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 A23: Biofluids: Active Fluids I |
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Chair: Jorn Dunkel, MIT Room: 300 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A23.00001: Nematic long-range ordering of topological defects in active liquid crystals Jorn Dunkel, Anand Oza Identifying the ordering principles of intracellular matter is key to understanding the physics of microbiological systems. Recent experiments demonstrated that ATP-driven microtubule-kinesin bundles can self-assemble into two-dimensional active liquid crystals that exhibit a rich creation and annihilation dynamics of topological defects, reminiscent of particle-pair production processes in quantum systems. This remarkable discovery has sparked considerable theoretical and experimental interest, yet a satisfactory mathematical description remains elusive. Here, we present and validate a continuum theory for this new class of active matter systems by merging universality ideas with the classical Landau-de Gennes theory. The resulting model agrees quantitatively with recently published data and, in particular, predicts correctly a previously unexplained regime of long-range nematic ordering of defects observed in experiments. Our analysis implies that active liquid crystals are governed by the same generic ordering principles that determine the non-equilibrium dynamics of dense bacterial suspensions and elastic bilayer materials. Moreover, the theory suggests an energetic analogy with strongly interacting quantum gases. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A23.00002: The swim force as a body force Wen Yan, John Brady Net (as opposed to random) motion of active matter results from an average swim (or propulsive) force. It is shown that the average swim force acts like a body force -- an {\em internal} body force [Yan and Brady, \textit{Soft Matter}, DOI:10.1039/C5SM01318F]. As a result, the particle-pressure exerted on a container wall is the sum of the swim pressure [Takatori \textit{et al., Phys. Rev. Lett.}, 2014, \textbf{113}, 028103] and the `weight' of the active particles. A continuum mechanical description is possible when variations occur on scales larger than the run length of the active particles and gives a Boltzmann-like distribution from a balance of the swim force and the swim pressure. Active particles may also display `action at a distance' and accumulate adjacent to (or be depleted from) a boundary without any external forces. In the momentum balance for the suspension -- the mixture of active particles plus fluid -- only {\textit{external}} body forces appear. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A23.00003: Diffusion in active suspension of microswimmers Eric CLIMENT, Blaise DELMOTTE, Franck Plouraboue, Eric KEAVENY, Matthieu MARTIN, Salima RAFAI, Philippe PEYLA, Eric BERTIN The presence of microswimmers in a fluid generates flow agitation due to multi-body hydrodynamic interactions. This agitation of the fluid leads to random trajectories of passive tracers particles and the swimmers themselves, and from a macroscopic point view, it can be interpreted as a diffusive mechanism. By means of experiments (videomicroscopy of suspensions of chlamydomonas reinhardtii)and numerical simulations (Stokesian fluid populated with squirmers), we investigate the evolution of the effective diffusion coefficient when the volumetric concentration of the active suspension varies. By comparing the experimental and numerical results, we quantify the role of active swimming on the measured diffusion and identify the physical mechanisms that lead to diffusion enhancement. Our results aim to provide a better understanding of how swimming organisms affect micron-scale transport in the environment. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A23.00004: Efficient Simulation of a Large Number of Microswimmers Using Fast Multipole Method Minghao Rostami, Sarah Olson Regularized Stokes formulation has been shown to be very effective at modeling fluid-structure interactions when the fluid is highly viscous. However, its computational cost grows quadratically with the number of particles immersed in the fluid. We demonstrate how fast multipole method can be applied to significantly reduce the computational cost of regularized Stokes method. Numerical results will be presented for simulating the dynamics of a large number of microswimmers immersed in 3D stokes flows. Furthermore, we also investigate the swimming efficiency of the microswimmers when they are placed in various geometric configurations. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A23.00005: Biogenic mixing induced by intermediate Reynolds number swimming at pycnoclines Shiyan Wang, Arezoo Ardekani Recently, there has been a debate regarding the contribution of marine organisms to ocean mixing. To address this question, we study fully-resolved motion of interacting swimmers in a density stratified fluids using a ``squirmer'' model to quantify their contribution to mixing. In the aphotic ocean (i.e. regions that are 200 m beneath the sea surface), zooplankton are the most abundant organisms leading to vertical fluid transport. Their body size ranges from millimeter to centimeter, and their Reyonlds number is in the range of O(1-100). Therefore, it is important to examine the biogenic mixing in this inertial regime. Our numerical results suggest that biogenic mixing increases with inertia, and in local hot spots, the vertical water transport induced by centimeter-sized organisms is comparable to the turbulent mixing. In the presence of background turbulence, the biogenic mixing is determined by the magnitude of dissipation of kinetic energy introduced by the organisms. