Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K59: Rheology of Active Fluids: From Active Polymers to Living MatterFocus
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Sponsoring Units: GSOFT Chair: Wylie Ahmed, California State University, Fullerton Room: BCEC 257B |
Wednesday, March 6, 2019 8:00AM - 8:36AM |
K59.00001: Symmetric shear bands and collective swarming of bacterial suspensions Invited Speaker: Xiang Cheng Active fluids are a novel class of non-equilibrium complex fluids with examples across a wide range of biological and physical systems such as flocking animals, swarming microorganisms, vibrated granular rods, and suspensions of synthetic colloidal swimmers. Different from familiar non-equilibrium systems where free energy is injected from boundaries, an active fluid is a dispersion of large numbers of self-propelled units, which convert the ambient/internal free energy and maintain non-equilibrium steady states at microscopic scales. Due to this distinct feature, active fluids exhibit fascinating and unusual flow behaviors unseen in conventional complex fluids. Here, by combining high-speed confocal microscopy, rheological measurements and biochemical engineering, we experimentally investigate the flow behaviors of E. coli suspensions, a premier example of active fluids. In particular, we show the microscopic dynamics of bacterial suspensions associated with the abnormal rheology and the emergence of collective swarming. Using theoretical tools of fluid mechanics and statistical mechanics, we develop a quantitative understanding of these interesting behaviors. Our study shows the general organizing principles of active fluids that can be exploited for designing “smart” fluids with controllable fluid properties. Our results also provide new insights into the fundamental transport processes of microbiological systems. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K59.00002: Activity effects on the non-linear mechanical properties of fire ant aggregations Michael Tennenbaum, Alberto Fernandez-Nieves Individual fire ants are inherently active as they are a living organism that convert stored chemical energy into motion. However, each individual ant is not equally disposed to motion at any given time. In an active aggregation, most of the constituent ants are active, and vice versa for an inactive aggregation. Here we look at the role activity plays on the non-linear mechanical behavior of the aggregation through large amplitude oscillatory shear measurements. We find that the level of viscous non-linearity can be decreased by increasing the activity or by increasing the volume fraction. The level of elastic nonlinearity is not affected by either activity or volume fraction. Applying too large of an applied strain amplitude removes the effect of activity but preserves the effect of the volume fraction. We also compare this to viscosity measurements where we see that the level of nonlinearity decreases with activity as well. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K59.00003: Spontaneous migration of cellular aggregates: from giant keratocytes to running spheroids Francoise Brochard-Wyart We introduce the broad field of entangled active matter. Unlike swarms of fish and flocks of birds, cells are bound by transient links and behave as active viscoelastic pastes. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K59.00004: WITHDRAWN ABSTRACT
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Wednesday, March 6, 2019 9:12AM - 9:24AM |
K59.00005: Impact of Motile Bacteria on Viscous Fingering Jane Chui, Harold Auradou, Karen Fahrner, Howard C Berg, Ruben Juanes Viscous fingering is a hydrodynamic instability that occurs when a less viscous fluid displaces a more viscous one. Instead of progressing as a uniform front, the less viscous fluid forms fingers to create complex patterns. Understanding how these patterns and their associated gradients evolve over time is of critical importance in characterizing the mixing of two fluids. Here, we investigate the impact of replacing the less viscous fluid with an active suspension of motile bacteria, relevant to applications such as microbial enhanced oil recovery and bioremediation. |
Wednesday, March 6, 2019 9:24AM - 10:00AM |
K59.00006: Looking into the rheology active microtubule suspensions Invited Speaker: Daniel Blair 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. In this talk we will present combined experimental results on 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 decrease 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. We will present a simple model that captures the connection between the local mechanics and the global viscoelasticity. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K59.00007: Interrogation and manipulation of active nematic films using colloidal probes David Rivas, Robert Henry, Daniel H Reich, Tyler Shendruk, Robert Leheny We report studies of the interactions between active nematic films and disk-shaped colloids. The active nematics are driven by molecular motors that cause the constituent microtubular bundles to slide past each other, generating extensional flows that continuously create pairs of +1/2 and -1/2 defects. Rotating magnetic microdisks in proximity to the films produce hydrodynamic flows that compete with the films’ intrinsic flow, leading to significant effects on the director field and defect landscape. Near the disks, the self-propelled +1/2 defects take on a preferred direction that is tangential to the direction of imposed stress, creating orientational ordering of the defect motion. At sufficient rotation rates, a more significant alteration of the director field is observed wherein a vortex-like structure within the director field with topological charge of +1 forms. Additionally, we are able to use the disks as effective microrheological probes to gain insight into the physical properties of the film. For example, by analyzing the velocity profile in the film produced by the spinning disk, we obtain an estimate of the film’s viscosity. