Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P16: Active Matter Under Confinement I |
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Sponsoring Units: GSOFT DBIO GSNP Chair: Yaouen Fily, Brandeis University Room: 275 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P16.00001: Probing Active Nematic Films with Magnetically Manipulated Colloids David Rivas, Kui Chen, Robert Henry, Daniel Reich, Robert Leheny We study microtubule-based extensile active nematic films using rod-like and disk-shaped magnetic colloids to probe the mechanical and hydrodynamic properties of this quasi-two dimensional out-of-equilibrium system. The active nematics are driven by molecular motors that hydrolyze ATP and cause sliding motion between microtubular bundles. This motion produces a dynamic nematic director field, which continuously creates pairs of $+$1/2 and -1/2 defects. In the absence of externally applied forces or torques, we observe that the magnetic rods in contact with the films align with the local director, indicating the existence of mechanical coupling between the film and probe. By applying known magnetic torques to the rods and observing their rotation with respect to the director, we gain insight into this coupling. We also find that by rotating magnetic microdisks using magnetic fields, hydrodynamic flows are produced that compete with the films' intrinsic flow, leading to significant effects on the director field and the defect landscape. At certain rotation rates, the disks produce a vortex-like structure in the director field and cause the creation and shedding of defects from the disk boundary. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P16.00002: Macroscopic motion of living organisms from internal microscopic stresses Shahrzad Yazdi, Alfredo Alexander-Katz The motion of living systems, particularly at the scale of a cell and above, depends on the collective motion of active agents constrained within a given boundary. For example, cell locomotion arises from the internal flows of filaments and active motors. However, it is not clear what the necessary conditions are to transfer microscopic stresses stemming from activity into macroscopic motion. Here, we study a system at a larger scale in which microscopic stresses are induced using ferromagnetic beads that are ingested by a model living system: the worm \textit{C. elegans}. To explore the relationship between internal microscopic activity and macroscopic motion, we apply a rotating magnetic field to induce torques on the beads and make them spin. Under other conditions, we also explore a combination of torques and point forces. Our system shows that the interfacial chemistry of the beads and the frequency of rotation are critical for observing macroscopic motion. Our study helps to elucidate the necessary ingredients to convert microscopic motions into macroscopic displacements. Furthermore, our work also helps in understanding how to manipulate \textit{in vivo} the activity of biological organisms, which can have important implications in cell analysis, drug discovery, and locomotion control. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P16.00003: Acoustic manipulation of bacteria cells suspensions Salomé Gutiérrez-Ramos, Mauricio Hoyos, Jean Luc Aider, Carlos Ruiz An acoustic contacless manipulation gives advantages in the exploration of the complex dynamics enviroment that active matter exhibits. \\ Our works reports the control confinement and dispersion of \textit{Escherichia coli}RP437-pZA3R-YFP suspensions (M9Glu-Ca) via acoustic levitation.The manipulation of the bacteria bath in a parallel plate resonator is achieved using the acoustic radiation force and the secondary radiation force. The primary radiation force generates levitation of the bacteria cells at the nodal plane of the ultrasonic standing wave generated inside the resonator. On the other side, secondary forces leads to the consolidation of stable aggregates.\\ All the experiments were performed in the acoustic trap described, where we excite the emission plate with a continuous sinusoidal signal at a frequency in the order of MHz and a quartz slide as the reflector plate. In a typical experiment we observed that, before the input of the signal, the bacteria cells exhibit their typical run and tumble behavior and after the sound is turned on all of them displace towards the nodal plane, and instantaneously the aggregation begins in this region. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P16.00004: Heavy tailed bacterial motor switching statistics define macroscopic transport properties during upstream contamination by E. coli N. Figueroa-Morales, A. Rivera, E. Altshuler, T. Darnige, C. Douarche, R. Soto, A. Lindner, E. Cl\'ement The motility of E. Coli bacteria is described as a run and tumble process. Changes of direction correspond to a switch in the flagellar motor rotation. The run time distribution is described as an exponential decay of characteristic time close to 1s. Remarkably, it has been demonstrated that the generic response for the distribution of run times is not exponential, but a heavy tailed power law decay, which is at odds with the motility findings. We investigate the consequences of the motor statistics in the macroscopic bacterial transport. During upstream contamination processes in very confined channels, we have identified very long contamination tongues. Using a stochastic model considering bacterial dwelling times on the surfaces related to the run times, we are able to reproduce qualitatively and quantitatively the evolution of the contamination profiles when considering the power law run time distribution. However, the model fails to reproduce the qualitative dynamics when the classical exponential run and tumble distribution is considered. Moreover, we have corroborated the existence of a power law run time distribution by means of 3D Lagrangian tracking. We then argue that the macroscopic transport of bacteria is essentially determined by the motor rotation statistics. