Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y48: Active Matter II |
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Sponsoring Units: GSNP DFD Chair: Igor Aronson, Biomedical Engineering, Pennsylvania State University Room: LACC 510 |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y48.00001: Motion reversal in Living Liquid Crystals Nuris Figueroa Morales, Igor Aronson
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Friday, March 9, 2018 11:27AM - 11:39AM |
Y48.00002: Detection of Bacterial Movements in a Picoliter-sized Droplet Using a Microcantilever Mustafa Karakan, Ahmet Kirlioglu, Kamil Ekinci Atomic Force Microscope (AFM) cantilevers have recently emerged as a promising tool for detecting nanomechanical movements of bacteria. In this approach, bacteria are adhered onto the surface of a microcantilever, and their metabolic activity results in excess low-frequency fluctuations of the microcantilever. Our work here is inspired by dip-pen nanolithography: we load liquid media onto a microcantilever from a bacteria (E. coli) solution and detect the excess microcantilever fluctuations due to the captured bacteria. We measure both the resonance frequency and the low-frequency fluctuations of the microcantilever before and after loading the liquid. Resonance frequency shifts allow us to estimate the mass and volume of the loaded droplet. Typical droplet sizes of 1-40 pL contain approximately 100 bacteria, which generate an observable increase in the microcantilever fluctuations. Our approach circumvents some of the disadvantages encountered in AFM measurements in liquids due to viscous damping and may allow for increased detection sensitivity. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y48.00003: Optical Diffusometry of Bacteria-laden Single Microbeads Ahmet Kirlioglu, Vural Kara, Kamil Ekinci Microcantilever transducers have been used to show that metabolic activity of bacteria can be correlated with their random nanomechanical fluctuations. Here, we demonstrate an alternative approach for measuring these bacterial fluctuations using an optical technique. In the experiments, we adhere various bacteria onto the surface of 3-8 μm microbeads via electrostatic forces. After bacteria adhesion, we image the Brownian motion of single microbeads suspended in liquid by high-resolution optical microscopy. Mean square displacements and diffusion constants are calculated from trajectories of the microbeads. We observe that, when a few motile bacteria such as E. coli are adhered to a bead, its diffusion constant increases significantly. Furthermore, the dynamics of the bead changes from diffusive to ballistic, similar to that of a Janus particle. Conversely, a microbead coated with non-motile bacteria such as S. epidermidis shows diffusive motion but with a subtle increase in the diffusion constant compared to that of a bead with no bacteria. Our approach allows for the measurement of the metabolic activity of various bacteria and can possibly be developed into a rapid antibiotic susceptibility test. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y48.00004: Collective stress tolerance during bacterial swarming Wenlong Zuo, Yilin Wu Bacterial populations such as biofilms and swarming colonies often survive under antibiotic treatment at concentrations lethal to individual cells. Here we used the model bacterium E. coli to investigate how motility affects the collective tolerance. We quantified the single cell motion pattern in swarms under stress and studied the relation between single cell motility and colony expansion. Our results suggest a general means of stress-response for bacterial swarms. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y48.00005: Focusing of Active Rods in a Converging Flow Andreas Kaiser, Mykhailo Potomkin, Leonid Berlyand, Igor Aronson
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Friday, March 9, 2018 12:15PM - 12:27PM |
Y48.00006: Hydrodynamic stability of magnetic swimmers in an external field Fabian Koessel, Sara Jabbari-Farouji Inspired by the dynamical behavior of magnetotactic bacteria, we present a minimal kinetic model for dilute suspensions of magnetic, self-propelled particles. Our kinetic theory couples a Fokker-Planck equation for active particles in an external magnetic field to the Stokes flow equation. Combining linear stability analysis and nonlinear 3D continuum simulations, we characterize the conditions under which instabilities occur. For sufficiently strong self-propulsion and magnetic field strengths, instabilities in density appear that make an orientationally aligned (polar) phase unstable. These hydrodynamic instabilities persist even for strong magnetic fields. The interplay between the hydrodynamic interactions and the coupling to an external magnetic field leads to emergent spatio-temporal patterns that depend on the type of swimmers. Examining the dynamics of pattern formation from initially homogeneous suspensions, we observe distinct types of instabilities for pusher and puller type swimmers. Pushers form wave-like structures perpendicular to the field that travel in its direction while pullers form wave-like lanes along the field. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y48.00007: Brownian self-driven particles on Riemannian surfaces Leonardo Apaza Pilco, Mario Sandoval-Espinoza We present the dynamics of overdamped Brownian self-propelled particles (swimmers) moving on a general Riemannian surface. In particular, we offer analytical results (mean-square displacement and variance) to characterize the effect of self-propulsion and geometry on the diffusion of swimmers moving on prolate and oblate spheroids. Our results are compared with Brownian dynamics simulations and an excellent agreement is obtained. Marginal probability density functions are also obtained. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y48.00008: The liquid solid transition of monodisperse active particles Daniel Vagberg, Ludovic Berthier We simulate dense systems of monodisperse self propelled particles in two dimensions. We explore how the character of the liquid solid transition changes as the system is gradually tuned from an equilibrium Brownian system to an active system with increasingly persistent particle motion. We are especially interested in how activity affects the existence and extent of the hexatic phase. We find that activity shifts the transition towards higher packing fractions and widens the interval where hexatic order is observed. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y48.00009: Forces in Inhomogeneous Open Active-Particle Systems Nitzan Razin, Raphael Voituriez, Jens Elgeti, Nir Gov We study the force that non-interacting point-like active particles apply to a symmetric inert object in the presence of a gradient of activity and particle sources and sinks. We consider two simple patterns of sources and sinks that are common in biological systems. We analytically solve a one dimensional model designed to emulate higher dimensional systems, and study a two dimensional model by numerical simulations. We specify when the particle flux due to the creation and annihilation of particles can act to smooth the density profile that is induced by a gradient in the velocity of the active particles, and find the net resultant force due to both the gradient in activity and the particle flux. These results are compared qualitatively to observations of nuclear motion inside the oocyte, that is driven by a gradient in activity of actin-coated vesicles. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y48.00010: Spin Lattices of Macroscopic Wave-driven Particles Pedro Saenz, Giuseppe Pucci, Alexis Goujon, Jorn Dunkel, John Bush We present experiments that demonstrate the spontaneous emergence of collective behavior in spin lattices of droplets walking on a vibrating fluid surface. Circular wells at the bottom of the fluid bath encourage individual droplets to walk in clockwise or counter-clockwise direction along circular trajectories centered at the lattice sites. A thin fluid layer between the wells enables wave-mediated interactions between neighboring walkers resulting in coherent rotation dynamics across the lattice. When the pair-coupling is sufficiently strong, interactions between neighboring droplets may induce local spin flips leading to ferromagnetic or anti-ferromagnetic order. Our results for different 1D and 2D lattice symmetries illustrate the effects of topological frustration. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y48.00011: Strength of Zero-stiffness Microactuators Qi Wang, Johannes Goosen, Hassan HosseinNia, Fred van Keulen Many popular microactuators, including piezoelectric actuators, suffer from incomplete conversion of input energy into mechanical output work. The culprit is their inherent stiffness. Energy used in the elastic deformation limits the output displacement and force and thus the work. At full stroke, all force is used to deform the material itself and all energy is stored as internal elastic energy. Therefore, microactuators with zero internal stiffness (in the direction of actuation) are desired for high energy conversion efficiency. These actuators use either field actuation (electrostatic, magnetic) or zero stiffness statically-balancing structures. Only when the system requires energy storage, e.g. in dynamic systems, should actuators with internal stiffness be considered. These microactuators must be integrated with proper force transfer structures to achieve optimal actuation configurations for different actuation purposes. Distributing these actuators in series and in parallel allows us to achieve the desired large displacement and force outputs resulting in distributed actuator structures with optimal energy performance. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y48.00012: Supersmarticle: a locomoting robot made of robots Shengkai Li, William Savoie, Ross Warkentin, Daniel Goldman To discover principles by which simple robots can be integrated to form more complex robots, we developed a stochastic locomotor composed of simple non-motile robots. These smarticles (14 cm long) are 3D printed, three-link 2-degree-of-freedom robots with simple sensory capabilities (sound/light). The outer link positions are controlled by servo motors and can perform gaits (periodic closed trajectories in 2D configuration space) or hold a shape. Each smarticle is incapable of individually displacing or rotating. However, when confined in an unanchored 20 cm plastic ring, the smarticle ensemble (which we refer to as a supersmarticle), can displace through collisions among active smarticles and the ring. When all smarticles are identical in mass and movement procedure, the system exhibits a diffusive motion due to the collisions between the smarticles and the ring. When one of them is deactivated (via light/sound cues) and maintained a straight shape, the system diffuses and drifts towards or away from the direction of the inactive particle. A 1D model treating the active smarticles as a single fluctuating point mass captures these behaviors. The model reveals how the magnitude and direction of the drift depend on the mass ratio of the confining ring and the inactive particle. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y48.00013: Stochastic control in microscopic nonequilibrium systems Steven Large, Raphael Chetrite, David Sivak From the manipulation of microscopic systems to the autonomous operation of molecular machines, quantifying nanoscale energy flows promises important insights in disciplines ranging from molecular biology to nanotechnology. A general understanding of the energetic costs of microscopic nonequilibrium processes would, for example, illuminate the design principles governing efficient biomolecular machines, and hence biological energy transduction. In recent years, considerable effort has gone into identifying deterministic control protocols that drive a system rapidly between states at minimum energetic cost. But for autonomous soft-matter systems, driving processes are themselves stochastic. Here we generalize a linear-response framework to incorporate such protocol variability, deriving a lower bound on the dissipation that is realized at finite protocol time. We illustrate our findings in several model systems. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y48.00014: Dynamics of Active Nematics on a Toroidal Surface Daniel Pearce, Perry Ellis, Alberto Fernandez-Nieves, Luca Giomi Two dimensional nematic textures can contain topological defects, singular regions where the orientational order is not defined. On a curved manifold, these defects interact energetically with the underlying local Gaussian curvature. Active nematics contain additional force generating components that generate an active stress; this stress generates a flow that causes +1/2 defects to self propel due to a lack of orientational symmetry. When the active stress is sufficiently large this leads to a turbulent regime, in which there are many defects moving chaotically. The local topological charge density of the defects is shown to be linear in the local Gaussian curvature of the underlying manifold. This can be observed experimentally using dense microtubule, kinesin suspensions on the surface of a toroidal water droplet. I will present here a theory describing the motion of the topological defects, and the hydrodynamics of the solvent and compare it directly with experimental observations. |
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