Visualization study on coalescence between pair of water drops on inclined surfaces

Visualization experimental investigation on the phenomena and dynamic behavior of coalescence between two equal-size water drops on inclined surfaces is reported in this paper. The effects of inclination angle of the surface and drop size on the dynamics of liquid bridge, fore/back contact angle and triple-phase contact line of the coalescent drop are discussed. The results show that highly concave interface around the liquid bridge and highly convex interface at the retracted top-side parts of two drops built surface tension gradient to drive the interior liquid flow in early stage of the coalescence, while changeable surface tension gradient between highly convex interface at the forepart and near flattened interface at the back part of the coalescent drop and adverse gravity induced the interior flow and the oscillation process later. Coalescence promoted the motion of the coalescent drop on the inclined surface. The released surface energy and potential energy compensated the internal viscous dissipation and the friction, and the fraction for friction increased with increasing inclination angle and drop size. For coalescence of small drops on the inclined surface, the triple-phase contact line, liquid bridge radius and fore/back contact angle of the coalescent drop behaved as periodic damped oscillations. Larger inclination angle of the surface induced severe oscillations for the displacement of back-end and fore contact angle of the coalescent drop while subdued oscillations for the displacement of front-end and back contact angle as well as higher frequency and smaller amplitude for liquid bridge radius. For large drops coalescence, the displacements of front-end and back-end monotonously increased, the liquid bridge radius reduced rapidly without oscillation after passing through a peak value, and the fore/back contact angle increased/decreased quickly with slight oscillation to reach a stable value.