Controlling Flow Patterns inside a Sessile Droplet using External Volatile Liquid*

Controlling flow patterns inside droplets has been the subject of many studies because it can be applied to mixing sample, uniform coating and particle separation. Usually, in droplet-based microfluidics, to control internal flows, an external energy is needed, e.g. acoustic, electric, and magnetic fields. Here, we show that when volatile liquid is placed next to the water droplet, vapor-driven solutal Marangoni effects are induced, which can create vortices inside a sessile droplet and mix samples without any active control device. Furthermore, we present that the total number of vortices (M) can be determined by the number of volatile components (N), i.e. M = 2N. The flow patterns are measured using particle image velocimetry. In this talk, physical arguments to support our experimental observations will be discussed.