Probing the dynamic surface deformation of droplet deposition on soft substrates using an interferometric nanostrain sensor*

Sessile droplets on soft substrate produce a distinct surface deformation consisting of a dimple due to Laplace pressure and a wetting ridge formed by surface tension. While many discoveries have been made on static soft wetting behavior, non-equilibrium soft wetting is much less investigated, especially the wetting process when the droplet first contacts the substrate. In this study, an interferometric nanostrain sensor is used to measure the dynamic surface deformation of droplets impinging on hydrophobic soft substrate. Three different stages are investigated: (A) initial droplet impingement onto the surface with a pipette tip, (B) formation, and (C) breaking of the liquid bridge. In Stage A, we observed a repeating ‘stick-growth, slip-shrink’ phenomenon whereby both the dimple and ridge height increased during contact line sticking and decreased during contact line slipping. In Stage B, a steady decrease for both the dimple and ridge height is observed when the liquid bridge is formed. Lastly, bridge breaking induced a large oscillation in the surface deformation that lasted for about 40 ms, indicating possible capillary wave interactions with the soft substrate. Comparative observations of drop wetting over a hydrophilic soft substrate will also be reported.