Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J15: Active Colloids II |
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Sponsoring Units: DFD DSOFT Chair: Quentin Brosseau, University of Pennsylvania Room: 210/212 |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J15.00001: Floor- or ceiling-sliding for chemically active, gyrotactic, sedimenting Janus particles Sayan Das, Zohreh Jalilvand, Mihail Popescu, William E. Uspal, Siegfried Dietrich, Ilona Kretzschmar Chemically active particles achieve force- and torque-free motility via catalytic chemical reactions promoted on parts of their surface. These lead to inhomogeneity in the chemical composition of the solution ("chemical field") and hydrodynamic flow of the solution. By means of coupling distortions of these fields back to its motion, a chemically active particle experiences effective interactions with boundaries; this can lead to the occurrence of, e.g., states of steady "sliding" along a wall. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J15.00002: Effects of Colloid Interactions on Active–Passive Collective Behavior Ian Madden, LinLin Wang, Juliane Simmchen, Erik Luijten Active colloidal matter can exhibit different phases, such as flocking, rafting, and spinning, in response to changes in active particle properties (Soft Matter 14, 6969–6973 (2018)). The links between these phenomena and the catalytic, hydrodynamic, electrostatic, and phoretic properties of the colloids are disputed, in part because it is difficult to separate the impact of each force in experiment. Whereas current models can capture some aspects of a system’s collective behavior, oversimplification can lead to incorrectly attributing a particular behavior to a specific interaction (The European Physical Journal E 41, 145–169 (2018)). Using experimentally informed models, we employ lattice-Boltzmann and molecular dynamics simulations to deconvolute these interactions and make direct connections between particle properties and observed collective phenomena. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J15.00003: New mechanism of motility-induced phase separation in active colloids Ricard Alert, Jie Zhang, Jing Yan, Ned Wingreen, Steve Granick At thermodynamic equilibrium, phase separation arises from attractive interparticle interactions. However, self-propelled particles can phase separate even if they have purely repulsive interactions. This phenomenon, called motility-induced phase separation (MIPS), has become a landmark of active matter physics. The conventional mechanism of MIPS is the decrease of particle speed due to repulsive interactions in high-density regions, which leads to further accumulation of particles. In this talk, I will demonstrate a new mechanism of MIPS, which instead of relying on a slowdown of particle motion with density, is based on interaction torques that reorient particles toward high-density regions. We show that such torques take place in suspensions of Janus colloids driven by an electric field. From the electrostatic interactions between the particles, we derive hydrodynamic equations that show how MIPS arises from orientational interactions in this system. Furthermore, we predict that, in contrast to the repulsion-based MIPS scenario, the phase diagram of torque-based MIPS exhibits reentrance, with the system reentering the uniform phase at high self-propulsion speed. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J15.00004: Extract Non-Thermal Fluctuations from an Experimentally Measured Histogram of a Confined Active Brownian Particle Chong Shen, Lanfang Li, Zhiyu Jiang, James F Gilchrist, H Daniel Ou-Yang Active Brownian particles (ABPs), colloidal particles driven into persistent motions by external fields, can serve as a model for understanding the non-thermal behavior of biological systems. Non-thermal fluctuations of an ABP are expected to behave differently from thermal Brownian motions in distinctive manners, e.g., exhibiting different diffusivities, effective temperatures, and fluctuation power spectral densities. While the non-thermal fluctuations can be separated from the thermal ones in their power spectral densities, it is non-trivial to decouple the non-thermal from thermal changes in a fluctuation histogram, because the experimentally measured fluctuation histogram is a convolution of the two fluctuations. We hypothesize that the non-thermal fluctuation could be extracted from the overall noise histogram by standard deconvolution algorisms when thermal fluctuations can be determined independently. This presentation reports an experimental study using a model ABP in a quadratic potential well to test the hypothesis and its limitations. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J15.00005: Transport phenomena and Green-Kubo relations in active media: Theory Jeffrey Epstein, Kranthi Mandadapu We derive Green-Kubo relations for the several viscosity terms that appear in a constitutive theory of viscous active fluids with and without an internal spin degree of freedom, starting from an Onsager regression hypothesis on the non-equilibrium steady state. In particular, we focus on the connection between time-reversal symmetry breaking and the emergence of the non-dissipative component of viscosity known as odd viscosity. Our results allow measurement of this odd viscosity in molecular dynamics simulations (for details see the corresponding Simulation talk). Time permitting, extensions of our results to liquid crystals will be discussed, with possible relevance to the modeling of active fluids such as suspensions or colonies of bacteria. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J15.00006: Transport phenomena and Green-Kubo relations in active media: Simulation Cory Hargus, Katherine Klymko, Jeffrey Epstein, Kranthi Mandadapu We compute the viscous transport coefficients of an active fluid composed of chirally rotated dumbbells with molecular dynamics simulations. We use recently obtained Green-Kubo relations to calculate the emergent viscous transport coefficients (for details see accompanying Theory talk), and find that odd viscosity arises due to the breaking of time-reversal symmetry at the level of stress correlations. We demonstrate agreement between viscosity coefficients obtained from Green-Kubo relations and non-equilibrium molecular dynamics simulations with imposed shear. This verifies the Green-Kubo relations derived using the application of Onsager’s regression hypothesis to non-equilibrium steady states of active systems, and provides an impetus for developing the theory of non-equilibrium thermodynamics of active media. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J15.00007: Nonlinear dynamics of chemically active microdrops grants an insight into interfacial chemistry Matvey Morozov, Laurence Rongy, Fabian Brau Active emulsions are complex physicochemical systems that may provide a model for biological phenomena such as RNA polymerase clusters. Theoretical understanding of the physical chemistry of active emulsions is key for reliable modeling of these systems. We consider the simplest "building block" of an active emulsion: a microdroplet undergoing gradual micellar solubilization in the bulk of surfactant solution. In experiments, dissolving droplets may spontaneously induce a flow in the surrounding fluid. Our model links the features of the flow around the drop with the characteristics of nonlinear surfactant sorption kinetics and nonlinear chemical reaction at the droplet interface. We show that continuous assembly of micelles may act as a cleaning mechanism preventing the formation of a continuous monolayer of surfactant monomers even when bulk surfactant concentration exceeds CMC. Our asymptotics reveals that the Marangoni flow velocity depends heavily on the reaction rate and micelle size, while numerical simulations indicate that nonlinear kinetics of micelles production allows for multistability of flow regimes. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J15.00008: Revisiting the equation of state of active Brownian particles Stewart Mallory, Ahmad Omar, John F Brady One of the unique feature of active systems is that they are able to phase separate without the presence of attractive interparticle interactions. Determining quantitatively the phase boundary for these fundamentally out-of-equilbrium systems is an on-going challenge within the community. The mechanically defined active pressure plays a central role in many of the recently proposed coexistence criteria. Using a combination of large-scale simulation and analytical theory, we reveal previously unknown features of this equation of state. Recognizing these features is essential when attempting to assess coexistence criteria by comparing the predicted phase diagram to simulation data. Upon using this refined equation of state, the predicted phase boundaries are qualitatively altered in comparison to previously work. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J15.00009: Three-body problem for Brownian particles at different temperatures Michael Wang, Alexander Y Grosberg A mixture of Brownian particles at different temperatures has been a useful model for studying out-of-equilibrium systems of many components with differing levels of activity (e.g. phase separation of mixtures of passive and active particles). This model was previously studied analytically up to the second virial coefficients using pair distributions in the dilute limit. We are interested whether or not the two-particle results can be extended to understand the three-particle distributions. By considering the solvable case of pairwise quadratic interactions, we show that, unlike the two-particle distribution, the three-particle distribution takes on an interesting form that is not in general Boltzmann-like with an effective temperature. We summarize some of our results here and briefly discuss an interesting connection with the Newtonian three-body problem. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J15.00010: Microscopic Origins of the Swim Pressure and the Anomalous Surface Tension of Active Matter Ahmad Omar, Zhen-Gang Wang, John F Brady The unique pressure exerted by active particles -- the ``swim" pressure -- has proven to be a useful quantity in explaining many of the seemingly confounding behaviors of active particles. However, its use has also resulted in some puzzling findings including an extremely negative surface tension between phase separated active particles. Here, we demonstrate that this contradiction stems from the fact that the swim pressure is not a true pressure. At a boundary or interface, the reduction in particle swimming generates a net active force density despite no external fields or forcing -- it is an entirely self-generated body force. The pressure at the boundary, which was previously identified as the swim pressure, is in fact an elevated (relative to the bulk) value of the traditional particle pressure that is generated by this interfacial force density. Recognizing this unique mechanism for stress generation allows us to define a much more physically plausible surface tension. We clarify the utility of the swim pressure as an ``equivalent pressure" (analogous to those defined from electrostatic and gravitational body forces) and the conditions in which this concept can be appropriately applied. |
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