Vapor mediated control of microscale flow in sessile droplets

Abstract

Controlling internal flow in evaporating sessile droplets is desirable across applications ranging from lab-on-chip medical diagnostics, DNA profiling to surface patterning. Diffusion limited evaporation in droplets exhibit very low internal flow velocities [∼O(10−6) m/s]. Enhancement of internal flow is useful for applications which demand in situ mixing at small scale fluidic systems but limited by the low Reynolds number. To overcome this limitation, we present a non-intrusive methodology to enhance flow inside the droplets without affecting its global evaporation pattern. A highly volatile ethanol droplet is positioned asymmetrically in the vicinity of a water droplet. The ethanol molecules are consequently adsorbed asymmetrically on the air-water interface creating a gradient in surface tension. This causes an internal Marangoni convection with flow rates ∼O (103) times higher than a naturally evaporating water droplet. The inter-droplet distance between ethanol-water is used as a control parameter to vary the strength of Marangoni convection. The flow pattern transitions through several regimes from asymmetric to symmetric double toroid once the ethanol droplet completely evaporates. Experimental flow visualization and quantification by micro-particle image velocimetry have been used alongside simple scaling arguments to quantify the physical mechanism at play. We can also switch between different flow patterns by strategic dispensing of ethanol droplets.