Droplet oscillation mechanism and its free surface behavior on impacting a heated hydrophobic surface at low Weber numbers

Oscillation mechanism and free surface behavior of micro-droplets (300, 400, and 500 µm in diameter), impinging on a dry, hydrophobic heated surface was investigated using the volume of fluid (VOF) method. It was observed that the droplets exhibit unique oscillation characteristics at low Weber numbers for different contact angles and surface temperatures. At higher contact angles, droplet deformation was retarded, and it tends to retain its shape. This increased its chances of bouncing away from the surface. With increasing surface temperature, the tendency of the droplet to bounce away from the surface also increased. The oscillation behavior was significantly influenced by droplet size, contact angle with the surface and by surface temperature. Larger sized droplets had lower oscillation frequency. An increase in contact angle or surface temperature led to the lowering of the oscillation frequency of droplets. However, droplet Weber number was found to have no significant effect on oscillation frequency. At these low value of Weber numbers, all droplet sizes investigated showed slightly under-damped harmonic motion with amplitude exponentially decaying toward zero.