Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K61: Active Matter VFocus
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Sponsoring Units: GSOFT DBIO GSNP Chair: Rui Zhang, University of Chicago Room: BCEC 258B |
Wednesday, March 6, 2019 8:00AM - 8:36AM |
K61.00001: Pushers, pullers, splitters: molecular processes governing the dynamics of active emulsions. Invited Speaker: Corinna Maass Active emulsions are a versatile microswimmer model system. We study a system of spherical oil droplets gradually dissolving in aqueous surfactant solutions, which move autonomously via self-sustaining Marangoni gradients at the oil-water interface. Owing to this interfacial driving, the droplets are direct experimental analogues to hydrodynamic squirmer models, with the interfacial velocity profile depending on the kinetics of surfactant micelles taking up oil from the swimming droplet. We have observed both hydrodynamic and chemical fields around the droplets and found that the system's geometry, length scales and solubilisation timescales govern various effects: pusher to puller type hydrodynamics; persistent to unsteady swimming; and, for large squeezed droplets, interfacial instabilities leading to deformation and division states. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K61.00002: Light-driven Assembly of Motile Colloidal Clusters from Immotile Building Blocks Falko Schmidt, Benno Liebchen, Hartmut Loewen, Giovanni Volpe Active matter, consisting of self-propelled units locally injecting energy into the system, opens new horizons for the creation of functional soft materials with designable properties. Experiencing a constant energy input, allows active matter to self-assemble into phases with a complex architecture and functionality such as living clusters which dynamically form, reshape and break-up but would be forbidden in equilibrium material by the entropy maximization (or free energy minimization) principle. The challenge to control this active self-assembly has evoked widespread efforts typically hinging on an engineering of the properties of individual motile constituents. Here, we provide a different route, where activity occurs as an emergent phenomenon only when individual building blocks bind together, in a way which we control by laser light. Using experiments and simulations of two species of immotile microspheres, we exemplify this route by creating active molecules featuring a complex array of behaviors, becoming migrators, spinners and rotators. The possibility to control the dynamics of active self-assembly via light-controllable nonreciprocal interactions will inspire new approaches to understand living matter and to design active materials. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K61.00003: Failures of defining Effective temperature in an active colloidal system Chong Shen, H Daniel Ou-Yang Active colloidal systems contain active particles, which convert energy from the environment into directed or persistent motion. Effective temperature, defined from active diffusion, fluctuation, and sedimentation profile, is used sometimes for quantifying the activity of active particles. However, non-thermal fluctuations were found to lead to failures when using effective temperature under some conditions, for examples, at short times and in strong confinement. The reason and conditions under which would the failure appear is not clear even for a single active particle. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K61.00004: Fluidic Metamaterials for Controlled Transport of Active Colloids Shahrzad Yazdi, Juan L. Aragones, Alfredo Alexander-Katz In metamaterials, one can design novel properties by tuning the links between constituent elements. Here, we leverage hydrodynamic interactions as designed links to engineer a fluidic metamaterial for non-equilibrium colloidal control. Our system consists of an active particle in a viscous fluid confined in a periodic array of posts. Controlled hydrodynamic and electrostatic interactions of the particle with posts give rise to interesting transport modes, reminiscent of those in Floquet-Bloch systems. We present a wide range of non-equilibrium transport states for various lattice structures and external field parameters. Using Brownian dynamics, we examine the robustness of transport states to thermal fluctuations and design defects. This novel system gives insight into design of new materials for smart transport of colloidal particles. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K61.00005: Propulsion of catalytic Janus spheres in viscosified solutions Patrick Underhill, Edmund Tang, Purba Chatterjee Many applications of Janus motors involve moving objects through complex environments that are mixtures of components. The flow properties of these mixtures could be Newtonian or non-Newtonian. In other applications, additives may be introduced as a way of controlling the motion of motors. The first step in understanding how the fluid mixtures alter their motion is to examine propulsion in fluids where additives change the shear viscosity while the fluid remains Newtonian. We show how solution viscosity affects Janus motor propulsion keeping all other factors (motor size, fuel concentration, temperature, etc.) constant. The velocity is shown to decay approximately inversely with viscosity. Further, the type of viscosifier used affects the interaction between fuel molecules and motor, which affects propulsion. This is part of the overall goal of understanding how solution properties impact propulsion independent of a particular application. When qualifying the propulsion in crowded environments, it is important to understand how to accurately quantify the response. We have used computer simulations to quantify the errors associated with particle tracking when extracting the propulsion of Janus motors. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K61.00006: Interrupted Motility Induced Phase Separation for aligning active colloids. Marjolein van der Linden, Dirk Aarts, Olivier Dauchot Switching on large activity in a rather dense system of active Janus colloids, we observe fast clustering, followed by clusters aggregation towards full phase separation. The phase separation process is however interrupted when large enough clusters start breaking apart. Following the cluster size distribution as a function of time, we identify four successive dynamical regimes. Tracking both the particle positions and orientations, we characterize the structural and alignment ordering present in the growing clusters, and thereby unveil the mechanisms at play in these regimes. In particular, we identify how alignment between the neighboring particles is responsible for the interruption of the full phase separation. This experimental study, which provides the first large-scale observation of phase separation in active colloids, combined with particle scale analysis of the local mechanisms, points at the new physics observed when both alignment and short-range repulsion are present. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K61.00007: The velocity of self-propelled Pt-coated colloids is determined by the substrate. Stefania Ketzetzi, Rachel Pamela Doherty, Daniela Jutta Kraft Active colloids are typically found self-propelling near a substrate. However, the effect of the substrate on the self-propulsion remains unexplored. Here, we investigate whether the substrate influences the self-propulsion by performing systematic experiments on different substrates. Interestingly, we find that the colloid velocities are considerably different on different substrates. We consider various physicochemical properties as the origin for this observation. Our results are useful for future modeling and might be helpful in understanding the details of the still debated self-propulsion mechanism. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K61.00008: Light Driven Fuel-Free Thermocapillary Microswimmers Matan Yah Ben Zion, Yaelin Caba, Alvin Modin, Paul M Chaikin Surface tension is a powerful agent for driving biological and artificial micron-sized particles. Surface tension gradients however, typically require a chemical reaction, making a capillary based swimmer fuel dependent. By combining a light absorbing bead with a fluid droplet we made a dimer that is propelled by light, and requires no chemical fuel. The 6 μm swimmers’ motility records over 10 μm/s and show good with measured thermocapillary and hydrodynamic parameters. We discuss the swimmers’ manipulation and interactions. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K61.00009: Tuning the motility of self-propelled droplets: from persistent to stochastic Adrien IZZET, Pepijn Moerman, Katherine A Newhall, Jasna Brujic Active droplets produce isotropic concentration gradients in the solution. These solute-mediated interactions depend on the size of the droplet and on the composition of the solvent, as shown in the case of di-ethyl phthalate (DEP) droplets in an aqueous solution of sodium dodecyl sulphate (SDS) [P. G. Moerman et al. PRE 96, 032607 (2017)]. When the fluctuations of these interactions are sufficient to break the symmetry of this dissolution gradient, a self-sustained motion is initiated. These droplets exhibit different motility profiles, from ballistic to diffusive. We use the rotational diffusion model to describe the motility of these swimmers. For a given droplet size, we tune the concentration of surfactant in the solution in order to control the droplet solubility. We show that contrary to Janus particles, the persistence length of the trajectory decreases when adding more fuel (surfactant) in the system: the sensitivity to fluctuations of local fuel concentration increases. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K61.00010: Exploring the origin of self-induced vertical oscillations of dust particles in a plasma Joshua Mendez, Guram Gogia, Justin Burton Micron-size charged particles can be easily levitated in a low-density plasma environment. Such "dusty plasmas" are often used to investigate traditional condensed matter and statistical physics at the single-particle level. We have recently observed a novel phenomenon where hundreds of particles can switch between crystalline and gas-like states over minutes-long time scales (Gogia et al., PRL, 2017). The constituent-level source of energy for this "active matter" system is sustained, large-amplitude vertical oscillations of the individual particles. Delayed charging, charge fluctuations, and variations in the plasma number density have been previously invoked to explain such behavior, however, we show that these mechanisms are unlikely to drive the oscillations we observe. Langmuir probe measurements suggest the plasma environment around the grains is time-invariant. Furthermore, grains carry charges on the order of 10^4 electrons, suggesting that square-root N fluctuations would be too small to drive such large amplitude oscillations. We hypothesize that particle oscillations arise not from electrostatic effects, but from a complicated interaction between the ion wakes streaming out in the lee of the particles and the underlying electrode at low pressures. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K61.00011: Diffusive ferromagnetic roller gas: velocity and displacement statistics Gašper Kokot, Alexey Snezhko Active colloids can display collective motion that is non-directional on average, akin to gas molecules. We experimentally observe and investigate such behavior for ferromagnetic colloidal rollers powered by a vertical alternating magnetic field. We show that the system has a bimodal velocity distribution. Furthermore, the displacement statistics transition from bimodal to quasi-Gaussian distributions (Gaussian core with power-law tails) for all densities. Overpopulated tails are observed at low densities. The system demonstrates the normal diffusive behavior for both active particles and inert glass beads of the same size. The density dependence of the diffusion constant shows a striking difference between active and inert particles. Our work provides insight into statistical properties of active Brownian-like non-equilibrium systems. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K61.00012: Collective dynamics and active turbulence in swarms of synchronized self-assembled spinners Koohee Han, Gašper Kokot, Alexey Snezhko Active magnetic colloids have proven to be an excellent model system to explore emergent out-of-equilibrium dynamics and structures. We demonstrate that ferromagnetic microparticles, suspended at an air/water interface and energized by an external rotating magnetic field, form dynamic ensembles of synchronized self-assembled spinners. The balance between the magnetic and viscous torques determines the size of an individual self-assembled spinner, which can be controlled by the frequency and strength of the applied magnetic field. Each spinner generates local hydrodynamic flows such that the collective interactions of the multiple spinners allow the formation of dynamic crystal lattices. We investigate active diffusion of passive cargo particles in such spinner ensembles and analyze the structure of the underlying self-induced surface flows. We show that induced flows exhibit properties of an active turbulence. The energy spectra of the active turbulence in such synchronized spinner ensembles reveal reverse energy cascade with the exponent significantly different from the classical 2D turbulence. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K61.00013: Solid-liquid transition in layers of deformable active droplets Benjamin Loewe, Davide Marenduzzo, M. Cristina Marchetti Layers of epithelial tissue have been modeled either as collections of spherical active particles or via the Vertex Model (VM) that describes cells as irregular polygons tiling the plane. The VM is appropriate to describe confluent layers where there are no gaps between cells and does not incorporate variations in the cell packing fraction that is by construction set to one. In contrast, the particle model can account for variation in packing fraction, but does not allow for deformations of individual cells. Both models predict a solid-liquid transition of cellular tissue tuned either by cell shape (VM) or by cell density (particle model). To bridge between these two models, we describe cells as deformable self-propelled droplets each characterized by a scalar field representing the cell’s density. We then examine the collective behavior of N droplets modeled as N interacting phase fields. Using this model, we examine the interplay of cell deformability, cell density and cell motility in controlling the solid-liquid transition. We quantify the melting of the hexagonal ground state, as well as the stability of metastable states. |
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