Mesoscopic Contact Dynamics of Emulsions: Influence of Droplet Size on Nanoindentation Behavior.

The complex interactions with viscoelastic materials in soft contact pose a challenge because effects that are often neglected in classical contact theories─such as viscosity, capillary forces, and surface stress─become significant. Here, we present a new rheometric approach combining ultrahigh-resolution nanoindentation with optical visualization to probe mesoscopic contact dynamics between a rigid spherical punch and concentrated oil-in-water emulsions with lamellar structures. Four emulsions with varying droplet sizes were prepared by adjusting the surfactant and alcohol concentrations. The results revealed two distinct contact regions: an oil-rich phase with force transmission pathways at the center of the indentation zone and a solvent-rich phase near the contact line. The former obeyed the conventional Johnson-Kendall-Roberts (JKR) theory, and the corresponding elasto-adhesion parameter increased with decreasing median droplet diameter. Additionally, emulsions with a larger elasto-adhesion parameter exhibited a short-range poroelastic process, consistent with gel-like microstructural characteristics observed in dynamic frequency sweeps. These findings provide new insights into the mesorheology of concentrated emulsions and highlight the utility of nanoindentation in probing their mesoscopic contact mechanics.