Effects of Surface Wettability on the Aerodynamics of Wind-Driven Droplets at the Verge of Shedding

An experimental study was conducted to investigate the aerodynamics of sessile droplets at the verge of shedding on hydrophilic and hydrophobic surfaces. A high-resolution particle image velocimetry (PIV) system was used to measure the velocity field and pressure distribution 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 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 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 droplets, at the verge of shedding, was independent of contact angle. This work shows that the drag coefficient is similar for droplets with different contact angles at the verge of shedding even though the droplet shape, flow Reynolds number and flow structures are different.