Wall shear rates generated during coalescence of pendant and sessile drops

Abstract

Coalescence is a process in which two or more drops contact each other and merge to form a single daughter drop. The process may occur in a fluid medium or on a solid surface. In both instances, coalescence is an intense dynamic process during which the fluid is momentarily set into motion. The present study compares the shear rates, and hence, shear stresses generated on the wall because of the coalescence of two drops in pendant and sessile configurations. In experiments, two drops of equal volume are placed adjacent to each other till a liquid bridge is formed with the drops just touching each other. The equilibrium contact angle considered is 110°. The liquid is water placed on and underneath a textured surface. Coalescence is driven by the negative curvature of the liquid bridge, leading to a pressure difference. The contact line moves and bridge relaxes as flow takes place from a region of higher to lower pressure. The entire process has been imaged by using a high speed camera. The image sequence is analyzed to find the instantaneous center of mass of the drop, which in turn, yields the velocity components. These are used to find the time-dependent wall shear rates. Experiments show that two timescales appear during the merging of the drops. Large shear stresses are momentarily developed at the wall with a magnitude that depends on the drop volume. Oscillations in the drop shape are stronger in the pendant configuration where gravity is an additional driving force.