Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X63: Out-of-Equilibrium Properties of Soft Materials and Biological SystemsFocus
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Sponsoring Units: GSOFT DBIO Chair: Stephen Whitelam, University of California, Berkeley Room: BCEC 259A |
Friday, March 8, 2019 8:00AM - 8:36AM |
X63.00001: Early Career Award for Soft Matter Research talk: Structure and dynamics of an active nematic fluid : Microscopic and hydrodynamic considerations Invited Speaker: Aparna Baskaran In this talk I will give an overview of the phenomenology of an active nematic fluid as understood from hydrodynamic theories and then identify questions that can only be addressed effectively by microscopic modelling and analysis. Finally I report results from a numerical simulation study that identifies the emergent signatures arising from the semiflexibility of the active nematogen. |
Friday, March 8, 2019 8:36AM - 8:48AM |
X63.00002: Designing active colloidal architectures from diffusiophoretic interactions Antoine Aubret, Jeremie Palacci In Nature, energy input is needed to develop advanced features, e.g. self-healing or self-regulation. I will show how we can extend this principle to the artificial world, using light to carve non-equilibrium interactions between synthetic microswimmers. Following sequential light-patterns, they autonomously assemble into robust self-spinning structures, or microgears. A rotor constitutes a dissipative building block that creates a repulsive, anisotropic potential of diffusiophoretic origin, which we characterize using Highly Inclined Laminated Optical |
Friday, March 8, 2019 8:48AM - 9:00AM |
X63.00003: " Light driven motion of active microswimmers " Pooja Arya, David Feldmann, Svetlana Santer We report on active colloids which can undergo light-driven self-propelled motion. The mechanism of self-propulsion is based on generation of local hydrodynamic flow at each micro-particle resulting from light-driven diffusioosmosis (LDDO) [1]. At the heart of this process is photosensitive azobenzene containing surfactant which undergoes a reversible trans-cis photo-isomerization with corresponding changes in hydrophobicity of the whole surfactant molecule. Active microparticles consist of porous silica and are negatively charged in water. When porous silica particles are dispersed in the solution of cationic photosensitive surfactant, they absorb partially the surfactant in trans-state, but the more hydrophilic cis-isomers are expelled out of the particles. Under illumination with blue light promoting trans-cis photo-isomerization, a continuous local hydrodynamic flow at a single particle is formed where cis-isomers leave the particle, while trans-isomers flow inside. The particles become active when one side of the porous surface is covered with a metal layer. |
Friday, March 8, 2019 9:00AM - 9:12AM |
X63.00004: Transport Processes in Active Fluids Katherine Klymko, Jeffrey Epstein, Kranthi K Mandadapu We perform a coarse-graining analysis of the paradigmatic active matter model, Active Brownian Particles, yielding a continuum description in terms of balance laws for mass, linear and angular momentum, and energy. The derivation of the balance of linear momentum reveals that the active force manifests itself directly as a continuum-level body force proportional to an order parameterlike director field, which therefore requires its own evolution equation to complete the continuum description of the system. We derive this equation, demonstrating in the process that bulk currents may be sustained in homogeneous systems only in the presence of inter-particle aligning interactions. Further, we perform a second coarse-graining of the balance of linear momentum and derive the expression for active or swim pressure in the case of mechanical equilibrium. |
Friday, March 8, 2019 9:12AM - 9:24AM |
X63.00005: Competition between chiral self-replicators is mediated by surface growth dynamics Ashish B. George, Kirill Korolev Phase separation and self-assembly of chiral components underlie many biological and industrial processes. We study how chirality of the components impacts these processes. Motivated by recent experiments with microbes, we focus on two-dimensional growth of auto-catalytic or self-replicating agents. The dynamics, described by a chiral reaction-diffusion equation, recapitulate experimental findings in homochiral populations. We predict very unusual behavior when there are two distinct chiral components. Depending on the relative chirality of each, the population evolves to a stable mixed state with both components or a homochiral state where one component goes extinct. To explain our results, we derive an effective theory that couples component competition and growth front dynamics. The theory reduces to a chiral extension of the KPZ equation coupled to a Burgers’ equation with multiplicative noise. The solution of these equations exhibits bulges and dips on the surface at boundaries between domains with different chirality. These undulations in turn alter the motion of the domain boundaries and determine composition and spatial structure. Our findings suggest a new class of surface growth phenomena and can explain the rapid evolution of chirality in biological populations. |
Friday, March 8, 2019 9:24AM - 9:36AM |
X63.00006: Self-oscillating poroelastic instabilities David Dykstra, Nigel Visser, Corentin Coulais Responsive materials changing shape in response to physical stimuli such as light, heat or chemical reactions are ubiquitous in nature and have recently sparked a myriad of applications, e.g. in smart coatings and soft robotics. So far, stimuli-responsive materials are not “active”: they cannot exhibit a perpetual motion unless the environmental conditions change. Here we show experimentally, numerically and theoretically how a simple system - a confined beam at the boundary of a solvent interface - can harness a combination of poroelastic swelling and elastic instabilities to generate perpetual oscillations. These oscillations are realised by rational design and optimisation of the time-dependent hysteretic loop coupling mechanical deformation to solvent transport within the structure. Our work provides general principles for active systems that operate autonomously, harnessing energy from their environment and as such offers new vistas for active metamaterials and soft robots. |
Friday, March 8, 2019 9:36AM - 9:48AM |