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P16.00005: Microscopic dynamics and velocity profiles of bacterial “superfluids” under oscillatory shear Xiang Cheng, Shuo Guo, Devranjan Samanta, Yi Peng, Xinliang Xu Bacterial suspensions—a premier example of active fluids—show an unusual response to shear stresses. Rather than increasing the viscosity of the suspending fluid, swimming bacteria can self-organize into collective flows under shear, turning the suspension into a ``superfluid'' with zero apparent viscosity. Although the existence of the bacterial superfluid has been demonstrated in bulk rheology measurements, little is known about the microscopic dynamics of such an exotic phase. Here, by combining sensitive rheology measurements with high-speed confocal microscopy, we study the detailed 3D dynamics of concentrated bacterial suspensions confined in narrow gaps under oscillatory shear. We find that sheared bacterial suspensions in the superfluidic phase exhibit velocity profiles with strong spatial heterogeneity, unexpected from the established hydrodynamic theory of active fluids. We quantitatively explain the observed velocity profiles by considering a balance of active stresses and shear stresses in an ensemble average. Our experiments reveal a profound influence of shear flows on bacterial locomotion and provide new insights to the origin of the unique flow behaviors of active fluids. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P16.00006: A minimal physical model for crawling cells Adriano Tiribocchi, Elsen Tjhung, Davide Marenduzzo, Michael E. Cates Cell motility in higher organisms (eukaryotes) is fundamental to biological functions such as wound healing or immune response, and is also implicated in diseases such as cancer. For cells crawling on solid surfaces, considerable insights into motility have been gained from experiments replicating such motion in vitro. Such experiments show that crawling uses a combination of actin treadmilling (polymerization), which pushes the front of a cell forward, and myosin-induced stress (contractility), which retracts the rear. We present a simplified physical model of a crawling cell, consisting of a droplet of active polar fluid with contractility throughout, but treadmilling connected to a thin layer near the supporting wall. The model shows a variety of shapes and/or motility regimes, some closely resembling cases seen experimentally. Our work supports the view that cellular motility exploits autonomous physical mechanisms whose operation does not need continuous regulatory effort. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P16.00007: Classifying and Analyzing 3d Cell Motion in Jammed Microgels Tapomoy Bhattacharjee, W. Gregory Sawyer, Thomas Angelini Soft granular polyelectrolyte microgels swell in liquid cell growth media to form a continuous elastic solid that can easily transition between solid to fluid state under a low shear stress. Such Liquid-like solids (LLS) have recently been used to create 3D cellular constructs as well as to support, culture and harvest cells in 3D. Current understanding of cell migration mechanics in 3D was established from experiments performed in natural and synthetic polymer networks. Spatial variation in network structure and the transience of degradable gels limit their usefulness in quantitative cell mechanics studies. By contrast, LLS growth media approximates a homogeneous continuum, enabling tractable cell mechanics measurements to be performed in 3D. Here, we introduce a process to understand and classify cytotoxic T cell motion in 3D by studying cellular motility in LLS media. General classification of T cell motion can be achieved with a very traditional statistical approach: the cell's mean squared displacement (MSD) as a function of delay time. We will also use Langevin approaches combined with the constitutive equations of the LLS medium to predict the statistics of T cell motion. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P16.00008: Hydrogel Crawlers in confined channels Franck Vernerey, Tong Shen Locomotion in confined spaces is common in nature: organisms such as cells and maggots often migrate through porous spaces by establishing contact and frictional forces to propel themselves forward. The development of synthetic particles that share these features is highly desirable in the context of chemical robots and drug delivery systems. In this presentation, we explore the migration of temperature-sensitive hydrogel particles that can crawl in narrow channels via a periodic oscillation of their body and the anisotropic frictional properties of the channels. Experimental measurements show that the particle motion is sensitive to both the presence asymmetric ratchet-like patterns on the channel and the particle confinement. These observations are supported by a model that identifies the underlying propulsion mechanisms and predicts the dependency of the particle velocity on its size, aspect ratio, and frictional properties of the substrate. Our results particularly suggest that particle velocity relies on a competition between the kinetics of particle-substrate friction and size-dependent swelling dynamics. We show that sub-micron sized particles are faster regardless of size while the speed of larger particles decreases with their size and stiffness. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P16.00009: Flexible active filaments confined to curved surfaces or in flexible encapsulation: simulation studies Michael Varga, Luca Giomi, Robin Selinger We model self-propelled flexible active filaments (FAFs) in complex geometries. First, we examine FAFs on a curved substrate with alternating regions of $+$/- Gaussian curvature. We study motility-induced phenomena including giant number fluctuations, anomalous diffusion, collective motion, and dynamic topology, and identify how collective behaviors are modified by variations in surface curvature. We compare these results to related models of active matter on curved substrates. Next, we consider FAFs encapsulated in a deformable ring and identify mechanisms of spontaneous symmetry breaking and pattern formation as a function of filament properties and encapsulation stiffness. Finally, we discuss how these mechanisms can be explored via potentially relevant experiments. