Evolution of Droplet Impact on Dry Surfaces with Different Surface Characteristics

Abstract

The temporal variations of the spreading diameter, in relation to the dynamic contact angle, are measured by a high speed camera while the droplet impacts onto dry solid surfaces of different surface characteristics. Immediately after impingement at high impact Reynolds numbers, there always exists a thin lamella already attached to the surface, leading to an acute initial contact angle. The dynamic contact angle increases dramatically and attains a maximum value before the maximum spreading diameter is reached. The dynamic contact angle decreases substantially toward the end of the receding stage; and will experience large fluctuation if the rebounding and the oscillating stages exist. The contact angle will eventually approach the equilibrium contact angle after completely wetted on the surfaces. During the kinematics stage, the spreading speeds are proportional to $( {\text{t}}^{*} )^{{{ - 0} . 5}}$. The spreading speed experiences a rapid and continuous deceleration and may be related to the non-regular variations of the dynamic contact angle before the maximum spreading diameter is reached. The receding speeds of the liquid film depend strongly on the surface characters. The more hydrophobic the surface is, the higher the receding speed of the liquid film.