Coalescence dynamics of sessile and pendant liquid drops placed on a hydrophobic surface

Experiments have been conducted in which two small water drops are placed on a chemically textured hydrophobic surface (apparent contact angle similar to 150 degrees), either in sessile or in pendant modes, just touching each other, under atmospheric conditions. Equal and unequal drop volumes have been studied, the Bond number of the combined drop falling within 0.01-0.04. The resulting coalescence process has been imaged by a high speed camera, till the combined drop reaches equilibrium. The position of the center of mass of the combined drop is determined by digital image processing of image sequences, with displacement yielding the velocity components. The centroid displacement data show that two time scales describe the harmonic content of flow oscillations. These are related to the high initial flow velocities generated, followed by the viscous relaxation of the fluid at later times. Scale analysis in terms of force pairs and energy components delineates experimental trends in velocity and wall shear stress. Shear stresses are momentarily developed at the wall at the short time scale, with its magnitude depending on the drop volumes and orientation. These are smaller in the pendant mode compared to the sessile. However, they are estimated to be large enough to overcome the yield stress of commercially used hydrophobic coatings. Published by AIP Publishing.