Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S14: Active Colloids |
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Sponsoring Units: GSNP GSOFT Chair: Michelle Driscoll, NYU Room: 273 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S14.00001: Hydrodynamic Torques and Rotations of Superparamagnetic Bead Dimers Christopher Pease, J. Etheridge, H.S. Wijesinghe, C.J. Pierce, M.V. Prikockis, R. Sooryakumar Chains of micro-magnetic particles are often rotated with external magnetic fields for many lab-on-a-chip technologies such as transporting beads or mixing fluids. These applications benefit from faster responses of the actuated particles. In a rotating magnetic field, the magnetization of superparamagnetic beads, created from embedded magnetic nano-particles within a polymer matrix, is largely characterized by induced dipoles $m_{ip}$ along the direction of the field. In addition there is often a weak dipole $m_{op}$ that orients out-of-phase with the external rotating field. On a two-bead dimer, the simplest chain of beads, $m_{op}$ contributes a torque $\Gamma_m$ in addition to the torque from $m_{ip}$. For dimers with beads unbound to each other, $m_{op}$ rotates individual beads which generate an additional hydrodynamic torque on the dimer. Whereas, $m_{op}$ directly torques bound dimers. Our results show that $\Gamma_m$ significantly alters the average frequency-dependent dimer rotation rate for both bound and unbound monomers and, when $m_{op}$ exceeds a critical value, increases the maximum dimer rotation frequency. Models that include magnetic and hydrodynamics torques provide good agreement with the experimental findings over a range of field frequencies. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S14.00002: Hydrodynamically synchronized motion of externally driven colloids Takashi TANIGUCHI, Kosuke Teshigawara, John Molina, Ryoichi Yamamoto, Norihiro Oyama Recent experiments by Kimura et al on externally driven colloidal particles have identified a unique mode of collective motion in three-particle systems. In the experiment, three colloidal particles are dispersed in water and trapped between two flat parallel plates. The particles are then driven by an optical tweezer along a predefined circular path. They have found a “doublet-singlet periodic motion”, in which a doublet (a pair of particles) approaches the remaining single particle from the back. The newly formed triplet will quickly break up into a new doublet at the front, and a singlet at the back. The doublet leaves the remaining particle behind. In this work, we numerically investigate the dynamics of various numbers of trapped spherical particles moving along a closed path under a constant tangential force. In particular, we have studied the dependence of the gap size, and the strength of the external driving force. With increasing tangential force, we found a transition in the most stable collective mode: from double-singlet periodic motion, to a newly predicted triplet state, which has not yet been observed experimentally. Our results indicate that this transition of the most stable mode of collective motion, from the doublet-singlet to the triplet, occurs continuously. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S14.00003: Walking the Tightrope: Colloidal surfers mimicking molecular motors Viva R. Horowitz, Michelle Driscoll, Melissa Ferrari, Mena Youssef, Stefano Sacanna, Paul Chaikin, Vinothan N. Manoharan We aim to understand cellular processes, particularly intracellular transport, at a physical level by building simple, well‐controlled systems that mimic the functions of a cell. We are inspired by molecular motors such as kinesin and myosin, which create a dynamic environment that is likely necessary for the biochemical reactions that take place in a eukaryotic cell. One approach we have taken is to investigate the superdiffusive environment created by platinum Janus swimmers encapsulated in artificial cells. Now we are investigating the motion of light-activated colloidal surfers. When they are activated, these particles are attracted to each other and to surfaces, and they are self-propelled, moving via self-diffusiophoresis. On a flat surface, these properties cause the particles to form active crystal structures [1]. When we introduce a wire to the geometry, the particles walk along a wire, reminiscent of the motion of molecular motors such as kinesin walking on a microtubule. When the wire is suspended in the center of a fluid chamber, the particles walk the tightrope. This bio-inspired research may lead to systems of particles walking networks of wires and carrying cargo through an artificial cell. [1] Palacci, J., et al. Science 339, 936–940 (2013). [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S14.00004: Large-scale dynamics of colloidal gyrofluids Sofia Magkiriadou, Vishal Soni, Theodore Hueckel, Benjamin C. van Zuiden, Vincenzo Vitelli, Stefano Sacanna, William T. M. Irvine We study the collective behavior of colloidal magnets in rotating magnetic fields. When spun by the field, these particles coalesce into large aggregates owing to their magnetic interactions. These aggregates, composed of millions of particles, behave like a chiral fluid with unusual properties. In this talk, we report our experimental observations of the fluid dynamics and instabilities of this curious fluid. