  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.
