Depletion Layer Dynamics of Polyelectrolyte Solutions under Poiseuille Flow*

The flow of complex fluids over solid surfaces remains an outstanding problem in fluid mechanics that is relevant for fields ranging from lubrication to nanofluidics. Direct experimental access to depletion layer dimension and composition has been prohibited due to the inherently short length scales associated with depletion layers. Here, we develop a novel adaptation of super-resolution microscopy based on stimulated emission depletion (STED) to directly measure depletion layer composition in real- space with 10s of nanometer resolution. The composition and dimension of depletion layers formed in solutions of high molecular weight poly(styrene sulfonate) at solid, non-adsorbing walls is measured at equilibrium and under Poiseuille flow. Using this novel approach, we 1) confirm concentration profile consistent with entropically driven depletion at the interface, 2) observe depletion layer narrowing at low to intermediate shear rates, and 3) observe depletion layer composition that approaches pure solvent at unexpectedly low shear rates.