X63.00007: Dynamic clustering of passive colloids in an active bacterial bath Shreyas Gokhale, Junang Li, Alexandre Solon, Nikta Fakhri, Jeffrey Gore Active or self-propelled particles such as motile bacteria often exhibit exotic forms of self-organization on account of their intrinsically nonequilibrium dynamics. Further, it is known that these nonequilibrium dynamics can be harnessed to manipulate passive objects such as microscopic gears and motors. Here, using video microscopy experiments and numerical simulations, we show that the nonequilibrium fluctuations in a bath of motile Pseudomonas aurantiaca bacteria can spontaneously drive the self-assembly of suspended passive colloidal silica particles. In contrast to the phase separation between active and passive particles reported in previous computational studies, we observe a dynamic clustering phenomenon with frequent formation and fragmentation events. We demonstrate that the mean cluster size increases with increasing bacterial density. Moreover, we extract an effective attractive interaction energy scale from the distribution of bond lifetimes and show that it correlates well with the mean cluster size. We hypothesize that a local transient circulation of the bacterial velocity field around colloidal particles is responsible for the observed attractive interactions. |
Friday, March 8, 2019 9:48AM - 10:00AM |
X63.00008: Large Deviation Functions for active Brownian particles Trevor GrandPre, David Limmer, Kranthi K Mandadapu, Katie Klymko Large deviation Functions (LDFs) characterize fluctuations of extensive observables away from equilibrium and offer a route to relate those fluctuations to the systems response to external perturbations. LDFs are difficult to calculate analytically and sometimes numerically for complex interacting systems. For a system of interacting active Brownian particles, we show how a weighted many body expansion can be used to calculate LDFs for a wide variety of relevant observables such as the mass current, active work, and activity. These in turn can be used to understand diverse phenomena such as the Motility Induces Phase Separation (MIPS) and active transport. |
Friday, March 8, 2019 10:00AM - 10:12AM |
X63.00009: Information, dissipation, and typical paths to self-assembling active materials Schuyler B. Nicholson, Rebecca A. Bone, Jason R. Green When chemically fueled, molecular self-assembly can sustain dynamic networks of polymeric fibers – hydrogels – with tunable properties. For a finite supply of fuel, the fibrous networks are transient, as competing reactions switch molecular subunits between active and inactive states, drive the assembly and collapse of fibers, and dissipate energy. Being a far-from-equilibrium process, the fibrous network and its mechanical properties can reflect the history of its preparation. This talk will describe our stochastic thermodynamic and information-theoretic framework to variationally identify nonequilibrium histories that are typical, among the myriad possibilities. It will also describe computer simulations of an experimentally-informed model that reproduces key features of recent gelators, including the observation via confocal microscopy of fast fiber growth and stochastic fiber collapse. Overall, this work is directed at understanding the paths that self-organizing systems travel, how nonequilibrium forces collectively drive structure formation, and how these paths contain clues about the design of dissipative self-assembling systems. |
Friday, March 8, 2019 10:12AM - 10:24AM |
X63.00010: Active diffusion of particles in a dynamic network Loren Hough, Kanghyeon Koo, Shankar Lalitha Sridhar, Jeffrey Dunagin, Franck Vernerey Diffusion is a phenomenon well understood for microscopic particles as arising from random molecular collisions. However, these interactions are typically non-specific and cannot be tuned. In contrast, macromolecular diffusion through networks can be controlled by binding and unbinding events between passaging molecules and flexible chains. Some biophysical examples include the central channel of the nuclear pore complex and liquid drops formed from multivalent interactions. The precise effect of properties such as binding and unbinding rates, number of binding sites and chain elasticity, on diffusion is still poorly understood. Following a statistical mechanics approach, we have developed a diffusion model which shows that the maximum diffusion occurs for few occupied binding sites independent of other parameter choices. We show the validity of our findings by comparing model predictions with a macroscopic diffusion experiment designed to contain similar driving mechanisms and to allow tuning of key parameters including the number binding sites, activity and kinetic parameters. These findings will drive future research work and understanding of controlled active diffusion in dynamic networks. |
Friday, March 8, 2019 10:24AM - 10:36AM |
X63.00011: Propagating Fronts in Columns of Fire Ants Caleb Anderson, Alberto Fernandez-Nieves For the past two decades, the study of active matter has revealed rich physics and universal behaviors, such as collective motion, across a wide variety of biological and synthetic systems with seemingly disparate particle interactions. Here we present our surprise observation and measurements of stable activity fronts propagating through 2 dimensional columns of thousands of fire ants. We then propose a model to explain the origin of these macroscopic non-linear fronts from simplified pairwise interactions between ants, and perform computer simulations that capture the experimental observations. Finally, we finish by comparing the fronts to other instances of collective motion observed in active systems. |
Friday, March 8, 2019 10:36AM - 10:48AM |
X63.00012: Mechanics of Active Foam: Local Energy Injection in Addressable 2D Foam Laurel Kroo, Manu Prakash The study of foam has inspired many insights into cellularized materials, including biological tissue. While cellular sheets superficially resemble passive foam structures, cells incorporate many layers of internal activity and feedback. To the best of our knowledge, no abiotic “active” foam experimental systems currently exist. We have developed such a material, where the activity of a single voxel is driven by volume oscillations. The platform allows air to be programmatically injected and removed from voxels in a 2D liquid soap foam in a Hele-Shaw cell. This cyclic energy injection triggers rearrangements in the structure involving neighbor swapping (T1), leading to cascades and global motion. We quantify this response of a single active voxel as a function of energy injected, symmetry and time course of evolution. Next, we study interactions between multiple active voxels with cyclic perturbations (in-phase and out-of-phase). Stroboscopic analysis is used when studying cyclic forcing. Just like our understanding of active fluids and active solids has brought new light to a multitude of problems - active foam provides a new platform to ask questions both in foam physics and, by analogy, in tissue mechanics. |
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