Quantification of dynamic water droplet impact onto a solid surface by using a digital image projection technique

In the present study, an experimental investigation was conducted to quantify the dynamics of the water droplet impact process on a solid surface in order to elucidate underlying physics to improve our understanding of the important microphysical processes pertinent to aircraft icing phenomena. The Reynolds number (Re) and Weber number (We) of the water droplets ranged from Re ≈ 1700 to 4900, and We ≈ 29 to 216, respectively, while all the experiments were done at a room temperature of T_ambinet=21°C. Digital image projection (DIP) was used to achieve time-resolved measurements of the water film thickness distributions during the entire impact process. Based on the time-resolved measurements of the water film/droplet thickness distributions, the droplet impact process was divided into three distinct stages, i.e., the spreading, receding, and oscillating stages. By comparing the droplet shape evolution under different impact velocities, the dynamics of droplet impact process at different Weber numbers or Reynolds numbers was analyzed in detail. With increasing impact velocity, the oscillation phenomenon would disappear rapidly, and thus the droplet impact process was shortened. Based on the time-resolved film thickness measurements, the liquid-air interface area was calculated, thus, the time evolution of the surface energy of water droplet during the entire impact process could be examined quantitatively.