Viscophobic turning dictates microalgae transport in viscosity gradients

Abstract

Gradients in fluid viscosity characterize microbiomes ranging from mucus layers on marine organisms and human viscera to biofilms. While such environments are widely recognized for their protective effects against pathogens and their ability to influence cell motility, the physical mechanisms controlling cell transport in viscosity gradients remain elusive, primarily due to a lack of quantitative observations. Through microfluidic experiments with a model biflagellated microalga (Chlamydomonas reinhardtii), we show that cells accumulate in high viscosity regions of weak gradients as expected, stemming from their locally reduced swimming speed. However, this expectation is subverted in strong viscosity gradients, where a novel viscophobic turning motility - consistent with a flagellar thrust imbalance - reorients the swimmers down the gradient and causes striking accumulation in low viscosity zones. Corroborated by Langevin simulations and a three-point force model of cell propulsion, our results illustrate how the competition between viscophobic turning and viscous slowdown ultimately dictates the fate of population scale microbial transport in viscosity gradients.

Swimming sheet in a viscosity-stratified fluid

“…Additionally, when our analysis is extended for large viscosity gradients, the contribution of the viscosity gradient dominates the homogeneous contribution and our theory predicts accumulation of the swimmers in regions of low or high viscosity depending on the Péclet number corresponding to the transport of the stratifying agent. Stehnach (2019) observed that swimming biflagellates accumulate in the high-viscosity region for longer flagella lengths. For their experimental analysis, the length scale is , the velocity scale is and the diffusion coefficient is , which results in a Péclet number of .…”

“…(2018) recently suggested that the viscous torque experienced by the swimmer due to its asymmetric shape could be the cause of the preferential alignment. Stehnach (2019) showed that swimming biflagellates (wild-type Chlamydomonas reinhardtii ) have a tendency to accumulate in high-viscosity regions due to the local reduction in the swimming speed.…”

Active noise-driven particles under space-dependent friction in one dimension

“…In fact, colloids can be exposed to almost any arbitrary external potential by using optical fields [41][42][43] and almost any kind of noise can externally be programmed by external fields [44,45]. A spacedependent friction can be imposed be a viscosity gradient in the suspending medium on the particle scale, a situation typically encountered for viscotaxis [46][47][48][49].…”