Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C31: Biological Fluid Dynamics: Active Fluids |
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Chair: Amir Nourhani, University of Akron Room: 613 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C31.00001: Transport control for apolar active suspension in a rectangular channel using circular cylinders Sheng Chen, Tong Gao We study the control for transport of a dilute apolar active suspension in a rectangular channel using circular cylinders. We adopt a coarse-grained active liquid crystal model to describe collective unstable dynamics of non-motile but mobile rod-like particles, i.e., the "Extensors", in Stokes flows. By using a Galerkin mixed finite element method, we previously revealed various patterns of spontaneous coherent flows that can be unidirectional, traveling-wave, and chaotic for the case without cylinders inside. To uncover the transport control using circular cylinders, we first study how fixed cylinders change the flow transitions in channel by systematically changing the cylinder size, separation distance, and the channel width. We further study if the flow transitions alter when the cylinders are free to rotate about a fixed axis. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C31.00002: Chaotic mixing of an active nematic in viscous environments Amanda Tan, Kevin Mitchell, Linda Hirst The unifying theme of active matter is that collections of subunits consume energy locally, translate that energy into movement, and ultimately produce large-scale flows. Such materials are out of equilibrium systems. A fascinating example of a tunable non-equilibrium system is the microtubule/kinesin-based active nematic that forms at an oil/water interface. This material, based on biological polymers and molecular machines, produces a steady-state in which we observe moving topological defects that braid around one another. Defects are continuously created and annihilate, generating fluid flows. A direction of interest in non-equilibrium systems is to study how the active material responds to changes in its environment. To generate external environmental changes, we change the viscosity of the oil that the network is in contact with and probe subsequent changes in the morphology and speed of the network. Lower viscosity oils give lower defect densities and produce higher network speeds, and vice versa for higher viscosity oils. Using analysis from chaotic advection theory, we explore how environmental changes affect various quantitative parameters such as the local fluid stretching rates within the network and the topological entropy as calculated from defect braiding. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C31.00003: Stress relaxation in active suspensions Prabhu Nott, Sankalp Nambiar, Phani Kanth Sanagavarapu, Ganesh Subramanian The rheology of `active' suspensions has been a subject of considerable interest in recent years, but mostly under steady shear. ~In a recent experimental study [1], the response of a dilute suspension of \textit{E. coli} was characterized for impulsive initiation and cessation of shear. Interesting features in the relaxation of the shear viscosity were found, as the shear rate was varied, including regimes of apparent superfluidity. Here, we report a theoretical study that attempts to explain and analyze the observed rheological response [2]. Starting from a kinetic equation appropriate for rod-shaped bacteria like \textit{E. coli}, we determine the evolution of the orientation distribution as a function of time following a step change in the shear rate, and thereby, the temporal evolution of the bacterial contribution to the shear viscosity. ~Our model predictions are in excellent agreement with the experiments in the limit of small shear rates, but the predicted relaxations differ qualitatively at higher shear rates. ~We offer plausible arguments to explain the disparity, and suggest courses for future experimental and analytical studies that will help understand relaxation phenomena in active suspensions. [1] Lopez et al., \textit{Phys. Rev. Lett.} \textbf{115}, 028301 (2015). [2] Nambiar et al. \textit{J. Fluid Mech. }\textbf{812}, 41--64 (2017); Nambiar et al. \textit{J. Fluid Mech. }\textbf{870}, 1072--1104 (2019). [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C31.00004: Self-propulsion of active colloids \textit{via} ion production Marco De Corato, Xavier Arqué, Tania Patiño, Ma Arroyo, Samuel Sánchez, Ignacio Pagonabarraga Active particles that harness chemical energy from the environment and turn it into directed motion attracted great interest in the recent years. Several applications have been envisaged for these particles from pollutant removal to anti-cancer therapies. To optimize active colloids for advanced applications, one has to achieve fundamental understanding of their dynamics in fluidic environments. Here, we develop a model for the self-propulsion of a chemically active colloid that asymmetrically releases ionic species. By solving the relevant equations using simulations and a perturbation expansion, we evaluate the velocity of the active particle as a function of the main parameters. Our results highlight several novel aspects that are qualitatively different from other mechanisms. The active particle can reverse direction of motion by changing the salt concentration in the solution and can propel even if it is not charged. We find an optimal condition for self-propulsion and a novel regime in which the velocity is independent of the ionic strength of the environment. The model quantitatively captures the salt-dependent velocity measured in experiments using active colloids that propel by decomposing urea via enzymatic reaction. