Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R57: Active Matter I |
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Sponsoring Units: GSOFT DBIO GSNP Chair: Enkeleida Lushi, Simons Foundation Room: LACC 518 |
Thursday, March 8, 2018 8:00AM - 8:12AM |
R57.00001: Microscopic engine powered by critical demixing Falko Schmidt, Alessandro Magazzù, Agnese Callegari, Luca Biancofiore, Frank Cichos, Giovanni Volpe During the last few decades much effort has gone into the miniaturization of machines down to the microscopic scale with robotic solutions indispensable in modern industrial processes and play a central role in many biological systems. There has been a quest in understanding the mechanism behind molecular motors and several approaches have been proposed to realize artificial engines capable of converting energy into mechanical work. These current micronsized engines depend on the transfer of angular momentum of light, are driven by external magnetic fields, due to chemical reactions or by the energy flow between two thermal reservoirs [1-5]. Here we propose a new type of engine that is powered by the local, reversible demixing of a critical binary liquid. In particular, we show that an absorbing, optically trapped particle performs revolutions around the optical beam because of the emergence of diffusiophoresis and thereby produces work. This engines is adjustable by the optical power supplied, the temperature of the environment and the criticality of the system. |
Thursday, March 8, 2018 8:12AM - 8:24AM |
R57.00002: Dynamics and Manipulation of Emergent Magnetic Roller Vortices Gašper Kokot, Alexey Snezhko Active colloids display complex collective behavior. We report the emergence of unconfined macroscopic vortices in the system of ferromagnetic colloidal rollers energized by a vertical alternating magnetic field. We demonstrate that observed spontaneous self-organization is not a consequence of a geometrical confinement or finite system size. By introduction of inert scatterers we elucidate the complex nature of interactions of the roller-vortex with the inert particles. The capability of certain inert particles to effectively pin the vortex and manipulate its dynamics is investigated. Furthermore, we demonstrate the potential of a magnetic roller vortex to effectively capture and transport inert particles. Our work provides new insights into behavior of a broad class of active systems where collective motion is caused by a fine interplay between long-range and short-range interactions. |
Thursday, March 8, 2018 8:24AM - 8:36AM |
R57.00003: Fast Crystallization driven by active dopants Sophie Ramananarivo, Etienne Ducrot, Mena Youssef, Stefano Sacanna, Jeremie Palacci Colloids have been instrumental in statistical mechanics to model the phase behavior of atomic liquids and solids. They notably played an important role in understanding the mechanisms at stake in crystallization, allowing for example to track individual particles, dislocations or grain boundaries, a daunting task for atomic systems. The theoretical framework of dissipative systems is open, and using active colloids -which continuously consume energy- allows us to probe dynamics out of traditional scope of equilibrium physics. We experimentally study the dynamics of a dense monolayer of passive colloids, relaxing after being quenched in a polycrystalline state with domains of mismatching orientations. A small fraction of active intruders navigates within the crystal. We study the evolution of the polycrystalline pattern, ensuing from the interplay between the self-propulsion of active particle highly constrained by lattice and the rearrangement of passive colloids induced by this internal activity. The intruders are shown to speed up grain growth leading to an ordered phase, with a reorganization dynamic dependent upon the number of intruder and their activity. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R57.00004: Mobility of Active Particle on Adhesive Surfaces at Nano- and Micro-Scales Yuan Tian, Zhen Cao, Heyi Liang, Andrey Dobrynin Self-propelled active particles capable of transducing energy to drive self-motion have enormous potentials in many applications such as cancer treatment and drug delivery. To understand the mobility of active particles at nano- and micro-scales, we performed molecular dynamics simulations of an active particle in contact with a rigid substrate. The active particle consists of a soft gel-like shell filled with a mixture of active and inactive beads. The transduction of the energy from active beads to the elastic shell could lead to stationary, steady rolling, and accelerating state depending on the strength of shell-substrate adhesion and the magnitude of external forces acting on active beads. In the stationary state, the limiting friction is larger than the external force generated by active beads and the active particle sticks to the substrate. In the steady rolling state, the rolling friction balances the driving force and active particle maintains a constant rolling speed. In this regime the elastic shell has a constant contact area with the substrate. In the accelerating state, the external driving force exceeds the friction force and the contact area of the elastic shell with the substrate decreases with increasing particle acceleration. |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R57.00005: Aggregate Suppression by Active Reorientation in a Benchtop Active Granular Experiment Powered by Toy Vibrobots Kyle Welch, Xinliang Xu, Xiang Cheng Motility-induced phase separation (MIPS) describes the tendency for