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 …

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 centreline, in contrast to well-known boundary accumulation in quiescent Newtonian …