Effects of droplet oscillation on the aerodynamic behavior of sessile droplets in shear flows

An experimental study was conducted to investigate the influence of droplet oscillation on the aerodynamic characteristics of sessile droplets on hydrophilic and hydrophobic surfaces. A hot-wire anemometer, a high-resolution high-speed camera, and particle image velocimetry (PIV) were employed to quantify the flow characteristics in the symmetry plane of both droplets and solid replicas, thereby isolating the effect of droplet oscillation on the surrounding flow fields. Since the solid replicas do not oscillate while the droplets exhibit significant oscillation, the observed differences in the flow field can be attributed to droplet oscillation. Periodic velocity fluctuations were observed in the wake of oscillating droplets, particularly in the shear layer, with a frequency matching that of the vertical droplet oscillation. In contrast, such fluctuations were absent in the case of solid replicas, indicating that droplet oscillation substantially alters the instantaneous flow structure in the wake. PIV results further revealed that droplet oscillation also affects the time-averaged flow structure in the symmetry plane. A notable difference is the shortened recirculation region behind oscillating droplets, suggesting that oscillation hinders flow separation growth and suppresses the formation of a large-scale wake region. Additionally, droplet oscillation destabilizes the shear flow originating from the droplet apex, leading to a surge in turbulence intensity closer to the droplet surface. The aerodynamic drag was estimated using a wake integration method applied to data from the droplet symmetry plane. Results show that although droplet oscillation modifies both the instantaneous and time-averaged flow fields, it does not significantly affect the time-averaged aerodynamic drag, even under varying droplet configurations and freestream velocities. For the widely studied case of wind-driven droplets, this finding shows that oscillation is negligible in time-averaged force analysis. The measurement uncertainty for the aerodynamic drag was carefully evaluated to be below 8.9%.