Super-Resolution Optical Imaging of Depletion Layer Dynamics

The dynamics of complex fluids flowing across solid surfaces remains an outstanding problem in fluid mechanics that is relevant for fields including nanofluidics and lubrications. The formation of depletion layers at the boundary has significant implications for the flow of the bulk fluid. However, the inherently small length scales associated with the interfacial physics has traditionally inhibited direct imaging. We developed a novel adaptation of super-resolution microscopy based on stimulated emission depletion (STED) to provide a real-space measurement of depletion layer composition and dynamics of a polyelectrolyte solution under flow with spatial resolution on the order of 50 nm. The composition and dimension of depletion layers formed from poly(styrene sulfonate) at a non-adsorbing wall are measured both at equilibrium and under flow. We observe a concentration profile consistent with entropically driven depletion of the polymer from the interface. Furthermore, under Poiseuille flow at low to intermediate shear rates, we observe depletion layer narrowing coupled with a change in the composition of the depletion layer to nearly pure solvent.