Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D23: Biological Fluid Dynamics: Locomotion - Active Suspensions |
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Chair: Sebastian Fürthauer, Simons Foundation Room: Georgia World Congress Center B311 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D23.00001: Navigation of magnetotactic bacteria is impaired by porous microstructure: Effects of porosity, pore scale, and geometrical order Amin Dehkharghani, Nicolas Waisbord, Thomas Coons, Jeffrey S. Guasto Microstructured quiescent environments host a variety of swimming microorganisms, which are crucial to many natural and engineering processes. Using microfluidics to precisely control the porosity, pore scale, and geometrical order of the environment, we systematically study the impact of the microstructure on both the diffusive and directed transport of swimming cells. Magnetotactic bacteria (MTBs) are used as a model biological system, because they share swimming mechanisms with other bacteria of interest, and their swimming direction is easily manipulated via an external magnetic field. We show that the cells’ effective diffusion coefficient without a magnetic field decreases markedly in the presence of porous microstructure compared to bulk fluid. Applying a guiding external field greatly enhances the mobility of the migrating cells through the media, an effect that is enhanced with increasing magnetic field strength. These results are an important step toward understanding the ecology of swimming cells in quiescent porous media as well as for controlling micro-robots in complex environments. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D23.00002: Asymptotic transport and dispersion of active particles in periodic porous media Roberto Alonso-Matilla, Brato Chakrabarti, David Saintillan We analyze the long-time transport of active Brownian particles flowing through a doubly-periodic porous lattice using generalized Taylor dispersion theory and Langevin simulations. The asymptotic spreading of a dilute cloud of microswimmers is shown to obey an obstacle-free advection-diffusion equation, which we use to elucidate the effects of motility, lattice geometry and applied fluid flow on transport properties. Our model suggests that the interplay of particle self-propulsion, entropic trapping by the obstacles, and shear-induced dispersion in the flow can be exploited for the control and directional transport of active swimmers through microstructured environments. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D23.00003: Fingering instabilities in tissue invasion: an active fluid model Michal Jozef Bogdan, Aude Mulard, Thierry Savin We study theoretically a possible physical mechanism for the formation of multicellular protrusions in carcinoma at the onset of metastasis. We suggest that these patterns may be a consequence of a mechanical instability, reminiscent of the classical viscous fingering instability, which results from tumor growth and cell's active traction. We first develop a simple model for the tissue's traction field, which we use to explore various regimes of instability. We notably show that, for a carcinoma growing in an external environment of comparable mechanical properties, even weak active traction can lead to multicellular protrusions. We also discuss the further evolution of the fingers within this model. Thanks to the simplicity and versatility of the proposed mechanism, we believe our model may be applicable in a wide range of scenarios, and may indeed rationalize fingering phenomena observed in vivo. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D23.00004: Active suspension of self-rotating particles Cody Reeves, Igor Aronson, Petia M Vlahovska Suspensions of self-propelled particles, such as bacteria, have received considerable attention. Recently there has been increased interest in suspensions of self-rotating particles, such as Quincke rotors driven by electric fields and ferromagnetic colloids in magnetic fields. While the individual particles are governed by relatively simple dynamics, the interaction of the particles can result in complex collective dynamics. Experiments show phase separation, macroscopic directed motion, and structure formation (e.g. vortices). Modeling these systems as discrete particles at the micro-scale (Yeo et al, PRL(2015)) is computationally expensive and limits the study of the rotors collective dynamics. We develop a continuum model based on the one for fluids with internal rotation (Rosensweig, J. Chem. Phys. (2004)). The model allows us to study properties of the fluid and the existence of active turbulence caused by the rotors. To study the effect of confinement, we include phase parameter to restrict the rotors inside a region with a defined diffuse interface. We then can study the interaction between the rotors and the interface for both a fixed and deformable interface |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D23.00005: Mixing & transport of microorganisms in 2D time-periodic flows Brendan C Blackwell, Boyang Qin, Paulo E. Arratia In this talk, we examine the effect of swimming