  "When the liquid phase of a particle-laden droplet evaporates, a ring of
solute is typically formed - what has become known as the "coffee ring effect".
A key focus of recent work has been the suppression of the coffee-ring effect
to leave behind more spatially-uniform particle coatings instead. Efforts to
suppress the coffee ring effect often focus on physical effects such as
Marangoni flows. Here we focus on a purely geometric effect - the effect of
substrate curvature - by evaporating suspension droplets on spherical surfaces
of different radius of curvature. We find that stains formed on more highly
curved substrates are more uniform. To understand this, we propose a model of
the evaporation-induced flow, combined with a detailed calculation of how
curvature modifies the local evaporation rate. This model shows that
evaporation in the centre of the droplet is enhanced, leading to increased
concentration of solute there and a reduction in the propensity for
ring-formation.
