Effect of Surface Stiffness on the Interfacial Dynamics of Dense Microgel Liquids

Many biological and engineering processes involve highly soft and deformable surfaces, ranging from biolubrication between synovial joints to aquaplaning of rubbery tires. In this talk, I will present our recent study of the effect of surface stiffness on the glassy dynamics of dense microgel liquids. Specifically, we investigate the interfacial dynamics of poly(N-isopropylacrylamide) (PNIPAM) microgels, whose particle stiffness can be tuned by polymer crosslinking degree upon polymerization, at a solid surface coated with PNIPAM microgels of matched or mismatched stiffness. By using confocal laser scanning microscopy, we analyze the mean-squared-displacement (MSD) of PNIPAM particles in the first 1-2 confined layers near the coated surface. The MSD shows strong dependence on surface stiffness and particles tend to approach the Brownian motion at the softest surface even at the microgel volume fraction approaching the one for glass transition. The correlation between dynamic heterogeneity and friction of confined dense PNIPAM suspensions is further examined with varied particle volume fraction and particle-to-surface stiffness ratio.