Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R14: Active Matter and Self-propelled Particles |
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Sponsoring Units: GSNP Chair: Aparna Baskaran, Brandeis University Room: 273 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R14.00001: Computational model for living nematic Mikhail Genkin, Andrey Sokolov, Oleg Lavrentovich, Igor Aranson A realization of an active system has been conceived by combining swimming bacteria and a lyotropic nematic liquid crystal. Here, by coupling the well-established and validated model of nematic liquid crystals with the bacterial dynamics we developed a computational model describing intricate properties of such a living nematic. In faithful agreement with the experiment, the model reproduces the onset of periodic undulation of the nematic director and consequent proliferation of topological defects with the increase in bacterial concentration. It yields testable prediction on the accumulation and transport of bacteria in the cores of $+$1/2 topological defects and depletion of bacteria in the cores of $-$1/2 defects. Our new experiment on motile bacteria suspended in a free-standing liquid crystalline film fully confirmed this prediction. This effect can be used to capture and manipulation of small amounts of bacteria. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R14.00002: Nematic Order Without Excluded Volume Interactions In a Confined Active Fluid. Daniel Goldstein, Bulbul Chakraborty Recent experiments in active filament networks reveal interesting rheological properties. This system consumes ATP to produce an extensile motion in bundles of microtubules. This extension then leads to self generated stresses and spontaneous flows. We propose a minimal model where the activity is captured by self-extending force dipoles that are part of a cross linked network. This network can reorganize itself through buckling of extending filaments and cross linking events that alter the topology of the network. The competition of emergent time scales and interaction with the boundary leads to nematic order without excluded volume interactions. A dynamic length scale emerges on which the motion of points in the active fluid are correlated. The competition between this length scale and the confinement allows for interesting patterns of nematic order to emerge. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R14.00003: Controlling Active Liquid Crystal Droplets with Temperature and Surfactant Concentration Jake Shechter, Peker Milas, Jennifer Ross Active matter is the study of driven many-body systems that span length scales from flocking birds to molecular motors. A previously described self-propelled particle system was made from liquid crystal (LC) droplets in water with high surfactant concentration to move particles via asymmetric surface instabilities. Using a similar system, we investigate the driving activity as a function of SDS surfactant concentration and temperature. We then use an optical tweezer to trap and locally heat the droplets to cause hydrodynamic flow and coupling between multiple droplets. This system will be the basis for a triggerable assembly system to build and couple LC droplets. [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 8:48AM |
R14.00004: Fluid dynamics in biological active nematics Amanda Tan, Linda Hirst We use biological materials to form a self-mixing active system that consists of microtubules driven by kinesin clusters. Microtubules are rigid biopolymers that are a part of the cytoskeleton. Kinesin motors are molecular motors that walk along microtubules to transport cellular cargo. In this system, microtubules are bundled together, and as the kinesin clusters walk along the filaments, the microtubule bundles move relative to each other. As microtubules shear against each other, they extend, bend, buckle and fracture. When confined in a 2D water-oil interface, the system becomes an active nematic that self-mixes due to the buckling and fracturing. To quantify this self-mixing, we attached beads to the microtubules, and tracked their motion. We quantify the quality of mixing using the bead trajectories. This new active material has potential applications as a self-mixing solvent. [Preview Abstract] |
Thursday, March 16, 2017 8:48AM - 9:00AM |
R14.00005: Investigating the effect of hydrodynamic and topological constraints on a confined active nematic material Michael Norton, Arvind Baskaran, Achini Opathalage, Aparna Baskaran, Michael Hagan, Seth Fraden Understanding the role of boundary conditions on non-equilibrium materials is key to creating systems with designed behaviors. In this work we numerically investigate the behavior of a 2D active nematic confined to a circular container. The evolution of the nematic order tensor is governed by Landau-deGennes free energy descent with convection and flow-alignment; hydrodynamics are driven by the active, extensile stress and balanced by viscous dissipation. Boundaries plays a dual role, enforcing both no-slip {\&} impermeable conditions, and setting the total topological charge of the system (for parallel anchoring the net charge is $+$1). We examine the dynamics of $+$/- 1/2 defects, which spontaneously form, as a function of nematogen density and activity. We identify an activity/density threshold below which the system coarsens into two co-rotating $+$1/2 defects; above this threshold, defects proliferate similar to a bulk nematic. We observe interplay between hydrodynamics and topology by introducing patches of perpendicular (rather than parallel) anchoring. While a net charge of $+$1 is maintained under strong activity, below a threshold activity the system transitions to a charge of $+$3/2. [Preview Abstract] |
Thursday, March 16, 2017 9:00AM - 9:12AM |
