Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D23: Biofluids: Active Fluids II |
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Chair: Francesco Carrara, MIT Room: 300 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D23.00001: A turbulence-induced switch in phytoplankton swimming behavior Francesco Carrara, Anupam Sengupta, Roman Stocker Phytoplankton, unicellular photosynthetic organisms that form the basis of life in aquatic environments, are frequently exposed to turbulence, which has long been known to affect phytoplankton fitness and species succession. Yet, mechanisms by which phytoplankton may adapt to turbulence have remained unknown. Here we present a striking behavioral response of a motile species -- the red-tide-producing raphidophyte Heterosigma akashiwo -- to hydrodynamic cues mimicking those experienced in ocean turbulence. In the absence of turbulence, H. akashiwo exhibits preferential upwards swimming (`negative gravitaxis'), observable as a strong accumulation of cells at the top of an experimental container. When cells were exposed to overturning in an automated chamber -- representing a minimum experimental model of rotation by Kolmogorov-scale turbulent eddies -- the population robustly split in two nearly equi-abundant subpopulations, one swimming upward and one swimming downward. Microscopic observations at the single-cell level showed that the behavioral switch was accompanied by a rapid morphological change. A mechanistic model that takes into account cell shape confirms that modulation of morphology can alter the hydrodynamic stress distribution over the cell body, which, in turn, triggers the observed switch in phytoplankton migration direction. This active response to fluid flow, whereby microscale morphological changes influence ocean-scale migration dynamics, could be part of a bet-hedging strategy to maximize the chances of at least a fraction of the population evading high-turbulence microzones. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D23.00002: Non-classical size-dependent particle diffusion in active fluids Arvind Gopinath, Alison Patteson, Paulo Arratia We experimentally investigate the effect of particle size on the motion of passive polystyrene spheres in suspensions of Escherchia coli, a flagellated bacterium that is approximately 2 microns long and swims using a sequence of runs punctuated by tumbles. Using particles covering a range of sizes from 0.6 to 39 microns, we probe particle dynamics at both short and long time scales. In all cases, the particles exhibit super-diffusive ballistic behavior at short times before eventually transitioning to diffusive behavior. Surprisingly, the long-time hydrodynamic effective diffusivity is non-monotonic with particle size; an anomalous response that is fundamentally different from classical thermal diffusion. Consistent with recent theory, we find that for fixed bacterial type, the active contribution to particle diffusion can be predicted by a single dimensionless parameter, the Peclét number. Combining our experimental results, we propose a minimal model that allows us to predict the requirements for a peak in the diffusivity as well as the location and magnitude of the peak as a function of particle size and bacterial concentration. Our results have broad implications on characterizing active fluids using concepts drawn from classical (passive) thermodynamics. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D23.00003: Spontaneous ordering and vortex states of active fluids in circular confinement Maxime theillard, Barath Ezhilan, David Saintillan Recent experimental, theoretical and simulation studies have shown that confinement can profoundly affect self-organization in active suspensions leading to striking features such as directed fluid pumping in planar confinement, formation of steady and spontaneous vortices in radial confinement. Motivated by this, we study the dynamics in a suspension of biologically active particles confined in spherical geometries using a mean-field kinetic theory for which we developed a novel numerical solver. In the case of circular confinement, we conduct a systematic exploration of the entire parameter space and distinguish 3 broad states: no-flow, stable vortex and chaotic and several interesting sub-states. Our efficient numerical framework is also employed to study 3D effects and dynamics in more complex geometries. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D23.00004: Subsonic to supersonic transition in density shocks of confined microswimmers Alan Cheng Hou Tsang, Eva Kanso Motile and driven particles confined in microfluidic channels exhibit interesting emergent behavior from propagating density bands to density shock waves. A deeper understanding of the physical mechanisms responsible for these emergent structures is relevant to a number of physical and biomedical applications. Here, we show in the context of an idealized model that a plug of microswimmers confined in a narrow channel and subject to a uniform external flow exhibit a transition of density shock waves from subsonic to supersonic regime depending on the intensity of the external flow. In the subsonic regime, density shock is formed at the back of the swimmers, whereas in the supersonic regime, density shock is formed at the front of the swimmers. This behavior results from a non-trivial interplay between hydrodynamic interactions and geometric confinement. We apply these findings to guide the development of novel mechanisms for controlling the emergent density distribution and average population speed, thus enabling processes such as sorting of cells in flow channels. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D23.00005: The Force on a Boundary in Active Matter John Brady, Wen Yan We present a general theory for determining the force (and torque) exerted on a boundary (or body) in active matter. The theory extends the description of passive Brownian colloids to self-propelled active particles and applies for all ratios of the thermal energy $k_BT$ to the swimmer's activity $k_sT_s = \zeta U_0^2\tau_R/6$, where $\zeta$ is the Stokes drag coefficient, $U_0$ is the swim speed and $\tau_R$ is the reorientation time of the active particles. The theory has a natural microscopic length scale over which concentration and orientation distributions are confined near boundaries, but the microscopic length does not appear in the force. The swim pressure emerges naturally and dominates the behavior when the boundary size is large compared to the swimmer's run length $\ell = U_0 \tau_R$. The theory is used to predict the motion of bodies of all sizes immersed in active matter. