Dislodging a sessile drop by a high-Reynolds-number shear flow at subfreezing temperatures.

Abstract

The drop, exposed to an air flow parallel to the substrate, starts to dislodge when the air velocity reaches some threshold value, which depends on the substrate wetting properties and drop volume. In this study the critical air velocity is measured for different drop volumes, on substrates of various wettabilities. The substrate initial temperatures varied between the normal room temperature (24.5∘C) and subfreezing temperatures (-5∘C and -1∘C). The physics of the drop did not change at the subfreezing temperatures of the substrates, which clearly indicates that the drop does not freeze and remains liquid for a relatively long time. During this time solidification is not initiated, neither by the air flow nor by mechanical disturbances. An approximate theoretical model is proposed that allows estimation of the aerodynamic forces acting on the sessile drop. The model is valid for the case when the drop height is of the same order as the thickness of the viscous boundary in the airflow, but the inertial effects are still dominant. Such a situation, relevant to many practical applications, was never modeled before. The theoretical predictions for the critical velocity of drop dislodging agree well with the experimental data for both room temperature and lower temperatures of the substrates.