Evolution of entrapped air under bouncing droplets on viscoelastic surfaces

We developed a technique based on total internal reflection to observe the entrapped air under an impacting droplet and provided experimental evidence of the formation of air film under the rebounded droplet on soft PDMS surfaces. Within a range of impact velocities, it was observed that the air trapped between the droplet and surface formed a thin film during the impact due to the shear-thinning property of the surface. The thin film reduced the surface energy and led to the rebound of the droplet. For sufficiently high impact velocities, the high kinetic energy of the droplet generated an air cavity and a bubble was entrapped inside the droplet. The bubble then coalesced with and released the air film underneath the droplet such that the surface energy became high and the rebound of the droplet was prohibited.