Interactions of free-swimming motile cells with surfaces*

Motile microorganisms frequently encounter and interact with solid surfaces. The physical mechanisms governing these cell-wall interactions are the subject of ongoing work. Elucidating the dynamic principles that govern such interactions is crucial in understanding biological processes such as bacterial colonization and biofilm formation. Previous work suggests that both hydrodynamic and steric forces play a role, but their relative importance remains an open question. Most experiments thus far have characterised cell motility and surface interactions in a two-dimensional environment, where the motion of the microorganisms is constrained between two parallel solid surfaces. Here, we track a suspension of motile cells in a relatively unconstrained three-dimensional domain, with a novel single-objective multi-camera three-dimensional tracking technique. Both puller (C. reinhardtii) and pusher (A. carterae) microswimmers of comparable dimensions are considered, and their interactions with a solid boundary characterized. Their statistical behaviour in the presence of walls is discussed as it relates to existing hydrodynamic theories and evidence from two-dimensionally constrained experiments.