Dynamics of a successive train of monodispersed millimetric-sized droplets impact on solid surfaces at low Weber number

Abstract

An experimental study of successive impingement of a single train of monodispersed millimetric-sized droplets at low Weber number (We < 200) is presented. A piezoelectric droplet generator which is capable of adjusting droplet impact frequency and droplet diameter is fabricated and used to produce monodispersed millimetric-sized droplets. The interaction phenomena between the droplet-surface and droplet-film were observed using a high-speed camera at 4000 fps. The variation of the dimensionless spread factor and flatness factor of the liquid film formed by the successive impacts of droplets over time were recorded. The impact frequency of droplets was varied between 40 Hz and 85 Hz. The Weber number ranged from 29.6 to 196.6. And three types of solid surface with different static contact angles (71.5°, 82.7°, 90.1°) were tested in this work. The results showed that the impact frequency of droplets in the selected range had a minor effect on the maximum spread factor and post-impact oscillation. The maximum dimensionless spread factor of the liquid film formed by the successive impacts at the same dimensionless time tends to be greater when Weber number was higher and static contact angle was lower. The post-impact oscillation was more violent when Weber number was lower and static contact angle was larger. Empirical correlations for the maximum spread factor and average flatness factor under different conditions were obtained based on the experiment results.