Alladi Ramakrishnan Hall

Enhanced velocity fluctuations in suspensions of interacting micro-swimmers

Sankalp Nambiar

Nordita, Stockholm, Sweden

Numerous experiments and simulations have shown that suspensions of rear-propelled microscopic swimmers such as bacteria E. coli and B. subtilis can exhibit long-ranged orientational order, enhanced tracer particle diffusivities as well as large-scale collective and coherent motion. Simulation of straight-swimmer suspensions have moreover revealed an interesting logarithmic divergence of the fluid velocity variance on account of hydrodynamic interactions. Motivated by these, in this talk I will present an analytical characterization of the fluid velocity covariance in a suspension of hydrodynamically interacting straight-swimmers and run-and-tumble swimmers. Modeling the swimmers as slender bodies, I will first briefly introduce the analytical framework for dilute non-interacting swimmer suspensions. Thereafter, I will highlight how pair-interactions lead to divergent fluid velocity fluctuations in a suspension of straight swimmers on account of slowly decaying pair-correlations. Next, I will elucidate how incorporating an orientation decorrelation mechanism for the swimmers in the form of run-and-tumble motion leads to the said divergence being cut-off, and how it modifies the covariance. The model we have developed treats swimmers of arbitrary run lengths, and both rear- and fore-propelled swimmers (pushers and pullers), the characteristics of which will be explained in the talk.