Upstream Swimming in Microbiological Flows

Abstract

Interactions between microorganisms and their complex flowing environments are essential in many biological systems. We develop a model for microswimmer dynamics in non-Newtonian Poiseuille flows. We predict that swimmers in shear-thickening (-thinning) fluids migrate upstream more (less) quickly than in Newtonian fluids and demonstrate that viscoelastic normal stress differences reorient swimmers causing them to migrate upstream at the centerline, in contrast to well-known boundary accumulation in quiescent Newtonian fluids. Based on these observations, we suggest a sorting mechanism to select microbes by swimming speed.

Emergent propagation modes of ferromagnetic swimmers in constrained geometries

“…26,27 However, biological swimmers continue to face parallel to their swimming direction as they are deflected by the surface, so do not experience a change in their propagation mode. The emergence of a new propagation mode in the ferromagnetic swimmers is due to differences in propulsion mechanisms.…”

“…However, biomedical and microfluidic applications will require swimmers initially moving in bulk fluids to interact with surface barriers, such as the cell membrane, blood vessel wall or the sides of a microfluidic chamber. Such considerations have been studied with regard to understanding biological low Reynolds swimmers, 26,27 but the implications for efficiency and functionality of artificial swimmers has received little attention. Here, we demonstrate that the magneto-elastic swimmers can move either in an unconstrained fluid, near a surface or in a channel, but that the proximity of surface barriers results in changes in the swimming propagation mode.…”

Force moments of an active particle in a complex fluid

“…The effect of fluid rheology on a single active particle with general deformation, or slip boundary conditions is far from understood [8,25,26]. The motion of two active particles in quiescent Newtonian fluids was recently studied by means of the reciprocal theorem, both for diffusiophoretic particles [27], and squirmers [28], likewise hydrodynamic interactions between two passive spheres in weakly nonlinear fluids [24,29], but hydrodynamic interactions amongst two (or many) active bodies in non-Newtonian fluids remain largely unexplored and can lead to quantitatively different dynamics [30]; the same can be said of active particles in non-Newtonian background flows [31]. If the particles are Brownian, fluid rheology can lead to significant differences in observed trajectories [32] and while it is possible to include a stochastic force in the above theory, one requires proper description of that forcing in nonlinear non-Newtonian flows [33,34].…”

Emergent behavior in active colloids

“…active particles do not cluster at specific lateral positions or focus in flow. In contrast, in the presence of swimmer-wall hydrodynamic interactions [210], bottom-heaviness [213], phototaxis [214,215], flexible body shape [216], or viscoelastic flows [217], the dynamics becomes dissipative in the sense of dynamical systems and particles aggregate at specific locations in the flow (see also the discussions in [211,218,219]). Although swinging and tumbling trajectories in microchannel Poiseuille flow have been observed experimentally with biological microswimmers [212,220], they have not yet been confirmed in experiments with active colloids.…”