Session K02: Biological Fluid Dynamics: Flows in Fluid Films and Biofilms (8:45am - 9:30am CST)

Abstract Withdrawn

It is known that soil bacteria such as \textit{Pseudomonas aeruginosa }can spread on flat surfaces by generating a spatial gradient of biosurfactants. However, in addition to flat surfaces, natural soils and other porous materials often consist of corners or other narrow geometries. Corners with angles smaller than a critical angle are known to induce surface-tension-driven flows without bacteria, but little is known about bacterial transport in the presence of corners. Here we show that \textit{P. aeruginosa} at high cell density can generate flows at corners by producing biosurfactants. By visualizing fluorescent bacterial solutions in transparent prism-shaped chambers, we observed bacterial flows with speeds on the order of 1 cm/hour, similar to the speeds of the well recognized bacterial swarming. We show that the flows only occur at corners with angles smaller than a critical angle, which depends on the concentration of the biosurfactants. We further demonstrate that bacterial corner flow is induced by the corner geometry, not by surface tension gradients as suggested by previous studies. This new bacterial transport mechanism is likely widely present in soils and other niches with corners or similar narrow geometries.   

On the relationship between velocities, tractions, and intercellular stresses in the migrating epithelial monolayer

The relationship between velocities, tractions, and intercellular stresses in the migrating epithelial monolayer are currently unknown. Ten years ago, a method known as Monolayer Stress Microscopy (MSM) was suggested from which the intercellular stresses could be computed given a traction field. The core assumption of MSM is that the intercellular stresses within the monolayer obey a linear and passive constitutive law such as a Hookean solid or a Newtonian fluid. Due to the lack of independently measured intercellular stresses, a direct validation of the 2D stresses predicted by MSM is presently not possible. An alternative approach, which we give here and denote as the Stokes method, is based on simultaneous measurements of the monolayer velocity field and the cell/substrate tractions. Using the same assumptions as those underlying MSM, the velocity field suffices to compute tractions, from which we can then compare with those measured by traction force microscopy. We find that the calculated tractions and measured tractions are uncorrelated. From which it follows that some serious modification of the underling rheology is needed. We will discuss a number of alternatives including the contribution of active stresses for which we have derived a novel constraint.   

A hydrodynamic instability drives protein droplet formation on microtubules to nucleate branches.

Liquid-liquid phase separation occurs not only in bulk liquid, but also on surfaces. In physiology, the nature and function of condensates on cellular structures remain unexplored. Here, we study how the condensed protein TPX2 behaves on microtubules to initiate branching microtubule nucleation, which is critical for spindle assembly in eukaryotic cells. Using fluorescence, electron, and atomic force microscopies and hydrodynamic theory, we show that TPX2 on a microtubule reorganizes according to the Rayleigh-Plateau instability, like dew droplets patterning a spider web. After uniformly coating microtubules, TPX2 forms regularly spaced droplets from which branches nucleate. Droplet spacing increases with greater TPX2 concentration. A stochastic model shows that droplets make branching nucleation more efficient by confining the space along the microtubule where multiple necessary factors colocalize to nucleate a branch.   

Instability of a chemotactic thin-film bacterial suspension

We formulate a thin-film flow driven by active stresses arising from the anisotropic orientation distribution of swimmers present in a chemotactic suspension. The film is found to be unstable to perturbations in the long-wave $(k<<1)$ regime and we identify two mechanisms associated with the instability. The hydrodynamic field of a pusher type swimmer works to reinforce perturbations to the density field and generates a mode of instability which has previously been predicted by Kasyap and Koch [2014]. In addition to this mode of instability, we find that the active stress allows for a jump in the viscous stress across the interface that is reminiscent of marangoni stresses in films, which drives a flow enhancing the interface perturbation. The perturbation of solely the interface or the density field is shown to be unconditionally stable for a suspension of pullers, while in the case of pushers, there exists a critical activity $(\beta)$ for the film to destabilize. However, the coupled system exhibits an instability for a suspension of pullers, wherein the marangoni like stresses at the interface work to reinforce the density perturbation. The stability characteristics of the system are further probed in the finite wave number regime.