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D23.00006: On the distribution and swim pressure of run-and-tumble particles in confinement Roberto Alonso Matilla, Barath Ezhilan, David Saintillan The spatial and orientational distribution in a dilute active suspension of non-Brownian run-and-tumble spherical swimmers confined between two planar hard walls is calculated theoretically. Using a kinetic model based on coupled bulk/surface probability density functions, we demonstrate the existence of a concentration wall boundary layer with thickness scaling with the run length, the absence of polarization throughout the channel, and the presence of sharp discontinuities in the bulk orientation distribution in the neighborhood of orientations parallel to the wall in the near-wall region. Our model is also applied to calculate the swim pressure in the system, which approaches the previously proposed ideal-gas behavior in wide channels but is found to decrease in narrow channels as a result of confinement. Monte-Carlo simulations are also performed for validation and show excellent quantitative agreement with our theoretical predictions. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D23.00007: Inertia changes the stability of synchronized states in hydrodynamically coupled oscillators Shanshan Jiang, Lisa Fauci We examine the hydrodynamic interaction of two oscillators in a 2D fluid driven by a geometric switch. Motivated by the work of Kotar et al (PNAS, 107:17, 2010), the colloidal oscillators are modeled by circular membranes that support tensile forces on their boundary and forces due to an external trap that switches between two spatial positions, depending upon the position of the oscillator. Numerical experiments are performed using an immersed boundary framework where the viscous, incompressible fluid is governed by either the inertia-free Stokes equations or the full Navier-Stokes equations. In the Stokes case, the anti-phase state is stable and the in-phase state is not. ~However, when a slight amount of inertia is added, we find that both states are stable to small perturbations. ~For higher, but still moderate Reynolds numbers we find that the anti-phase state is unstable and all perturbations tend to in-phase oscillations -- a dramatic change from zero Reynolds number [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D23.00008: Fluctuation spectra underlie the behaviour of non-equilibrium systems Alpha Lee, Dominic Vella, John Wettlaufer A diverse set of important physical phenomena, ranging from hydrodynamic turbulence to the collective behaviour of bacteria, are intrinsically far from equilibrium. Despite their ubiquity, there are few general theoretical results that describe these non-equilibrium steady states. Here we argue that a generic signature of non-equilibrium systems is nontrivial fluctuation spectra. Based on this observation, we derive a general relation for the force exerted by a non-equilibrium system on two embedded walls. We find that for a narrow, unimodal spectrum, the force depends solely on the width and the position of the peak in the fluctuation spectrum, and will, in general, oscillate between repulsion and attraction. We demonstrate the generality of our framework by examining two apparently disparate examples. In the first we study the spectrum of wind-water interactions on the ocean surface to reveal force oscillations underlying the Maritime Casimir effect. In the second, we demonstrate quantitative agreement with force generation in recent simulations of active Brownian particles. A key implication of our work is that important non-equilibrium interactions are encoded in the fluctuation spectrum. In this sense the noise becomes the signal. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D23.00009: Dynamics and structure of simple suspensions of active dipoles Tong Gao, Meredith Betterton, An-Sheng Jhang, Michael Shelley We analyze what is perhaps the simplest active fluid with complex dynamics: a suspension of non-motile, but mobile, ``extensor'' or ``contractor'' rods that exert active dipolar stresses on a fluid in which they are immersed. This is relevant to several experimental systems, including growing filaments in isotropic to smectic phase transitions, bundles of cytoskeletal filaments driven by motor proteins, and trimetallic gold-platinum rods immersed in hydrogen peroxide solutions. We first describe the system through a kinetic theory based on microscopic modeling. The stresses produced by particle activity produces long-ranged hydrodynamic coupling, and for extensors can lead to complex time-dependent flows and, depending upon flow geometry, to a form of singularity dynamics through disclination defects production, propagation, and annihilation. We then study useful closures of the kinetic theory, particularly the ``Q-tensor'' Bingham closure that has been used to study suspensions of passive micrscopic rods. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D23.00010: Fluctuations of Bacteria-laden Microbeads in a Liquid Vural Kara, Charles Lissandrello, Joan O'Connor, Jose Alberto Romero Rodriguez, Le Li, Kamil Ekinci The motion of bacteria adhered on surfaces may lead to powerful approaches for novel diagnostic tests. Examples were recently shown using microcantilevers on which bacteria were adhered using surface chemistry [1,2]. In these experiments, the presence of bacteria led to an increase in the fluctuations of the microcantilevers in the frequency range 1-100 Hz. After administering antibiotics, the fluctuations returned to their control value. Here, we build on these studies by monitoring the fluctuations of micro-beads with bacteria adhered on their surfaces. We coat the micro-beads with Poly D Lysine (PDL) in order to attach \textit{Escherichia coli. } We measure the fluctuations of the beads in motility buffer media using an optical microscope with and without bacteria. We calculate the diffusion coefficients from the mean square displacements (MSD) and correlate these with the presence of bacteria on the beads. These studies lay the foundation for the development of a rapid antibiotic susceptibility test based on bacterial activity. \\[4pt] [1] Lissandrello, C. et al. Nanomechanical motion of Escherichia coli adhered to a surface. Appl. Phys. Lett. 105, 113701 (2014).\\[0pt] [2] Longo, G. et al. Rapid detection of bacterial resistance to antibiotics using AFM cantilevers as nanomechanical sensors. Nat. Nanotechnol. 8, 522--526 (2013). [Preview Abstract] |
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