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Paper IPM / NanoSciences / 16996  


Abstract:  
We present a quantitative analysis on the response of a dilute active suspension of selfpropelled rods (swimmers) in a planar channel subjected to an imposed shear flow. To best capture the salient features of the shearinduced effects, we consider the case of an imposed Couette flow, providing a constant shear rate across the channel. We argue that the steadystate behavior of swimmers can be understood in the light of a population splitting phenomenon, occurring as the shear rate exceeds a certain threshold, initiating the reversal of the swimming direction for a finite fraction of swimmers from down to upstream or vice versa, depending on the swimmer position within the channel. Swimmers thus split into two distinct, statistically significant and oppositely swimming majority and minority populations. The onset of population splitting translates into a transition from a selfpropulsiondominated regime to a sheardominated regime, corresponding to a unimodaltobimodal change in the probability distribution function of the swimmer orientation. We present a phase diagram in terms of the swim and flow PÃ©clet numbers showing the separation of these two regimes by a discontinuous transition line. Our results shed further light on the behavior of swimmers in a shear flow and provide an explanation for the previously reported nonmonotonic behavior of the mean, nearwall, paralleltoflow orientation of swimmers with increasing shear strength.
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