Role of Friction in Microswimmer and Active Filament Motion

Abstract

Friction is pervasive in all fields of science. It is the key factor that emerging technologies involving autonomous motion at micron scales such as micro-bots need to overcome in order to be efficient. They fall in the new paradigm of active matter which in its scope also covers the biological world. In this talk, I will present my thesis work on the role of frictional stresses in the motion of two model biological systems: (a) a microswimmer, Chlamydomonas which swims through the fluid by using the motion of its two anterior flagella/cilia, and (b) an active filament, the cell-free isolated cilium from the same microswimmer and reactivated in the presence of an external energy source. These prototypical active systems, driven by oscillatory motion of the cilia, generate fluid motion at the micron-scale. Naturally, these systems operating in low Reynolds number regime are expected to be governed by the ambient fluid friction. We, therefore, explore the role of hydrodynamics and other sources of friction, if any, in these model systems through simultaneous measurements of their motion and flow fields. In the first part of my thesis, I discuss the role of confinement in coupling cell motility and fluid flow of the microswimmer. Extreme confinement of this swimmer between rigid boundaries often arises in natural and technological contexts, yet measurements of its mechanics in this regime are absent. We show that strongly confining Chlamydomonas between two parallel plates not only inhibits its motility but also leads, for purely mechanical reasons, to inversion of the surrounding vortex flows due to contact friction with the walls. This contrasts with expectations based on the source-dipole description of confined swimmers. Insights from the experiment lead to a simplified theoretical description of flow fields based on a quasi-2D Brinkman approximation to the Stokes equation than the usual method of recursive images. We argue that this vortex flow inversion provides the advantage of enhanced fluid mixing despite higher friction. In the second part of the thesis, I study the mechanics of the active cilium which undergoes spontaneous oscillations by continuously consuming chemical energy and dissipating them through mechanical motion. Therefore, stable oscillations require that the elastic stresses due to the active energy input must be balanced by a significant source of dissipation. Conventionally, it stems from the external fluid. We show, in contrast, that external fluid friction is negligibly small to counteract the passive elastic stresses within the isolated and active Chlamydomonas cilium beating near the instability threshold. Consequently, internal friction emerges as the sole source of dissipation for ciliary oscillations. We combine these experimental insights with theoretical modeling of active filaments to show that an instability to oscillations takes place when active stresses are strain softening and shear thinning. Together, these results demonstrate that it is not always the external fluid friction, but friction from the external boundaries as well as internal degrees of freedom, which govern confined microswimmer motion and active ciliary oscillations, respectively.