Passive mixing enhancement of microliter droplets in a thermocapillary environment

Fast and efficient mixing of reagents and bioassays is of great importance in micro total analysis systems and in particular, open-surface microfluidics. This is the first study of droplet collision and mixing phenomenon of levitated droplets on an immiscible thin liquid substrate undergoing Marangoni convection. The mixing percentage was determined by monitoring the changing color during chemical reaction that occurs in the resultant coalesced droplet. The thermocapillary effect of the liquid substrate plays a significant role in mixing in that the liquid surface velocity and the droplet velocity increase with an increase in the surface temperature gradient. This, in turn, increases the mixing of the coalesced droplet, which is further enhanced when the drop size is smaller. Since the collision is convection dominated, an appropriate time scale is proposed, which provides a universal correlation of the non-dimensional mixing time in terms of Reynolds number.