Evaporation-driven liquid flow in sessile droplets

TL;DR

This review paper consolidates current knowledge on evaporation-driven internal flows in sessile droplets, highlighting recent experimental and theoretical advances, and discusses open questions and future research directions across multiple scientific disciplines.

Contribution

It unifies diverse perspectives on evaporation-driven flows in sessile droplets and summarizes recent progress and open challenges in the field.

Findings

Recent experimental techniques have advanced understanding of flow patterns.

Theoretical models now better predict evaporation-induced flows.

Open questions remain regarding flow control and material deposition.

Abstract

The evaporation of a sessile droplet spontaneously induces an internal capillary liquid flow. The surface-tension driven minimisation of surface area and/or surface-tension differences at the liquid-gas interface caused by evaporation-induced temperature or chemical gradients set the liquid into motion. This flow drags along suspended material and is one of the keys to control the material deposition in the stain that is left behind by a drying droplet. Applications of this principle range from the control of stain formation in the printing and coating industry, to the analysis of DNA, to forensic and medical research on blood stains, and to the use of evaporation-driven self-assembly for nanotechnology. Therefore, the evaporation of sessile droplets attracts an enormous interest from not only the fluid dynamics, but also the soft matter, chemistry, biology, engineering, nanotechnology and mathematics communities. As a consequence of this broad interest, knowledge on evaporation-driven flows in drying droplets has remained scattered among the different fields, leading to various misconceptions and misinterpretations. In this review we aim to unify these views, and reflect on the current understanding of evaporation-driven liquid flows in sessile droplets in the light of the most recent experimental and theoretical advances. In addition, we outline open questions and indicate promising directions for future research.