(411c) Spatiotemporal Elastohydrodynamic Deformation of Supported Elastomer Films

The repulsive hydrodynamic forces present when two objects are brought closer in a fluid environment can cause out-of-contact elastohydrodynamic deformation of soft materials or materials with compliant coatings beyond those of surface asperities. An important challenge in studying the coupling between a material’s elasticity and viscous forces has been the difficulty to measure simultaneously both the hydrodynamic interactions and the local deformation. Here we characterize both the hydrodynamic force and the out-of-contact spatiotemporal deformation profile of a soft elastomer coating caused by normal viscous forces. We explore both the limit where the deformation (w) is small and large compared to the local fluid film thickness (x), measuring values of w/x that vary by three orders of magnitude. We use lubrication theory coupled with linear elasticity and viscoelasticity to predict the full spatiotemporal surface profile and hydrodynamic forces. In general we observe good agreement between the experiments and predictions with the main source of error originating from the effective stiffening due to the underlying rigid substrate. The layering effects due to the underlying substrate is more pronounced than for indentation at the same applied load; even when the interacting surfaces are out of contact and in the limit where the deformation radius is small compared to the film thickness.  We attribute the enhanced effect of geometric stiffening in EHD to the fact that the applied load due the hydrodynamic forces is more concentrated than in the case of indentation performed in typical JKR experiments. Finally, we characterize the elastic relaxation when the surfaces stop moving and find good agreement with predictions that were derived for droplets.