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C31.00005: 'Superfluid' vs collective motion in model active suspensions Alexander Morozov, Viktor \v{S}kult\'ety Systems comprising active particles often exhibit non-zero steady-state currents, and possess unique mechanical and transport properties that are absent from their passive counterparts. One of the most striking examples of such properties is the recent experimental observation of a vanishingly small shear viscosity in suspensions of swimming bacteria. \\ Here, we present our recent results on the connection between this phenomenon and the onset of collective motion in bacterial suspensions. We find that confinement strongly influences both phenomena, and that the apparent viscosity drops to zero before the transition to collective motion. We compare our predictions against the active gel theory, and discuss their relevance to recent experiments. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C31.00006: Spontaneous symmetry breaking of an active magnetic fluid Amir Nourhani, David Saintillan We develop a continuum kinetic model for the dynamics of self-propelled micromagnets in a Newtonian fluid. Starting from a uniform random configuration, long-range interactions between magnetic dipoles lead to spontaneous symmetry breaking and unidirectional flow of the active suspension. By exploring the parameter space we investigate the order-to-disorder transition and obtain a phase diagram as a function of magnetic flux, number density, and diffusivity. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C31.00007: Melting of vortex crystals in a minimal model for active fluids Martin James, Dominik Anton Suchla, Jorn Dunkel, Michael Wilczek Two-dimensional crystals show several intriguing properties. For instance, crystals in equilibrium systems lack long-range positional order and their melting is a complex phenomenon. Here we investigate the formation and melting of a nonequilibrium crystal, an active vortex crystal, in a minimum continuum model for active matter. Using simulations, we find spontaneously emerging vortex crystal solutions after an extended turbulent transient. These active vortex crystals melt into a turbulent active fluid as the activity or the advection, the two free parameters in the model, are changed. We map out the phase diagram and systematically characterize the melting transition as a function of both the parameters. Depending on the path through the parameter space, the melting exhibits diverse transition scenarios; it may proceed through a hysteretic marginal stability region or an intermediate hexatic phase. Our results indicate that crystalline phases in active matter share similarities with their equilibrium counterparts. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C31.00008: Experimental characterization and modeling of contractile behavior and fluid flows in an optically-controlled microtubule network Zijie Qu, Jialong Jiang, Jack Stellwagen, Zitong Wang, Matt Thomson Cells perform physical tasks (genome segregation, movement) by organizing the activity of force-generating, “active” molecules in time and space. Most experimental active matter systems of biological or synthetic molecules are capable of spontaneously organizing into structures and generating global flows while lacking the spatiotemporal control found in cells, limiting their utility for studying non-equilibrium phenomena and bioinspired engineering. Here, we use an optically-controlled active matter system, consisting of stabilized microtubule filaments and kinesin motors, to demonstrate a series of simple operations by projecting various light patterns including both concave and convex polygons. The light patterns activate a reversible link between the kinesin motors which pull on microtubules. A two-phase contracting behavior is observed. The first phase includes a fast formation of microtubule network and its uniform contraction. The second phase is dominated by the steady state flow established afterwards. Two separate mathematical models are proposed to study these behaviors. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C31.00009: Engineering microfluidic flow networks and probing self-organisation with light-controlled active suspensions Xingting Gong, Arnold JTM Mathijssen, Zev D Bryant, Manu Prakash The transport and self-regulation of biomolecular constituents is essential for cellular pathways but also for autonomous microfluidic devices. In Chara cells, among the longest cells in nature, this self-organisation is facilitated by motor-driven vesicles that generate large-scale flows. Here, we consider reconstituting this system with light-controlled molecular motors in order to probe the emergent properties of this active suspension, and to engineer microfluidic flow networks with optogenetic control. We model myosin-coated colloids that can bind with the surfaces of an actin-patterned microchamber, together creating an active carpet. The attached colloids generate flows that in turn can advect detached particles towards the walls, forming a feed-back loop. Switching the motor velocities with light, we perturb this feed-back and create a rich design space of flow patterns. We derive the possible mode structures and use this theory to optimise transport and chaotic mixing. Our results pave the way towards understanding and controlling active fluids. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C31.00010: Active yielding in extensile active gels David Gagnon, Claudia Dessi, Zvonimir Dogic, Daniel Blair Active gels are stiff biopolymer networks driven by molecular motors. In contrast with other non-equilibrium active suspensions, active gels form ephemeral networks with long-range, temporary mechanical contacts. These active crosslinks imbue the gel with fluid-like properties over long timescales and solid-like properties at short timescales. In this talk, I will discuss how activity modifies the mechanical properties of an extensile active gel comprised of microtubules driven by walking kinesin motors. We find an active gel's apparent resistance to flow is non-monotonic with increasing externally-applied shear rate. When the internal active rates are faster than the applied shear rate, the network exhibits active yielding and provides little additional resistance to flow. However, at applied shear rates faster than the internal rates, the active gel stiffens and then yields as if it were a passive biopolymer network. [Preview Abstract] |
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