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P16.00010: Scale-invariant transition from turbulent to coherent flows in 3D confined active fluids. Kun-Ta Wu, Jean Bernard Hishamunda, Daniel T.N. Chen, Stephen J. DeCamp, Ya-Wen Chang, Alberto Fernandez-Nieves, Seth Fraden, Zvonimir Dogic Far-from-equilibrium, kinesin-driven active microtubules (MT) consume ATP and form extensile bundles. The bundles provide active stress, driving background fluids. Here we found that confining these fluids in a toroid triggers a transition from turbulent to coherent flow. The criterion for the transition is the aspect ratio of the toroid's channel cross-section, disregarding its absolute size. The underlying mechanism for such scale-invariant transition remains unclear. To gain insight, we measured the profiles of fluid flows as well as MT nematic order parameters when in coherent and incoherent states. We found that such flow transition is accompanied with a formation of MT nematic layer wetting the boundaries, while MT structure remains disordered in the bulk, indicating that such a coherent flow is a surface-driven phenomenon. In particular we found that the layer thickness is increased with the local shear rate of background fluid flow, reinforcing the connection between the surface layer and fluid motion. Our finding paves the path to outlining principles of interaction between active particles and fluid flows as well as the coherent transition caused by their collective dynamics. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P16.00011: Reconfigurable mechanical properties of fire ant aggregations Michael Tennenbaum, Alberto Fernandez-Nieves Fire ant aggregations are inherently active materials. Each ant converts its own chemical energy into motion, and it is the overall motion of all individual ants that contributes to the bulk material properties of the aggregation. So far we are unable to affect the activity level of the ants themselves. However, the ants go through cycles of activity which we can monitor by measuring the normal force exerted by the aggregation on the plate of a rheometer. We can then examine the properties of the aggregation as it evolves through these cycles and learn how the activity level affects the overall aggregation properties. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P16.00012: Simultaneous 3D tracking of passive tracers and microtubule bundles in an active gel Yi Fan, Kenneth S. Breuer Kinesin-driven microtubule bundles generate a spontaneous flow in unconfined geometries. They exhibit properties of active matter, including the emergence of collective motion, reduction of apparent viscosity and consumption of local energy. Here we present results from 3D tracking of passive tracers (using Airy rings and 3D scanning) synchronized with 3D measurement of the microtubule bundles motion. This technique is applied to measure viscosity variation and collective flow in a confined geometry with particular attention paid to the self-pumping system recently reported by Wu et al. (2016). Results show that the viscosity in an equilibrium microtubule network is around half that of the isotropic unbundled microtubule solution. Cross-correlations of the active microtubule network and passive tracers define a neighborhood around microtubule bundles in which passive tracers are effectively transported. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P16.00013: Mechanics of Active Microtubule Gels: Can confinement determine elasticity? Claudia Dessi, Daniel Blair, Daniel Chen, Zvonimir Dogic A rheological characterization of the viscoelastic properties of active microtubule based biopolymer gels is presented. Passive in-vitro biopolymer networks have been intensively characterized using bulk- and micro-rheology. However, active networks remain largely unexplored. Using stabilized microtubules, kinesin motor proteins, and adenosine triphosphate (ATP), we explored the dynamic viscoelastic transition from active to passive states of a unique class of biologically derived extensile active materials. By means of our coupled confocal-microscopy rheometer platform (con-rheo) we directly determine the bulk network response while simultaneously quantifying the microscopic dynamics based on the activity magnitude as driven-force. Our preliminary results indicate that these materials may be simply viscous in the active state despite the existence of long-lived spanning filaments. However, we observe a clear transition to elastic behavior that occurs as the magnitude of the activity is gradually reduced in time. We will discuss how the magnitude and the nature of active-to-passive dynamics transition depends on the geometry confinement. This is related to the observed different structural arrangement due to different fluid dynamics regime. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P16.00014: Brownian self-propelled particles on a sphere Leonardo Felix Apaza-Pilco, Mario Sandoval We present the dynamics of a Brownian self-propelled particle at low Reynolds number moving on the surface of a sphere. The effects of curvature and self-propulsion on the diffusion of the particle are elucidated by determining (numerically) the mean-square displacement of the particle's angular (azimuthal and polar) coordinates. The results show that the long time behavior of its angular mean-square displacement is linear in time. We also see that the slope of the angular MSD is proportional to the propulsion velocity and inverse to the curvature of the sphere. The angular probability distribution function (PDF) of the particle is also obtained by numerically solving its respective Smoluchowski equation. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P16.00015: Behavior of self-propelled acetone droplets in a Leidenfrost state on liquid substrates Stoffel Janssens, Eliot Fried It is demonstrated that non-coalescent droplets of acetone can be formed on liquid substrates. The fluid flows around and in an acetone droplet hovering on water are recorded to shed light on the mechanisms which might underlie non-coalescence. For sufficiently low impact velocities, droplets undergo a damped oscillation on the surface of the liquid substrate but at higher velocities clean bounce-off occurs. Comparisons of experimentally observed static configurations of floating droplets to predictions from a theoretical model for a small non-wetting rigid sphere resting on a liquid substrate are made and a tentative strategy for determining the thickness of the vapor layer under a small droplet on a liquid is proposed. That strategy is based on the notion of effective surface tension. The droplets show self-propulsion in straight line trajectories in a manner which can be ascribed to a Marangoni effect. Surprisingly, self-propelled droplets can become immersed beneath the undisturbed water surface. This phenomenon is reasoned to be drag-inducing and might provide a basis for refining observations in previous work. [Preview Abstract] |
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