Dynamics of Confined Microgel Liquids between Compliant Surfaces

While much research has strived to understanding the dynamics of confined liquids by rigid solid surface, the behaviors of liquids confined between compliant surfaces remains unclear. In this work, we employed poly(N-isopropylacrylamide) (PNIPAM) microgels of varied crosslinking density as the confined liquids as well as confining surface coatings. We analyzed the packing structure and mobility of confined PNIPAM microgel particles between PNIPAM coated surfaces of varied particle-to-surface compliance ratio and gap spacings. We observed that confining surfaces of low compliance show much weaker effect on the dynamics of confined microgel particles of varied elasticity, where confined microgels at the gap spacing approximately smaller than 10 particle layers exhibit little deviation from their bulk behaviors. Intriguingly, we observed that no confinement effect is observed when the compliance of microgel particles is equal to or comparable to that of confining surfaces. Hence,commensurate compliance between confining liquid and surface could give new insight to design low-friction lubricants and lubricious coatings.