Gravitational effects on coffee-ring formation during the evaporation of sessile droplets

We consider the role of gravity in solute transport when a thin droplet evaporates. Under the physically relevant assumptions that the contact line is pinned and the solutal Peclet number, Pe, is large, we identify two asymptotic regimes that depend on the size of the Bond number, Bo. When Bo = O(1) as Pe ->infinity, the asymptotic structure of solute transport follows directly from the surface-tension-dominated regime, whereby advection drives solute towards the contact line, only to be countered by local diffusive effects, leading to the formation of the famous 'coffee ring.' In the distinguished limit in which Bo = O(Pe(4/3)) as Pe ->infinity, this interplay between advection and diffusion takes place alongside that between surface tension and gravity. In each regime, we perform a systematic asymptotic analysis of the solute transport and compare our predictions to numerical simulations. We identify the effect of gravity on the nascent coffee ring, providing quantitative predictions of the size, location and shape of the solute mass profile. In particular, for a fixed Peclet number, as the effect of gravity increases, the coffee ring is diminished in height and situated further from the contact line. Furthermore, for certain values of Bo, Pe and the evaporation time, a secondary peak may exist inside the classical coffee ring. The onset of this secondary peak is linked to the change in type of the critical point in the solute mass profile at the droplet centre. Both the onset and the peak characteristics are shown to be independent of Pe.

Automated summary

This study investigates the influence of gravity on solute transport during evaporation of thin droplets. Two asymptotic regimes are identified based on the size of the Bond number. The results provide quantitative predictions of the size, location, and shape of the solute mass profile, taking into account the interplay between advection, diffusion, surface tension, and gravity.