Effect of a rupturing encapsulated bubble in inducing the detachment of a drop.

Droplet-based microfluidics is inherently based on the ability to control the motion of liquid drops. In most situations, drops are required to be controlled individually. Here, we examine how the rupture of an encapsulated bubble causes the detachment of a drop previously pinned on an incline. When the drop is located on a horizontal surface with a low liquid-solid adhesion energy (such as water on a superhydrophobic surface), the entire drop is propelled vertically off the surface without the input of an external energy source. From an energy balance, we determined that the majority of the stored surface energy is consumed by the formation of a large jet. When a surfactant is introduced into the system, the adhesion energy is then too large to overcome, resulting in a pinned oscillating drop. We also show that the process can be used to selectively cause drops to slide (at usually stable inclines) on a hydrophobic surface. The required sliding angle was decreased by almost 20° for a 48 μL water drop and a 10 μL bubble. This process enables the selective pinning and depinning of drops, a method that may prove useful for future droplet control techniques.