Session M18: Complex Suspensions in Complex Fluids: Passive and Active Matter

Live

Fluid-mediated interactions lead to new modes of locomotion in suspensions of swimming cells, from small bacteria in confined spaces to larger aquatic organisms in bulk fluids. In this talk, I will present recent work in my group where we study theoretically the impact the hydrodynamic interactions on motility. First, in collaboration with C. Fradin and K. Dalnoki-Veress at McMaster University, I will report on a new type of collective motion where a uniform distribution of magnetotactic bacteria is rendered unstable by a magnetic field due solely to hydrodynamic interactions among the cells and with boundaries. I will next explain how hydrodynamic interactions between swimmers and surfaces can be used to separate motile cells that have different types of far-field hydrodynamic signatures. I will finally consider biased swimmers (e.g. phototactic or gyrotactic) and show how this bias can lead to new instabilities in cell suspensions.   

Live

Microorganisms often propel themselves through biological fluids that exhibit complex responses to deformations, responses which may include elastic and even anisotropic (direction-dependent) effects. Mammalian spermatozoa, for instance, encounter several complex fluids throughout the female reproductive system, including glycoprotein-based cervical mucus, mucosal epithelium inside the fallopian tubes, and actin-based viscoelastic gel outside the ovum. These complex fluid phenomena can either enhance or retard a microorganism's swimming speed, and can even change the direction of swimming, depending on the body geometry and the properties of the fluid. We will discuss analytical and numerical insights into swimming through model viscoelastic (Oldroyd-B) and in particular liquid-crystalline (Ericksen-Leslie) fluids, with a special focus on the important and in some cases dominant roles played by the presence of nearby boundaries.   

Live

The organized movement of intracellular material is part of the functioning of cells and the development of organisms. These flows can arise from the action of molecular machines on the flexible, and often transitory, scaffoldings of the cell. That sounds complex and multiscaled, as is much of biology, but its study is also becoming a beautiful sub-branch of biophysical fluid dynamics characterized by geometric complexity, confinement, and microscale activity. Understanding phenomena in this realm has necessitated the development of new simulation tools, and of new coarse-grain mathematical models to analyze and simulate. In that context, I'll discuss how a symmetry-breaking "swirling" instability of a motor-laden cytoskeleton may be an important part of the development of an oocyte, and what models of active, immersed polymers tell us about chromatin dynamics in the nucleus.   

Live

Elastic filaments and semiflexible polymers are ubiquitous in both biology and engineering, where they play a key role in cell mechanics and locomotion and are the constituents of many non-Newtonian fluids. In this talk, I will discuss the dynamics of elastic filaments in microscale flows, with focus on buckling instabilities that arise when viscous stresses overcome bending rigidity in the presence of thermal fluctuations. In simple shear flow, a series of morphological transitions is found to occur with increasing elastoviscous number, a dimensionless measure of flow strength relative to elasticity. The tumbling motion seen in weak flows and typical of rigid Brownian rods gives way to elastoviscous buckling followed by a tank-treading regime. We characterize these transitions using microfluidic experiments with F-actin along with numerical simulations and a reduced-order theoretical model, which provides a prediction for the onset of tank-treading with no fitting parameter. In uniaxial compressional flow, buckling also occurs above a critical flow strength and is found to give rise to helicoidal morphologies for high elastoviscous numbers. This phenomenon is also characterized in detail and explained using a weakly nonlinear model based on the post-buckling nonlinear interaction of linearly unstable planar deformation modes. I will conclude by discussing the implications of these instabilities for the rheology of elastic polymer suspensions, where the onset of buckling is found to enhance shear-thinning and normal stress differences.   

Live

The interaction of motile microorganisms and surrounding fluids is of importance in a variety of biological and environmental phenomena including the development of biofilms, colonization of microbes in human and animal bodies, and formation of marine algal blooms. Complex fluids are pervasive in microbial environments and can have profound effects on the motility of microbes, affecting fundamental microbial processes such as their ability to take up nutrients and colonize surfaces. In this study, we scrutinize the role of rheological properties of the surrounding fluid on the motility, spatial distribution, and collective behavior of microswimmers in complex fluids.