic sphere. We analytically calculate the concentration field for the torus-sphere pair and discuss the resulting motion of the pair. We consider both stable configurations as well as configurations in which the sphere passes through the torus. Finally, we show that the results can straightforwardly be extended to a self-phoretic spheroid-torus pair. |
Wednesday, March 16, 2022 10:24AM - 10:36AM Withdrawn |
M11.00013: Automating Bayesian inference and design to quantify acoustic particle levitation Kiran Dhatt-Gauthier Self-propulsion of micro- and nanoparticles powered by ultrasound provides an attractive strategy for the remote manipulation of colloidal matter using biocompatible energy inputs. Quantitative understanding of particle motion and its dependence on size, shape, and composition requires accurate characterization of the acoustic field, which depends sensitively on the experimental setup. Here, we show how automated experiments based on Bayesian inference and design can accurately and efficiently characterize the acoustic field within resonant chambers used to propel acoustic nanomotors. Repeated cycles of observation, inference, and design (OID) are guided by a physical model that describes the rate at which levitating particles approach the nodal plane. Using video microscopy, we observe the relaxation of tracer particles to this plane following the application of the acoustic field. We use sequential Monte Carlo methods to infer model parameters such as the amplitude and frequency of the resonant chamber while accounting for particle-level measurement noise and population-level heterogeneity in the field. Guided by simulated outcomes, we select the optimal design for the next experiment as to maximize the information gain in the relevant parameters. We show how this iterative process serves to discriminate between competing hypotheses and efficiently converges to accurate parameter estimates using only few automated experiments. We discuss the need for model criticism to ensure the validity of the guiding model throughout automated cycles of observation, inference, and design. This work demonstrates how Bayesian methods can learn the parameters of nonlinear, hierarchical models used to describe video microscopy data of active colloids. |
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