R14.00006: Defect dynamics of the 2D confined active nematic liquid crystals. Achini Opathalage, Michael Norton, Michael Juniper, Seth Fraden, Zvonimir Dogic We study the role of boundary conditions on a simplified experimental model of biological active matter system composed of extensile filamentous bundles of microtubules driven by clusters of kinesin motors, to elucidate the structure and dynamics of active nematic liquid crystals. These bundles form a dense quasi-2D active nematic liquid crystals when sediment onto a surfactant-stabilized oil-water interface. We further confine this system onto different boundary conditions, imposing total topological charge and obstructing the natural length scales of the bundles. We observe unique dynamical behavior under high confinement in the order of hundred micrometers. The system produces two circulating $+$/- 1/2 defects, drive the material toward the edge of the circle. The circulating behavior is eventually destroyed by buckling of the nematic at the container wall which nucleates a $+$/- \textonehalf defect pair. This behavior is remarkably periodic until the energy supply of the system; ATP is drained. We also study the defect-defect and defect-boundary in the confinement of annuli for a range of inner diameters and widths. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:24AM |
R14.00007: Active Liquid Crystals with variable Elasticity Nitin Kumar, Rui Zhang, Jennifer Ross, Juan de Pablo, Margaret Gardel Actin filaments driven by myosin motors provide an ideal system to study active matter. We perform experiments on a quasi-two-dimensional sheet of actin filaments ranging from 1 - 10 $\mu$m in length. The addition of myosin II motors to a dense sheet of short filaments ($<$2 $\mu$m) results in extensile flow patterns and nematic defect propagation, characteristics of an active liquid crystal. We form liquid crystals with variable actin filament length and find that the shape of +1/2 defect changes from \textit{circular} to \textit{triangular} as the filament length is gradually increased. By comparing the experimental shapes with simulations, we show that the filament length controls the relative values of splay and bend elastic moduli in the liquid crystal. We find that another means to tune liquid crystal elasticity is through the addition of microtubules. We also show that the orientation and velocity correlation lengths in the active nematic phase scale linearly with the activity, consistent with our simulation results. Our experiments demonstrate active liquid crystals with tunable elasticities. [Preview Abstract] |
Thursday, March 16, 2017 9:24AM - 9:36AM |
R14.00008: Disorder Induced Transport Joshua Steimel, Tal Kachman, Juan Aragones, Alfredo Alexander-Katz Transport of active or driven particles plays a crucial role in a myriad of processes ranging from biological systems to quantum phenomena. Here we study the transport of active spinning particles in a confined substrate that contains fixed obstacles. Except for a handful of systems, a disordered environment in the form of impurities or obstacles in a material will inhibit transport, and under some circumstances lead to localization. Such phenomena has been directly seen in transport of light in disordered photonic crystals. This is an important question because many vital biological processes depend on the active transport of molecules inside cells and organisms, from molecular motors to cellular transport. In particular, it is vital to know whether disorder leads to the inhibition of transport and localization, or enhances transport. We demonstrate with experiments and simulations that, contrary to intuition, active spinning matter exhibits a disorder-induced delocalization transition dependent on the local order of the obstacles on the substrate. For the regimes studied, we always find anomalous super-diffusive transport that slowly approaches the diffusive regime in the limit of high activity. These results shed light on the effect of hydrodynamic boundary conditions and optimal transport processes in active matter in disordered environments. [Preview Abstract] |
Thursday, March 16, 2017 9:36AM - 9:48AM |
R14.00009: Flocking through disorder Alexandre Morin, Nicolas Desreumaux, Jean-Baptiste Caussin, Denis Bartolo How do flocks, herds and swarms proceed through disordered environments? This question is not only crucial to animal groups in the wild, but also to virtually all applications of collective robotics, and active materials composed of synthetic motile units. In stark contrast, appart from very rare exceptions, our physical understanding of flocking has been hitherto limited to homogeneous media. Here we explain how collective motion survives to geometrical disorder. To do so, we combine experiments on motile colloids cruising through random microfabricated obstacles, and analytical theory. We explain how disorder and bending elasticity compete to channel the flow of polar flocks along sparse river networks akin those found beyond plastic depinning in driven condensed matter. Further increasing disorder, we demonstrate that collective motion is suppressed in the form of a first-order phase transition generic to all polar active materials. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R14.00010: Effect of CorrelatedRotational Noise Benjamin Hancock, Caleb Wagner, Aparna Baskaran The traditional model of a self-propelled particle (SPP) is one where the body axis along which the particle travels reorients itself through rotational diffusion. If the reorientation process was driven by colored noise, instead of the standard Gaussian white noise, the resulting statistical mechanics cannot be accessed through conventional methods. In this talk we present results comparing three methods of deriving the statistical mechanics of a SPP with a reorientation process driven by colored noise. We illustrate the differences/similarities in the resulting statistical mechanics by their ability to accurately capture the particles response to external aligning fields. