Viscous Effects in Collision Outcomes of a Falling Drop Impinging on a Sessile Drop

A growing number of technologies rely on the collision outcomes of falling liquid drops colliding with sessile drops. Applications include rapid additive manufacturing, novel electronics packaging approaches, spray cooling of electronics, coatings, inkjet printing of polymers, and tissue engineering. These applications incorporate drops sequentially deposited onto a partially wetting substrate, and the conditions under which the deposition occurs may lead to varying drop placement. The drop placement outcome investigated here occurs at relatively low impact speed, and the result of the collision may either be that the colliding drops coalesce into a single larger drop, or that the falling drop bounces off of the sessile drop. Very different physical mechanisms may lead to these two results, and the focus of this investigation is the effect of drop viscosity. Collision experiments were performed with drops having comparable properties to water in air, except the viscosities were one and three orders of magnitude greater than water. The sessile and falling drops had volumes of either 3μL or 6 μL. The collision phenomena were visualized with highspeed imaging, and a map of the collision outcomes was presented.