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 L22: Biological Fluid Dynamics: Locomotion - Bacteria and Microswimmers |
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Chair: Thomas Solomon, Bucknell University Room: Georgia World Congress Center B310 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L22.00001: Hydrodynamic simulations of noncanonical flagellated bacteria Henry Shum Hydrodynamic models of flagellar propulsion have been successful in reproducing and explaining many of the features of bacterial locomotion, such as the circular trajectories often observed under a microscope. The most common model for a bacterium is a rod-shaped or spheroidal cell body propelled by a single flagellum behind the cell and aligned with the long axis. Simulations taking into account the precise shape of the cell body and flagellum of a model swimmer show that these details strongly influence the hydrodynamic interaction between a swimmer and nearby surfaces. When the cell body is nearly spherical, there is a strong tendency to swim towards surfaces. The close proximity to the surface is expected to result in tight, circular trajectories. This is not consistent with observations of Magnetococcus marinus, which is an unusual bacterium in many ways. It is a magnetotactic bacterium that swims with two sheathed bundles of flagella located on the same side of its nearly spherical body. Using numerical simulations, we explore some generalizations of the canonical, monotrichous model to determine which features are important to the characteristic motion of microorganisms in the presence of flat surfaces. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L22.00002: Regulation of bacterial locomotion at oil-water interfaces Jiayi Deng, Mehdi Molaei, Kathleen J Stebe The locomotion of P. aeruginosa at the interface between oil and bacterial suspensions is studied. At solid surfaces, it is well established that bacteria experience hydrodynamic and adhesion forces that inhibit flagellar rotation. Such changes in flagellar motility initiate different processes such as EPS secretion and ultimately biofilm formation. However, hydrodynamic boundary conditions and interfacial forces differ from those on solid boundaries. Here, we study effect of fluid interfaces and structure formation on the regulation of the bacterial motility. The rheology of the interface evolves over time due to the adsorption of polysaccharides and bio-surfactant. These changes in interface mechanics alter the motion of cells adhered to and adjacent to the interface. Trajectories of bacteria are studied to analyze their velocity, reorientation, and dispersion coefficient. We investigate 3 different strains and mutations of P. aeruginosa to study effect of interfacial rheology on the bacterial behavior. These include (i) PA14ΔpelA, selected as a control strain as it does not restructure the interface; (ii) PA01 ΔpelA which does not secret polysaccharides but forms viscoelastic film; and (iii) wild type PA01 which makes a highly elastic film of bacteria at the interface. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L22.00003: Active mixing of motile bacteria in hyperbolic and vortex flows Michael A. Gerber, Christina J. Yu, Kevin A Mitchell, Thomas Solomon We present experiments and simulations on the motion of self-propelled microbes in imposed laminar fluid flows. The flows are either a hyperbolic flow in a microfluidic cross or T-cell, or a vortex-dominated flow. The microbes are bacillus subtillus, either wild-type -- characterized by run-and-tumble trajectories in the absence of a flow -- or a mutated ``smooth swimmer'' strain in which the tumbling is suppressed. We analyze the results in conjunction with a theory that predicts the existence of ``Swimming Invariant Manifolds'' (SwIMs) that act as one-way barriers that impede the trajectories of the bacteria. We explore how the shape and location of the SwIMs vary with the imposed flow, along with the different ways in which wild-type and smooth-swimming bacteria are affected by these SwIMs. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L22.00004: Impacts of multiflagellarity on stability and speed of bacterial locomotion Frank T. M. Nguyen, Michael D. Graham Trajectories and conformations of uni- and multiflagellar bacteria are studied with a coarse-grained model of elastic flagella connected to a cell body. The elasticities of both the hook protein (connecting cell body and flagellum) and flagella are varied. Flexibility plays contrasting roles for uni- and multiflagellar swimmers. For a uniflagellar swimmer, hook and/or flagellar buckling occurs above a critical flexibility relative to the torque exerted by the flagellar motor. Addition of a second flagellum greatly expands the parameter regime of stable locomotion, because flexible hooks that would lead to buckling instability in the uniflagellar case provide the flexibility required for flagellar bundling in the biflagellar case. Similar observations hold for tri- and quadriflagellar swimmers. Indeed the stability regimes for uni- and quadriflagellar swimming are virtually inverted -- to a first approximation what is stable in one case is unstable in the other. Swimming speed is also examined: it increases very weakly with number of flagella and a simple theory is developed that explains this observation. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L22.00005: A computational study of amoeboid cell migration through 3D matrices Eric J. Campbell, Prosenjit Bagchi Locomotion of amoeboid cells is mediated by cellular protrusions known as pseudopods which grow, bifurcate, and retract in a dynamic fashion. This type of motion is observed in leukocytes, embryonic cells, and metastatic cancer cells. It is a complex and multiscale process, involving bio-molecular reactions, cell deformation, and cytoplasmic and extracellular fluid motion. Additionally, cells within the human body are subject to a confined 3D environment known as the extra-cellular matrix (ECM). We present a multiphysics computational approach coupling fluid mechanics, solid mechanics, and a pattern formation model to simulate locomotion of amoeboid cells through a porous matrix composed of a viscous fluid and an array of finite-sized spherical obstacles. The model is able to recreate squeezing and weaving motion of cells through the matrix. We study the influence of matrix porosity, and cell deformability on the motility behavior. It is found that below certain values of these parameters, cell motion is completely inhibited. Phase diagrams are presented depicting such motility limits. The results show a strong coupling between cell deformability and ECM properties, and provide new fluid mechanical insights on amoeboid motility in confined medium. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L22.00006: Hook and flagellar deformations in bacterial flicks Mehdi Jabbarzadeh, Henry Chien Fu Flexibility of the hook and flagellum affects the bacterial motility and run-reverse-flick motility of single-flagellated bacteria. Previously we have neglected hydrodynamic interactions between the cell body and flagellum and developed an efficient spring model with bending and torsional stiffnesses for the hook by linearizing Kirchhoff rod model. We have reported critical hook parameter that describes transition from stable orbits to precession. Then, we included the flexibility of the flagellar filaments and found that these flexibilities initiate dynamical buckling and flick in single-flagellated bacteria. Many other previous studies replaced the hook by a linear spring which does not have torsional response. Here, we study how including the torsional spring effects of our linearization alters swimming dynamics of organisms. We also investigate the role of hydrodynamic interactions between the cell body and flagellum and find that including hydrodynamic interactions makes hooks more susceptible to buckling instabilities. Finally, we model complete flick events, investigating the buckling angle and reorientations of the swimming cells due to time dependent hook flexibility while including flexibility of the flagellar filament |
Monday, November 19, 2018 5:23PM - 5:36PM |
L22.00007: Oscillatory dynamics of swimming E coli bacteria at walls in Poiseuille flow Andreas Zöttl, Arnold JTM Mathijssen, Nuris Figueroa Morales, Gaspard Junot, Eric Clément, Anke Lindner Swimming microorganisms respond to flows in highly diverse and complex environments, at scales ranging from open oceans to narrow capillaries. The combined effects of fluid flow and boundaries lead to preferred swimmer orientation breaking the up/down-stream and left/right symmetry. To date, this so-called bacterial surface rheotaxis has been quantified by measuring instantaneous orientation distributions or average transport velocities, but a complete picture is still missing. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L22.00008: Effect of cell size on bacterial motion in micropillars arrays Pooja Chopra, David A Quint, Ajay Gopinathan, Bin Liu Although there has been tremendous focus on the fundamental principles of bacterial cell body morphology, little is known about how this morphology couples bacterial dynamics to their local environment. We have focused on investigating bacterial dynamics with regard to changes in bacterial size in a fixed background environment of micro-pillar arrays. Utilizing our high throughput tracking and image analysis tools, we are able to better quantify changes in bacterial motion while they interact with the micro-pillar environment. Our findings highlight that non-tumble E. coli mutants display bimodal transportation behavior that is bacterial size dependent. For small bacteria (4 $\mu$m), behavior becomes more tortuous that correlates with circular motion around pillars, while larger (7 $\mu$m) bacteria display high diffusivity, rarely interacting with pillars. To further investigate the role of bacterial size with micro-pillar environments we have studied bacterial motion through multiple pillar size arrays in order to elucidate mechanisms that control bacterial locomotion and its coupling to cell body size.
