Title: Droplet impact dynamics: floating on air and vapor

Lecturer: Dr. Chao Sun, Faculty of Science and Technology, University of Twente, the Netherlands

Time: September 25, from 15:30

Place: No. 1 Conference Room, Department of Thermal Engineering

Abstract:

A drop impacting on a solid surface deforms before the liquid makes contact with the surface. We directly measure the time evolution of the air layer profile under the droplet using high-speed color interferometry, obtaining the air layer thickness before and during the wetting process. Based on the time evolution of the extracted profiles obtained at multiple times, we account for the wetting mechanism and bubble entrapment. Additionally, entrapped bubble size as a function of the impact velocity is experimentally quantified and compared with theory and numerical simulations.

When a liquid droplet impacts on a surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surfaces (contact boiling), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (film boiling). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading of impacting droplets on the heated surfaces in film boiling regime shows a universal scaling with the Weber number We, which is steeper than for impact on non-heated (hydrophilic or hydrophobic) surfaces. This enhanced spreading factor is explained with a model. Furthermore, the effect of surface structures on impact dynamics will be discussed in this talk.