Dissipation and microstructure in sheared active suspensions of squirmers

TL;DR

This study investigates how shear flow affects energy dissipation, viscosity, and microstructure in suspensions of squirmers, revealing complex rheological behaviors driven by activity and shear interactions.

Contribution

It provides new insights into the microstructural and rheological responses of active squirmers under shear, highlighting the roles of activity type and shear rate.

Findings

Shear increases total dissipation but decreases relative viscosity.

Pushers dissipate more energy than pullers at low shear rates.

Microstructure shows enhanced nematic order and anisotropic correlations under shear.

Abstract

We study the energy expenditure and structural correlations in semi-dilute to concentrated suspensions of squirmers using active fast Stokesian dynamics simulations. Specifically, we simulate apolar active suspensions of squirmers, or 'shakers,' and show that shear enhances the total dissipation but reduces the relative viscosity for both puller- and pusher-type shakers. At low shear rates where activity dominates, pushers dissipate more energy than pullers, and more so at higher volume fractions, in contrast to bacterial suspensions displaying a 'superfluid' transition. At high shear rates where shear dominates, pullers and pushers behave effectively as passive spheres, generating negative normal stress differences due to shear-induced collision. Remarkably, the rate-dependent rheological responses are accompanied by unusual microstructural signatures of an enhanced nematic order and anisotropic pair correlation, both of which contribute to a higher viscosity under shear. Further simulations of self-propelled, neutral squirmers exhibit similar but weaker shear-thinning, highlighting the importance of activity over motility, underpinned by hydrodynamic interactions. Overall, our results elucidate the interplay of internal activity and external flow on the dissipation and microstructure in sheared active suspensions of squirmers.