Hydrodynamic persistence within very dilute two-dimensional suspensions of squirmers

In this study, we consider suspensions of swimming microorganisms in situations where we might expect the promotion of two-dimensional flow, such as within thin fluid films. Given that two-dimensional, inertialess flows are notoriously long-ranged (although not afflicted by Stokes paradox in the case of self-motile bodies), this raises interesting questions around the care which must be taken with the semi-dilute assumption in such situations. Adopting the prototype squirmer as a model of a swimming microorganisms of the type previously considered, we find that although the flowfield decays algebraically with the characteristic separation distance between microorganisms, there remains a finite interaction between the squirmers even at asymptotically large distances. This finding is further borne out by asymptotic analysis, which confirms that the limiting form of the far-field interaction depends solely upon the relative orientation between the microorganisms. Those which swim in the same general direction are seen to experience very large lateral displacements (many times the size of the displacements experienced owing to interactions between less well-aligned swimmers). This clearly has potential implications for very dilute suspensions in which squirmers become broadly aligned in their swimming direction (e.g. during chemotaxis). We show that hydrodynamically enhanced cell spreading, previously reported for denser suspensions, can persist even at extreme dilutions. Moreover, we demonstrate that this induced spreading can continue in the presence of potentially decohering Brownian effects.R. J. Clarke, M. D. Finn and M. MacDonal