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A23.00006: Collective motion of microswimmers in viscoelastic fluids Gaojin Li, Arezoo Ardekani The dynamics of suspension of self-propelled microorganisms show fascinating hydrodynamic phenomena, such as, large scale swarming motion, locally correlated motion, enhanced particle diffusion, and enhanced fluid mixing. Even though many studies have been conducted in a Newtonian fluid, the collective motion of microorganisms in non-Newtonian fluids is less understood. The non-Newtonian fluid rheological properties, such as viscoelasticity and shear-dependent viscosity in saliva, mucus and biofilm, significantly affect the swimming properties and hydrodynamic interaction of microorganisms. In this work, we use direct numerical simulation to investigate the collective motion of rod-like swimmers in viscoelastic fluids. Two swimming types, pusher and puller, are investigated. The background viscoelastic fluid is modeled using an Oldroyd-B constitutive equation. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A23.00007: Numerical study of the generation of metachronal waves in layers of beating cilia using a Lattice Boltzmann method. Application to the generation of fluid motion at the cell scale. Jean Mercat, Zhe Li, Julien Favier, Umberto d'Ortona, Sebastien Poncet Cilia are flexible elongated whip-like structures which are ubiquitous in nature. Indeed, the collective beating of arrays of thousands of cilia can transport fluid (mucus in airways) or induce locomotion on microorganisms swimming in water. From a purely hydrodynamical point of view, cilia do not beat randomly, but rather generate typical metachronal waves at their surface. In this work, we study the self-organization of the beating motion of large fields of beating cilia in a two-component flow environment, made of water and a much more viscous fluid. The numerical solver is based on an immersed boundary-lattice Boltzmann method in the context of single- and multi-component fluid flows, and in the presence of fixed or moving solid boundaries. The solver has been validated in previous studies. Various parameters are varied, such as length, spacing and phase motion of individual cilia. The energetic performances of different kind of waves are studied to understand the emergence of antiplectic metachronal waves, commonly observed in nature. It is found that a purely hydrodynamical coupling between fluid and cilia can explain the onset of metachronal waves in cilia arrays, and that these waves are maximizing a performance ratio. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A23.00008: A fast method to compute triply-periodic Brinkman flows Hoang-Ngan Nguyen, Karin Leiderman, Sarah Olson A fast method is developed to efficiently compute three-dimensional Brinkman flows induced by triply-periodic arrays of points forces and regularized forces. For point forces, we decompose the periodic Brinkman velocity into the sum of two series: one in real space and one in Fourier space. To do the splitting, we make use of a regularized solution with special decay properties so that both summands will decay in a Gaussian manner. For regularized forces, the same methodology is used to split the regularized velocity, and again, Gaussian decay of the summands is achieved. When there are $N$ forces ($N$ periodic arrays), the overall complexity is $O(N^2)$. We discuss different ways to reduce the complexity to $O(N^{3/2})$ and to $O(N\log N)$. Finally, we present two sets of numerical results. The first validates the computational complexity of the algorithm and the second illustrates how this method can be used to study microscopic flows of organisms in a porous medium. A simple dumbbell model of swimmers is implemented that exhibits a large scale flow varying as a function of resistance within the porous medium. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A23.00009: Confinement of active systems: trapping, swim pressure, and explosions Sho Takatori, Raf De Dier, Jan Vermant, John Brady We analyze the run-and-tumble dynamics and motion of living bacteria and self-propelled Janus motors confined in an acoustic trap. Since standard optical tweezers are far too weak, we developed an acoustic trap strong enough to confine swimmers over distances large compared to the swimmers' size and run length. The external trap behaves as an ``osmotic barrier'' that confines the swimmers inside the trapping region, analogous to semipermeable membranes that confine passive Brownian particles inside a boundary. From the swimmers' restricted motion inside the trap, we calculate the unique swim pressure generated by active systems originating from the force required to confine them by boundaries. We apply a strong trap to collect the swimmers into a close-packed active crystal and then turn off the trap which causes the crystal to ``explode'' due to an imbalance of the active pressure. We corroborate all experimental results with Brownian dynamics simulations and analytical theory. [Preview Abstract] |
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