Vapor Mediated Interaction in 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 exhibits very low internal flow velocities [and#8764;O(10^and#8722;6) m/s]. Enhancement of internal flow is helpful for applications that demand in situ mixing at small-scale fluidic systems but are limited by the low Reynolds number. To overcome this limitation, we present a non-intrusive methodology to enhance flow inside the droplets without affecting their 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 and#8764;O(10^3) 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. Mixing at small fluidic length scales is especially challenging in viscous and non-volatile droplets frequently encountered in biochemical assays. In situ mixed methods, which depend on diffusion or evaporation-driven capillary flow, are typically slow and inefficient, while thermal or electro-capillary methods are either complicated to implement or may cause sample denaturing. As a consequence of increased velocity by vapor-mediated interactions, we can use it to enhance mixing in droplets.