Colloidal hydrodynamic interactions in viscoelastic fluids

The motion of suspended colloidal particles generates fluid disturbances in the surrounding medium that set up interparticle interactions. While such colloidal hydrodynamic interactions (HIs) have been extensively studied in viscous Newtonian media, comprehensive understanding of HIs in viscoelastic fluids is lacking. We develop a framework to quantify HIs in viscoelastic fluids with exquisite spatiotemporal precision by trapping colloids and inducing translation-rotation hydrodynamic coupling. Using solutions of wormlike micelles (WLMs) as a case study, we discover that HIs are strongly time-dependent and depend on the structural memory generated in the viscoelastic fluid, in contrast to "instantaneous" HIs in viscous Newtonian fluids. We directly measure "time-dependent" HIs between a stationary probe and a driven particle during transient start-up, developing on the WLM relaxation tim…