self-propelled particles to form coexisting dense aggregates and diffuse, gas-like regions at sufficiently high density. Here we show, by way of a benchtop active granular system and numerical simulations, that this behavior can be suppressed with the introduction of an internal rotational degree of freedom. Our grains are circular aluminum cups made active when placed upside down over toy vibrobots. Because the vibrobot is not constrained to a particular orientation inside the cup, when one of these active particles collides with an obstacle (another particle, for example), the robot actively reorients until the particle can move away. This mechanism stands in contrast to an active brownian system in which the particle’s escape depends on rotational diffusion, which is much slower. Thus, no aggregates form in our system, even as the density approaches the maximum packing fraction for disks in two dimensions. We explore the suppression of MIPS and other emergent dynamics in our benchtop experiment as well as in numerical simulations, and draw parallels to real macroscopic systems of motile agents capable of active reorientation. |
Thursday, March 8, 2018 9:00AM - 9:12AM |
R57.00006: Bacterial swarms drive the propagation of active-passive boundaries through emergent vortical flows coupled to boundary curvature Alison Koser Patteson, Arvind Gopinath, Paulo Arratia Many species of bacteria exhibit a collective behavior known as swarming, which features long-range self-organized flows on agar substrates. We found that swarming colonies of Serratia marcescens are capable of remodeling their physical environment by dissolving large domains of passive particles that obstruct the path of the expanding colony. Passive domains form when a swarm is exposed to high intensity light that locally immobilizes bacteria. Post-exposure, bacteria penetrate, shape, and erode the passive domain. We interpret the phenomena by identifying a propagating active-passive interface with an emergent interfacial stiffness generated by the action of vortical flows of bacteria colliding with the interface. Our results show that the evolution of the interface is governed by the local speed of bacteria coupled to the interface curvature, a result which suggests the existence of an active analogue to the Gibbs-Thomson-Stefan boundary in passive interphase systems. These observations broaden our understanding of swarming dynamics and provide insight into how bacteria compete for environmental niches permeated by particles such as dirt, spores, and other microbial species. |
Thursday, March 8, 2018 9:12AM - 9:24AM |
R57.00007: Collective forces of black soldier fly larvae Olga Shishkov, John Brady, David Hu Black soldier fly larvae are edible maggots that transform tons of food waste into sustainable protein per day. Although they are known to have a collective motion around and inside food sources, a physical understanding of their high eating rates is missing. We show that quorum sensing, not chemotaxis, is responsible for larvae finding food, and investigate a zone of high activity of larvae that forms near food. Using constant strain compression experiments, we demonstrate that these larvae exert an active pressure in addition to the elastic pressure of their bodies. We treat larvae as an active matter system and model their pressure with a mechanical theory. Larvae pressure increases when the larvae become more active, for example when they are consuming food. The high activity of larvae near food increases their interactions with each other, which helps larvae find food even though they are unable to smell it. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R57.00008: Light driven fluid micro-swimmers Matan Yah Ben Zion, Yaelin Caba, Paul Chaikin Many biological and artificial micro-swimmers use capillary forces for motility. Those forces rely on a chemical reaction, making the swimmers fuel dependent. Here we present a light driven Marangoni micro-swimmer requiring no fuel. Combing a light absorbing bead with a fluid droplet we made a micro-dimer that is propelled by light. The motion shows ballistic dynamics and correlated with the dimer orientation. |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R57.00009: Characterising phase transitions in experimental active matter systems with compression algorithms Melissa Ferrari, Stefano Martiniani, Stefano Sacanna, Paul Chaikin Light activated colloidal swimmers have been shown to self-organize into dynamic crystalline structures. For our phoretic swimmers, the separation into a high density clustering phase and a low density gas-like phase happens at a surface area fraction above 7%, with a phase transition characterized by particle number fluctuations. The need for ad-hoc order parameters makes locating and characterizing the nature of such dynamical transitions difficult for this class of systems. We will show how compression algorithms can be used to systematically locate and characterize phase transitions in this experimental active matter system, as well as to study the relaxation of the system back to equilibrium. