Moses Effect: Splitting a Sessile Droplet Using a Vapor-Mediated Marangoni Effect Leading to Designer Surface Patterns.

In this work, we showcase a mechanism of rapid and focused solvent depletion using vapor-mediated interaction that can nonintrusively cleave a sessile water droplet reminiscent of Moses parting the Red Sea. The Marangoni effect is induced by the differential adsorption of vapor from a nearby pendant droplet of ethanol, leading to an exponential increase in surface velocity inside the water droplet. The Marangoni convection leads to the drainage of liquid from the central section of the water droplet and consequently splits it. By encoding the position of the ethanol (vertical as well as horizontal) droplet, an array of liquid motion is observed (split, shift, and slosh) in the water droplet. This method is further extended to nanocolloidal systems, where the liquid motion can be exploited to generate a wide gamut of deposit patterns ranging from uniform precipitate to sporadic islands without resorting to the more traditional evaporation-driven capillary flows ("coffee stains") or custom engineering of the shape of the nanoparticles. We further provide a detailed exposition of the physical mechanisms responsible for the splitting of the liquid drop and consequent particle deposition. The concept can be extended to liquid actuation in open channel microfluidic chips and surface patterning as in medical diagnostics, optoelectronics, and thermal management.