Nanoscale Optical Imaging of Soft Matter Under Flow*

Far-field fluorescence microscopy provides an enticing platform for spatially resolving equilibrium and nonequilibrium dynamics of soft matter systems, ranging from thin films to polymer solutions. However, several long-standing challenges, including limited spatial resolution and poor image contrast in densely labeled systems, have limited its application to such systems. Here, we develop a novel adaptation of super-resolution microscopy based on stimulated emission depletion (STED) coupled with fluorescence anisotropy detection to image concentration profiles of high molecular weight polyelectrolytes in semi- dilute solutions. We use this technique to study depletion layer formation and dynamics under Poiseuille flow, which remains an outstanding problem in fluid mechanics. While the inherently short length scales associated with depletion layer dynamics have traditionally prohibited direct imaging, STED-anisotropy imaging provides sufficient spatial resolution and contrast to measure small changes in polymer concentration over 10s of nanometers.