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R57.00010: Photo-gravitaxis in Synthetic Microswimmers William Uspal, Dhruv Singh, Mihail Popescu, Laurence Wilson, Peer Fischer We study the dynamics of active Janus particles that self-propel in aqueous solution by light-activated catalytic decomposition of chemical “fuel.” In experiments, the particles, initially sedimented at a bottom wall, exhibit wall-bound states of motion, dependent on the size of the particle, when illuminated from underneath the wall. Upon increasing the intensity of the light above a threshold value, which is also dependent on the size of the particle, the particles lift off the wall and move way from it, i.e., they exhibit a photo-gravitactic behavior similar to some planktonic microorganisms. The dependencies on the particle size are rationalized by using a theoretical model of self-phoresis that explicitly accounts for the “shadowing” effect of the opaque catalytic face of the particle. Our model allows us to unequivocally identify the photochemical activity and phototactic response as the key mechanisms beyond the observed phenomenology. Consequently, one has the means to design photo-gravitatic particles that can reversibly switch between operating near a boundary or in the volume away from the boundary by judiciously adjusting the light intensity, i.e., simply by “turning a knob." |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R57.00011: Swimming Emulsion Droplets Adrien IZZET, Pepijn Moerman, Katie Newhall, Eric Vanden-Eijnden, Jasna Brujic Active droplets create concentration gradients in their surroundings and therefore modify the local composition of their solvent. These solute-mediated interactions depend on droplet size and solute concentration, as we have 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)]. Around a DEP droplet, the concentration gradient is initially isotropic but fluctuations can give rise to self-sustained motion in a random direction. These droplets exhibit different motility profiles, from ballistic to sub-diffusive, due to the dynamical interplay between the concentration gradient they generate and their autophoretic motion. We explore a wide range of experimental control parameters, such as the fuel concentration, droplet size, and droplet-droplet interactions, in search of a unifying framework to understand this active system. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R57.00012: Dynamics of Janus Colloids in Phase Separating Solvent Mixtures Lewis Sharpnack, Thomas Zinn, Theyencheri Narayanan Self-propelled colloids are a model system for probing the dynamics of active matter at micron and submicron length scales. Common systems utilize the asymmetric catalytic activity of Janus particles to generate local chemical gradients that drive the motion in a catalytic medium. Previous studies showed that analogous behaviors were obtained by suspending certain Janus particles in a phase separating binary mixture due to the preferential adsorption of one of the liquid species on one side of the colloids. Utilizing the recently upgraded beamline ID02 at ESRF and fast Eiger pixel detector (up to 20,000 fps), we studied the dynamics of silica-nickel Janus colloids in phase separating quasi-binary mixture of 3-methylpyridine, water and heavy water using x-ray photon correlation spectroscopy in the ultra-small angle scattering regime. This allowed the investigation of more concentrated systems than studied by optical microscopy. Results suggest rapid arrest of active motion by dynamic clustering of Janus particles. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R57.00013: Quantifying mixing in active fluids Amanda Tan, Eric Roberts, Kevin Mitchell, Linda Hirst Active matter consists of individual particles that consume energy and move collectively in bulk, forming emergent patterns. We study an active system composed of semi-flexible biopolymers (microtubules), and clustered molecular motors (kinesin) that self-mixes. In this system, microtubules are bundled together, and as kinesin clusters walk along the filaments, the bundles move relative to each other where they extend, bend, buckle, and fracture. When confined in 2D at an oil-water interface, the active network is an extensile active nematic, we can consider the defects to be virtual stirrers and the microtubules/kinesin system is the fluid. We quantify the quality of mixing, or topological entropy, in this self-mixing system, by coupling beads to the microtubule bundles and tracking their motion as it is mixed. Bead trajectories are used to measure the rate of separation in the material to calculate the topological entropy and these results are compared with a line-stretching method. We then change the rate of local extension, by varying the ATP concentration to see the effect on the topological entropy. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R57.00014: Effective temperature defined by diffusivity and fluctuation of an active Brownian particle Chong Shen, H Daniel Ou-Yang This talk addresses an ongoing debate on whether effective temperature (Teff) is a state function of an active colloid. The debate arises because effective temperatures (Teff) defined by different dynamic variables may not be the same. We address this debate using active Brownian particles comprising metallic Janus particles driven by induced-charge electrophoresis. We determined Teff1 from the ratio of the active diffusivity over the passive diffusivity. We also determined Teff2 from the low-frequency limit of the noise power spectral density of an ABP in an optical trap. The Teff1 and Teff2 are found to be equal, both are linearly proportional to the drift speed squared, suggesting they are a state function for the ABP system. |
Thursday, March 8, 2018 10:48AM - 11:00AM |
R57.00015: Hierarchical Self-Assembly of Spinning Microgears Antoine Aubret, Mena Youssef, Stefano Sacanna, Jeremie Palacci In this work, we demonstrate a hierarchical approach to create complex, robust, and dynamical superstructures using synthetic particles. |
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