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Monday, November 19, 2018 5:49PM - 6:02PM |
L22.00009: A singularity model for the dynamics of externally driven microswimmers Jake Buzhardt, Phanindra Tallapragada In recent years, artificial microswimmers have received significant research attention due to their potential biomedical and engineering applications. The propulsion of magnetic micro-robots by an external magnetic field seems particularly promising, as it presents the capability to control these swimmers remotely. Specifically the dynamics of magnetic microswimmers composed of spheres have recently been explored, as these geometries are easily experimentally realized and may be configured to possess the necessary asymmetries to couple a translational motion to an external torque. In this talk we present a simplified Stokes singularity based model for such microswimmers. This model allows the investigation of the interaction of such swimmers with walls and other microswimmers. While singularity models have become commonplace in the mathematical modeling of biological locomotion, these methods have not been readily used for the modeling of these externally driven swimmers, in part due to the difficulty of computing the time-varying singularity strengths. We expect that the proposed singularity models will have applications in the development of control strategies for externally driven teams of micro-robots and path planning in the presence of complex boundaries. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L22.00010: Phototactic three-dimensional motion of active Janus particles William E. Uspal, Laurence G. Wilson, Dhruv P. Singh, Mihail N. Popescu, Peer Fischer We study the dynamics of active Janus particles that self-propel in solution by light-activated catalytic decomposition of chemical "fuel." In experiments, the particles, illuminated from below, swim upward against gravity. Using holographic microscopy, we track the three-dimensional positions of particles at different incident light intensities. A statistical analysis reveals a nonlinear dependence of the mean vertical velocity on intensity. Theoretically, we develop a model of a photo-active self-phoretic particle that accounts for "self-shadowing" of the light by the opaque catalytic face of the particle. We find that self-shadowing can drive "phototaxis" (rotation of the catalytic cap towards the light source) or "anti-phototaxis," depending on the properties of the particle. Incorporating the effect of thermal noise, we show that the distribution of particle orientations is captured by a Boltzmann distribution with a nonequilibrium effective potential. Furthermore, the mean vertical velocity of phototactic (anti-phototactic) particles exhibits a superlinear (sublinear) dependence on intensity. Overall, our findings show that photo-active particles exhibit a rich "tactic" response to light, which could be harnessed to program complex three-dimensional trajectories. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L22.00011: Dynamics of a model microswimmer in an anisotropic fluid Abdallah Daddi-Moussa-Ider, Andreas M. Menzel Several recent experiments investigate the orientational behavior of self-propelled bacteria and colloidal particles in anisotropic fluids such as nematic liquid crystals. Correspondingly, we study theoretically the dynamics of a simple model microswimmer in a uniaxially anisotropic fluid. The behavior of both puller- and pusher-type swimmers in the anisotropic fluid is analyzed. Depending on the propulsion mechanism as well as the relative magnitude of different involved viscosities, we find alignment of the microswimmer parallel or perpendicular to the anisotropy axis. The observed swimmer reorientation results from the hydrodynamic coupling between the self-induced fluid flow and the anisotropy of the host fluid. Our theoretical predictions are found to be in qualitative agreement with recent experiments on swimming bacteria in nematic liquid crystals. They support the objective of utilizing the anisotropy of a surrounding fluid to guide individual swimmers and self-propelled active particles along a requested path, enabling controlled active transport. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L22.00012: Physical bounds on velocities in soft magnetic microswimmers Kiarash Samsami, Seyed Amir Mirbagheri, Farshad Meshkati, Henry Chien Fu Magnetic microswimmers are being studied for their biomedical applications and manipulation and sensing abilities in microscales. We study rigid magnetic microswimmers propelled rotationally using a rotating magnetic field as a commonly examined class of these microswimmers. Based on current understanding of paramagnetic and hard ferromagnetic swimmers, it is commonly thought that higher velocities can generally be achieved by increasing the magnetic field magnitude. Here we investigate, instead, soft magnetic microswimmers, and show analytically that there is a maximum limit for velocity depending on the material's property and geometry. We also perform numerical simulations demonstrating this effect for two examples: first, a helical microswimmer with an ellipsoidal head, which has a relatively simple magnetization model, and second, a helical microswimmer with a thin square plate head, which is similar to several experimental realizations of magnetic microswimmers. |
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