Effects of surface wettability on the aerodynamics of wind-driven droplets at the verge of shedding

An experimental study was conducted to investigate the time-averaged aerodynamics of sessile droplets at the verge of shedding on hydrophilic and hydrophobic surfaces. A high-resolution particle image velocimetry system was used to measure/reconstruct the velocity and pressure fields in the droplet symmetry plane and obtain the time-averaged aerodynamic loading. It was found that the stagnation angle (the angle bounded by the substrate and the ray emanating from the droplet center connecting to the stagnation point) decreases with decreasing contact angle due to the shrinking size of the horseshoe vortex. The air pressure reaches the maximum near the stagnation point and its minimum near the droplet apex where flow separation occurs. In the near wake of droplets, a recirculation region, where the velocity reduces to nearly zero and the pressure is low, is generated due to the flow separation. The normalized length of the recirculation region decreases with increasing contact angle since droplets with higher contact angles need flows with lower Reynolds number to reach the point of shedding. In addition, the aerodynamic drag over droplets was evaluated by the wake integral method, analyzing the contribution of momentum deficit, Reynolds stress, and pressure deficit. The drag coefficient of the droplets, at the verge of shedding, was independent of the contact angle. This work shows that the drag coefficient of droplets with different contact angles at the verge of shedding can be similar even though the droplet shape, Reynolds number, and flow structures are different.