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R14.00011: Chemotaxis and auto-chemotaxis of self-propelling artificial droplet swimmers Chenyu Jin, Carsten Krueger, Corinna Maass Chemotaxis and auto-chemotaxis are key mechanisms in the dynamics of micro-organisms, e.g. in the acquisition of nutrients and in the communication between individuals, influencing the collective behavior. However, chemical signalling and the natural environment of biological swimmers are generally complex, making them hard to access analytically. Simple experimental systems showing similar features could provide vital insights. We present such a swimmer system, as well as controlled assays to study chemotactic effects quantitatively and reproducibly. In our experiments, we let auto-chemotactic droplet swimmers pass through bifurcating microfluidic channels and record anticorrelations between the branch choices of consecutive droplets. We present an analytical model based on balancing stochastic forces versus a diffusing chemical gradient matching the experimental data. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R14.00012: Could dividing active droplets provide a model for protocells? Rabea Seyboldt, David Zwicker, Christoph Weber, Anthony Hyman, Frank J\"ulicher Macromolecular aggregation and phase separation into droplets has been proposed as a mechanism to organize chemical reactions that could have been a key precursor at the origin of the first living cells. However, it remains unclear how early protocells could have proliferated and divided - deformed droplets usually relax towards a spherical shape and do not easily divide. Our theoretical study shows that in the presence of chemical reactions that produce and destroy droplet material, a chemically active droplet may undergo a shape instability and subsequently divide into two daughter droplets, which may then grow and divide again. We also find that when considering the effects of hydrodynamics which tend to stabilize spherical droplets, the shape instability can still occur for sufficiently small droplets. Our work suggests that chemically active droplets that divide and propagate could serve as a model for prebiotic protocells. [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 10:36AM |
R14.00013: Programmable Active Matter: Dynamics of active filaments on patterned surfaces Vikrant Yadav, Daniel Todd, Peker Milas, Paul Ruijgrok, Zev Bryant, Jennifer Ross Interfaces are ubiquitous in biology. For a sub-cellular component moving inside the cell, any change in its local environment across an interface whether chemical concentration, density, or any other physical variables can produce novel dynamics. Recent advances in bioengineering allow us to control motor proteins' velocities when prompted by an optical trigger. Using an optical diaphragm and a gear-shifting myosin XI construct containing a photoactive LOV domain, we can spatially pattern light to create interfaces across which speed of a gliding actin filament can differ by as much as a factor of two. We observe that when a gliding actin filament crosses an interface that has a discontinuous velocity jump, it buckles and changes its angle of orientation due to the velocity mismatch. Our preliminary data suggests that for small angels of incidence, the angle of emergence increases linearly. If we increase the angle of incidence further we observe that the angle of emergence saturates. For some actin filaments approaching the interface near-tangentially we observe total internal reflection as they fail to crossover the boundary. We have modeled our system using Cytosim software package and find excellent agreement with experimental data. [Preview Abstract] |
Thursday, March 16, 2017 10:36AM - 10:48AM |
R14.00014: Heterogeneous Active Matter Thomas Kolb, Daphne Klotsa Active systems are composed of self-propelled (active) particles that locally convert energy into motion and exhibit emergent collective behaviors, such as fish schooling and bird flocking. Most works so far have focused on monodisperse, one-component active systems. However, real systems are heterogeneous, and consist of several active components. We perform molecular dynamics simulations of multi-component active matter systems and report on their emergent behavior. We discuss the phase diagram of dynamic states as well as parameters where we see mixing versus segregation. [Preview Abstract] |
Thursday, March 16, 2017 10:48AM - 11:00AM |
R14.00015: Cross-stream migration of active particles William Uspal, Jaideep Katuri, Juliane Simmchen, Albert Miguel-Lopez, Samuel Sanchez For natural microswimmers, the interplay of swimming activity and external flow can promote robust directed motion, e.g. propulsion against (‘upstream rheotaxis’) or perpendicular to the direction of flow. These effects are generally attributed to their complex body shapes and flagellar beat patterns. Here, using catalytic Janus particles as a model system, we report on a strong directional response that naturally emerges for spherical active particles in a channel flow. The particles align their propulsion axis to be perpendicular to both the direction of flow and the normal vector of a nearby bounding surface. We develop a deterministic theoretical model that captures this spontaneous transverse orientational order. We show how the directional response emerges from the interplay of external shear flow and swimmer/surface interactions (e.g., hydrodynamic interactions) that originate in swimming activity. Finally, adding the effect of thermal noise, we obtain probability distributions for the swimmer orientation that show good agreement with the experimental probability distributions. Our findings show that the qualitative response of microswimmers to flow is sensitive to the detailed interaction between individual microswimmers and bounding surfaces. [Preview Abstract] |
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