Fine-tuning the dispersion of active suspensions with oscillatory flows.

The combined effect of axial stretching and cross-stream diffusion on the downstream transport of solute is termed Taylor dispersion. The dispersion of active suspensions is qualitatively distinct: viscous and external torques can establish non-uniform concentration fields with weighted access to shear, modifying mean drift and effective diffusivity. It would be advantageous to fine-tune the dispersion for systems such as bioreactors, where mixing or particle separation can improve efficacy. Here, we investigate the dispersion of active suspensions in a vertical channel driven by an oscillatory pressure gradient-Womersley flow-using gyrotactic swimmers (bottom-heavy cells subject to viscous torques). Preliminary experimental results reveal interesting dispersion phenomena that are highly dependent on the oscillation parameters, motivating theoretical investigation. Employing Lagrangian simulations, we find that oscillatory flows can induce drift and increase lateral and downstream dispersion, with periodic mixing between left and right sides. Such flows can also be used to separate species with different motile behaviour. Eulerian numerical schemes typically require an approach to averaging in orientational space, such as generalized Taylor dispersion (GTD), with assumptions on translational and rotational time scales. For an oscillatory time scale commensurate with cell dynamics, we reveal the limitations of such approximations, beyond which the averaging techniques collapse.This article is part of the theme issue 'Biological fluid dynamics: emerging directions'.