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Enhanced hydrodynamic diffusion in dense binary active suspensions

Zhouyang GeShervin BagheriGwynn J. Elfring
Nov 2023
Active suspensions, fluids loaded with energy-injecting microswimmers such as cells or microrobots, have potential applications in diverse areas ranging from energy to sustainability and health. Here, we study the effect of incorporating passive particles into a dense suspension of active ones. Specifically, we focus on individually immotile active particles, or shakers, whose collective dynamics arise from the activity-induced many-body interactions. In the absence of Brownian motion, both active and passive particles fluctuate vigorously in space, displaying pronounced hydrodynamic diffusion. The rotational diffusivity of all particles decreases monotonically with the amount of passive ones; however, surprisingly, the translational diffusivity is non-monotonic and can be significantly enhanced. This suggests a hydrodynamic coupling of the two dynamics, which we find are attributed to a local ordering of the active particles under their hydrodynamic interactions. Using a toy model where passive particles are treated as defects in a lattice of aligned force dipoles, or stresslets, we reveal a simple mechanism by which spatial correlations and inactivity give rise to enhanced diffusion. Our results uncover an unexpected effect of passive particles on the dynamics of active suspensions, providing new possibilities to enhance mixing and control mass transport in active fluids.
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