Characterisation and control of the dynamical properties of swimming microorganisms under confinement

• Main Author: Contino, Matteo
• Published: University of Warwick 2017
• Subjects: 530; QR Microbiology
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.737707

Due to their importance for scientific and technological purposes, interactions between swimming microorganisms and solid boundaries is raising a growing interest in the scientific community but, despite recent efforts, many of their aspects are still unknown. In this thesis we investigate through experiments carried out in PDMS microfluidic devices and simulations the effect that the presence of solid walls produce on the motility of a well known puller-type microswimmer Chlamydomonas reinhardtii (CR). We present the study of the scattering of CR against circular pillars in a microfluidic device. Exploiting the curvature of the pillar surface, we are able to determine the contribution of hydrodynamic and contact forces to the scattering interaction. We find that lubrication forces tend to make algae swim parallel to the pillar surface and flagellar contact break this entrapment, making CR to scatter away at a fixed angle determined by the ratio between flagellar and body length independently of their original swimming direction. We also report the existence of random scattering, which, as the name suggests, results in an outgoing trajectory probability uniformly distributed across all possible directions. We find that these interaction rules also apply flat and concave surfaces. Through these results we are able to describe the effect of geometrical confinement in narrow straight channels and circular pools. We find that in channels it is possible to change the cellular transport efficiency simply by varying the distance between channel walls and we discover the existence of an optimal width for the transport of wild type CR. We also describe the escape dynamics of cells from circular pools through narrow apertures; once again we find that the attraction to solid boundaries is essential in describing the results, as cells can find the exit on the perimeter by sliding out of the pools while swimming along its perimeter.