Heterogeneity-induced retraction in viscoelastic fluids following cessation of flow

Heterogeneous shear flows, where the microstructure and rheological properties of a flowing material vary spatially within the flow geometry, occur frequently in complex fluids. In shear flows of entangled wormlike micelles (WLMs), substantial flow heterogeneity results in the formation of shear bands, regions of fluid with distinct shear rates, microstructures, and viscosities. However, measuring flow heterogeneity is nontrivial and while methods for determining spatially-dependent microstructures in flow exist, quantifying spatially-dependent rheological properties remains an outstanding challenge. To address this challenge, cessation of flow is performed following shear startup on two polymeric WLMs with starkly different rheology, exhibiting gel-like vs. viscoelastic behavior. Flow is stopped at key times in the shear stress evolution. Velocity profiles are measured before and after the cessation of flow via particle tracking velocimetry (PTV). We show that cessation of flow induces retraction in viscoelastic fluids with flow heterogeneity, and that measuring retraction in cessation of flow can be used to determine spatially-dependent rheological properties. Subsequent simulations of cylindrical Couette flow using the Germann-Cook-Beris model qualitatively capture the experimental fluid retraction. These findings highlight the capability of cessation of flow paired with rheo-PTV to quantify flow heterogeneity in viscoelastic complex fluids, providing spatial information not accessed by other techniques.