bacteria on transport, diffusion, and mixing in time-periodic flows. An oscillatory two-dimensional flow is generated by driving a sinusoidal current through a conducting fluid (salt water) that is situated atop an array of magnets. We perform experiments with an ordered lattice to create regular vortices and with a random configuration to create a spatially disordered flow pattern; the Reynolds number ranges from 0.1 to 100. Two types of fluids are used: (1) a simple Newtonian liquid (water) and (2) a mixture of water and bacteria (Vibrio cholerae). Velocimetry data are used to calculate the flow stretching fields and Lyapunov exponents. Results with plain salt water are compared to results with the addition of varying concentrations of V. cholerae. We find that the addition of active bacteria, even in dilute quantities, results in significant changes to the stretching fields even though the Eulerian velocity fields remain quite similar. We also perform experiments with fluorescent dye, both with and without bacteria, to more directly characterize mixing. These data also show a substantial difference between the active suspension and the control case, even with small concentrations of bacteria. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D23.00006: Run-and-Tumble-like dynamics of Quincke rollers actuated by an AC electric field Hamid Karani, Gerardo Pradillo, Petia M Vlahovska Run-and-tumble dynamics is a canonical example of swimming strategy in self-propelled microswimmers such as E. Coli. It is characterized by swimming on straight line at almost constant velocity (runs), followed by a sudden complete random reorientation of swimming direction (tumbles). Here, we experimentally show how the Quincke rollers, previously studied mainly as an active Brownian particles, can perform Run-and-Tumble-like locomotion. We achieve this by modulating the intensity and duration of the applied electric field. Through single-particle-tracking analysis, we characterize the short-term and long-term dynamics of the mean-squared-displacement of the Quincke random walkers. More specifically, it is shown how the directed motion at short times and enhanced Brownian diffusion at longer time-scales are linked to the frequency and intensity of the applied electric field. We further demonstrate how we can engineer AC Quincke rollers to create a novel artificial particle system with well-controlled tunable properties to investigate anomalous diffusion in Run-and-Tumble dynamics. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D23.00007: A network of mutually propelled rods: theory and experiment Sebastian Fuerthauer, Bezia Lemma, Peter J Foster, Claire E Walczak, Stephanie C Ems-McClung, Zvonimir Dogic, Daniel J Needleman, Michael John Shelley Cellular components such as cytoskeletal filaments and motors are the essential constituents of a new class of materials: so called active fluids. We present experiments and theory on a system of stabilized microtubules driven by the molecular motor protein XCTK2. Through photobleaching experiments, we demonstrate that in this system microtubules are aligned along the long direction of the system and travel through the gel at a velocity independent of the local average polarity. We show that this result is most naturally understood in the frameworks of an active gel theory that goes beyond pairwise microtubule interactions and treats the gel as highly cross-linked. Our theory bridges the length scales from the microscopic mechanical behavior of motor-filament interactions to the large scale behavior of the active gel and generalizes to describe different kinds of cytoskeletal assemblies.
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Sunday, November 18, 2018 4:01PM - 4:14PM |
D23.00008: Rheology and dynamics of active microtubule suspensions David A. Gagnon, Claudia Dessi, Zvonimir Dogic, Daniel L. Blair Biofilm formation, mammalian reproduction, and bacterial infection are ubiquitous and practical examples of suspensions containing self-driven particles. These active suspensions are inherently out-of-equilibrium and can possess anomalous bulk rheological properties. Previous experimental and numerical studies suggest organisms with extensile swimming behavior (e.g. Escherichia coli) can decrease the apparent viscosity of a fluid, while those with contractile swimming behavior (e.g. Chlamydomonas reinhardtii) can increase the apparent viscosity of a fluid. Here, we systematically explore the rheology and dynamics of an active suspension of microtubules and kinesin motors driven by ATP. We use a custom-built confocal rheometer to provide simultaneous macroscale rheological measurements and fluorescent imaging of local microtubule dynamics. We find increasing ATP concentration, and therefore increasing activity, yields a significant increase in the apparent viscosity of the suspension. Simultaneously, using velocimetry techniques, we find significant increases in local velocity fluctuations and deformation rates, suggesting underlying microscale mechanisms for the observed macroscale rheology. |
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