Controlling Marangoni flow directionality: patterning nano-materials using sessile and sliding volatile droplets

Abstract

Controlling the droplet shape and the corresponding deposition patterns is pivotal in a wide range of processes and applications based on surface phenomena, such as self-assembly of different types of nanomaterials and fabrication of functional electronic devices. In this paper we study different flow regimes and deposition patterns from volatile sessile droplets and droplets sliding over inclined solid substrates. The directionality and intensity of the Marangoni flow was controlled by vapor composition in a sealed chamber enclosing the evaporating droplets. Two types of volatile droplets are investigated: single component droplets and binary solution droplets. Binary solution droplets can exhibit either inward or outward Marangoni soluto-capillary flow, depending on a surface tension dependence on the concentration of the fast evaporating component. We carried out a detailed experimental study of the micro-rivulet (μ-R) regime in different binary solutions. The μ-R formation in a certain range of Ca proved to be a universal phenomenon subject to the occurrence of inward Marangoni flow. We propose a simplified mathematical model for the shape of μ-R based on the lubrication approximation. The resulting μ-R profile shows a good agreement with the experimental results.