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K59.00008: Shearing Living Liquid Crystals Hend Baza, Taras Turiv, O D Lavrentovich, Antal Istvan Jakli Flagellated bacteria of pusher type, such as B. subtilis and E. coli, change the rheological properties of the surrounding isotropic medium. B. subtilis dispersed in a water-based lyotropic chromonic liquid crystals (LCLCs), form a “living liquid crystal” (LLC). Here, we explore the response of the LLC to shear, in particular, how the effective viscosity of LLC changes with the shear rate and how the flow-aligning effect of shear and liquid crystal-mediated elasticity compete with the activity of bacteria that destabilize the orientational order. Studies with an optical rheometer and polarizing optical microscope demonstrate that upon cessation of shear, the activity of bacteria leads to a progressive development of periodic director undulations (with the wave vector parallel to the shear direction), nucleation and multiplication of disclination pairs that produce the regime of topological turbulence. When the gap height increases, disclinations transform from short segments connecting opposite plates to elongated segments in the plane of the cell. The studies reveal a wealth of intriguing phenomena when the system transforms from 2D to 3D-like confinement. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K59.00009: On the relationship between velocities, tractions, and intercellular stresses in the migrating epithelial monolayer Yoav Green, Jeffrey Fredberg, James P. Butler Cells migrate collectively in physiological phenomena including cancer metastasis, development, and asthmatic airway remodeling. Within an epithelial monolayer, for example, each constituent cell exerts intercellular stresses on neighboring cells, and exerts traction forces on its substrate. While traction forces exerted by a monolayer have been measured for more than a decade, their relationship to measured cellular velocities remains unknown. Additionally, the relationship between intercellular stresses and tractions also remains unresolved. In passive systems, traction stresses and velocities are linked in principle through a constitutive law. The simplest and commonly assumed form of such a law is a Newtonian fluid, which implies a linear relationship between stress and strain rate. In this talk, I will address the question: To what extent are tractions related to velocities through such a linear constitutive law? Using a newly derived theoretical model, I will show that the tractions predicted from the measured velocity field are uncorrelated with the measured tractions. This implies that current methods for intercellular stress recovery need to be corrected where one such rectification is the inclusion of an active stress term, for which we derive a novel constraint. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K59.00010: Rheological response and direct visualization of collective patterns formation leading to shear-bands formation in a suspension of active E.coli at the super-fluidity transition Eric Clement, Vincent M Martinez, Jochen Arlt, Carine Douarche, Adama CREPPY, Harold Auradou, Angela Dawson, Jana Schwarz-Linek, Wilson Poon Suspensions of motile Escherichia coli were found to display `negative viscosity increment' at low shear rate viz., adding such bacteria lowers the shear viscosity of the system (H.M.Lopez , J. Gachelin, C. Douarche, H.Auradou, E. Clément, Phys. Rev. Lett. 115, 028301 (2015)). Furthermore at higher concentrations a regime of zero viscosity, akin to a "superfluidity" transition, can be reached. Here, for a strain of very active bacteria, we report a full exploration of the rheological response changing confinement, shear rate and concentration, both in a low-shear Couette rheometer and in a cone-plane rheometer. The last allows a direct visualization of the collective organization under shear and the exploration of the different regimes leading the “superfluidity transition” in relation with the dynamics of shear-band formation. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K59.00011: The active force spectrum of a microswimmer - modeling and experiments Wylie Ahmed Chlamydomonas reinhardtii are a widely-studied microswimmer that propel themselves by converting chemical energy to mechanical motion of their flagellum in a breast stroke motion. Fluid dynamics approaches have revealed much about the importance of hydrodynamics at the micron-scale and its role in microswimmer transport. However, the stochastic dynamics which are dominated by active non-thermal fluctuations are not well understood. We use optical tweezers and the photon momentum method to directly measure the stochastic forces generated by a trapped Chlamydomonas microswimmer. We model the microswimmer using the generalized Langevin equation approach with active stochastic forcing. Our combined experimental and theoretical approach, based on microrheological techniques, isolates the active force spectrum generated by Chlamydomonas to quantify their nonequilibrium dynamics. We seek to use this framework to test recent developments in stochastic thermodynamics. |
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