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S14.00005: Passive colloids work together to become Active Hima Nagamanasa Kandula, Wei Wang, Jie Zhang, Huanxin Wu, Ming Han, Erik Luijten, Steve Granick In recent years there is growing body of research to design self-propelled colloids to gain insights into non-equilibrium systems including living matter. While most active colloids developed hitherto entail prefabrication of Janus colloids and possess single fixed active site, we present one simple system where active colloids are formed in-situ naturally with multiple active sites and are reversible as well as reconfigurable. A binary mixture of Brownian colloids which have opposite polarizations when subjected to an AC electric field spontaneously assemble into clusters which are propelled by asymmetric induced charge electro osmosis. We find that tuning the relative sizes of the two species allows for the control over the number of active sites. More interestingly, the patches are dynamic enabling reconfiguration of the active cluster. Consequently, the clusters are active not only in motion but also in their structure. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S14.00006: Segregation of colloidal swimmers by their activity Melissa Ferrari, Mena Youssef, Michelle Driscoll, Stefano Sacanna, David Pine, Paul Chaikin We study a system of micron sized self-propelled colloidal swimmers whose activity can be switched on or off with the flick of a light switch. We have designed a system where an external LED source reflects light off of an array with hundreds of thousands of independently controlled tiny mirrors, through an optical microscope, and onto the plane of the swimmers. By exposing a collection of particles to a spatial or dynamic light field, we have the ability to control the speed of a particle based on its position, and therefore the density of the collection of particles in space. Theoreticians in the field have been building a framework that describes systems which are out-of-equilibrium and we will show how our system can be useful tool in mapping these theories to experiment. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S14.00007: Phase Behavior, Synchronization, and Emergent Flows of Spinning Magnetic Colloids Vishal Soni, Sofia Magkiriadou, Benny C. van Zuiden, Theodore Hueckel, Vincenzo Vitelli, Stefano Sacanna, William T.M. Irvine We study the collective motion, phase behavior, and synchronization of colloidal particles spun by a rotating magnetic field. We tune the inter-particle interactions, spinning speed, and particle shape, while imaging the orientation of each particle over time. I will discuss the observed phase transitions in structure and synchronicity, as well as the collective dynamics of the system. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S14.00008: When push comes to shove: Contact-triggered active particles Mayank Agrawal, Isaac Bruss, Sharon Glotzer Active matter is inherently out-of-equilibrium, and therefore exhibits phenomenon not observed at thermodynamic equilibrium. In general, active particles convert energy from the environment into directed motion. For some biological and synthetic systems, this energy conversion is triggered by a particle-particle contact event. For example: neural crest cells chase placodal cells by chemotaxis; chemically different droplets in contact can generate asymmetrical flows due to a surface tension gradient that induces propulsion; and electrohydrodynamic flow fields around dissimilar colloids can generate propulsive forces on dimers. To understand such systems we extend the standard models used to study active matter so that activity is now triggered on contact. We numerically implement and study this model on a binary mixture of particles. We show that this system phase separates into dense and dilute regions similar to self-propelled particles. However, there exists 4-fold and 6-fold ordering within the clusters. We further resolve the dense phase into inter-penetrating lattice structures of the two particle types. The understanding of these phenomena can be employed to synthesize novel materials with reconfigurable lattice structures. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S14.00009: Curvature-induced microswarming and clustering of self-propelled particles Isaac Bruss, Sharon Glotzer Non-equilibrium active matter systems exhibit many unique phenomena, such as motility-induced phase separation and swarming. However, little is known about how these behaviors depend on the geometry of the environment. To answer this question, we use Brownian dynamics simulations to study the effects of Gaussian curvature on self-propelled particles by confining them to the surface of a sphere. We find that a modest amount of curvature promotes phase separation by altering the shape of a cluster's boundary. Alternatively, particles on surfaces of high curvature experience reduced phase separation and instead form microswarms, where particles share a common orbit. We show that this novel flocking behavior is distinct from other previously studied examples, in that it is not explicitly incorporated into our model through Vicsek-like alignment rules nor torques. Rather, we find that microswarms emerge solely due to the geometric link between orientation and velocity, a property exclusive to surfaces with non-zero Gaussian curvature. These findings reveal the important role of local environment on the global emergent behavior of non-equilibrium systems. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S14.00010: The Bumper Boats Effect: Effect of Inertia on Self Propelled Active Particles Systems Chengyu Dai, Isaac Bruss, Sharon Glotzer Active matter has been well studied using the standard Brownian dynamics model, which assumes that the self-propelled particles have no inertia. However, many examples of active systems, such as sub-millimeter bacteria and colloids, have non-negligible inertia. Using particle-based Langevin Dynamics simulation with HOOMD-blue, we study the role of particle inertia on the collective emergent behavior of self-propelled particles. We find that inertia hinders motility-induced phase separation. This is because the effective speed of particles is reduced due to particle-particle collisions—much like bumper boats, which take time to reach terminal velocity after a crash. We are able to fully account for this effect by tracking a particle's average rather than terminal velocity, allowing us to extend the standard Brownian dynamics model to account for the effects of momentum. This study aims to inform experimental systems where the inertia of the active particles is non-negligible. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S14.00011: Measuring the osmotic pressure of active colloids Michael Wang, Vishal Soni, Sofia Magkiriadou, Melissa Ferrari, Mina Youssef, Michelle Driscoll, Stefano Sacanna, Paul Chaikin, William Irvine We study the behavior of a system of colloidal spinners, consisting of weakly magnetic colloids driven by a rotating magnetic field. First the particles are allowed to sediment to an equilibrium density profile in a gravitational field, from which we measure the equilibrium equation of state. By spinning the particles at various frequencies, we introduce activity into the system through the hydrodynamic interactions between particles. We observe that the activity expands the sedimentation profile to a new steady state, from which we measure the pressure as a function of the density and activity. We compare the effects of activity on the pressure and mean-squared displacement of spinners and tracer particles. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S14.00012: Light driven assembly of active colloids Antoine Aubret, Youssef Mena, Sophie Ramananarivo, Stefano Sacanna, Jeremie Palacci Self-propelled particles (SPP) are a key tool since they are of relative simplicity as compared to biological micro-entities and provide a higher level of control. They can convert an energy source into motion and work, and exhibit surprising non-equilibrium behavior. In our work, we focus on the manipulation of colloids using light. We exploit osmotic and phoretic effects to act on single and ensemble of colloids. The key mechanism relies on the photocatalytic decomposition of hydrogen peroxide using hematite, which triggers the motion of colloids around it when illuminated. We use hematite particles and particles with photocatalytic inclusions (i.e. SPP). We first show that the interactions between hematite and colloidal tracers can be tuned by adjusting the chemical environment. Furthermore, we report a phototaxic behavior (migration in light gradient) of the particles. From this, we explore the effect of spatio-temporal modulation of the light to control the motion of colloids at the single particle level, and to generate self-assembled colloidal structures through time and space. The so-formed structures are maintained by phoretic and hydrodynamic forces resulting from the motion of each particles. Ultimately, a dynamic light modulation may be a route for the creation of act [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S14.00013: Manipulating colloidal assemblies with active dopants Sophie Ramananarivo, Antoine Aubret, Jeremie Palacci The dynamics of a densely packed 2D layer of colloids can be significantly altered upon introducing a small amount of active microparticles. Those motile intruders drive the system out-of-equilibrium, which produces a variety of new complex phenomena such as the accentuation of density heterogeneities or the reorganization of crystalline colloidal structures. We investigate the altered dynamics of the passive spheres, as well as the behavior of micro-swimmers propelling in such crowded environment where interactions with passive obstacles or other active units become important. Ultimately, understanding and controlling such mixed systems could open new routes toward activity-assisted manipulation of colloids, potentially guiding the design of materials able to self-anneal their defects. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S14.00014: From microscopic rules to macroscopic dynamics with active colloidal snakes. Jie Zhang, Jing Yan, Steve Granick Seeking to learn about self-assembly far from equilibrium, these imaging experiments inspect self-propelled colloidal particles whose heads and tails attract other particles reversibly as they swim. We observe processes akin to polymerization (short times) and chain scission and recombination (long times). The steady-state of dilute systems consists of discrete rings rotating in place with largely quenched dynamics, but when concentration is high, the system dynamics share features with turbulence. The dynamical rules of this model system appear to be scale-independent and hence potentially relevant more generally. [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S14.00015: Dynamics of Active Spinners Somayeh Farhadi, Sergio Machaca, Justin Aird, Paulo Arratia, Doug Durian We have performed experiments on a system of active spinning particles. The particles are placed on a mesh, and an upward flow of air drives them to spin. The air flow induces spin due to the internal design of particles, which consists of turbine-like blades. We then study the collective dynamics of this model system as the packing fraction is varied. Our measurements show that the velocity distribution function (VDF) deviates from Gaussian behavior, which is typical for dissipative systems. For $0.28<\phi<0.34$, a robust exponential distribution is observed. As the packing fraction is increased beyond $\phi>0.34$, the VDF starts to deviate from exponential, and approaches Gaussian distribution. We then use a modified Langevin equation to explain this transition. We also study the “spin segregation” of particles. Our observation indicates that the mixture of particles with opposite spin directions, immediately form clusters which are relatively stable over time. [Preview Abstract] |
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