From Coffee Rings to Uniform Deposits: Insights from 1D Modelling #

Nathan Coombs, Mykyta Chubynsky, James Sprittles

12:10 Wednesday in 3Q16.

Part of the Evaporation session.

Abstract #

Coffee ring formation, a well-known phenomenon observed in drying sessile droplets, has been understood theoretically in the past with aid of 1D modelling. These models typically assume infinitely fast vertical diffusion of the solute, and so are formulated in terms of depth averaged quantities. The coffee ring can then be understood as a is a shock front, beyond which solute particles are immobile, that propagates into the droplet interior.

In a study by Li et al. it was found that the ring can be attenuated simply by increasing the ambient temperature. This has the effect of increasing the evaporation rate and hence the free surface recession speed, meaning more particles in the droplet bulk will adhere to the surface (rather than being transported towards the contact line). Here, we make the first steps towards a 1D model that incorporates this behaviour. Building on models in existing coffee ring literature, we have lifted the assumption of fast vertical diffusion. Using some preliminary results in diffusive boundary layer theory, we present a model that is capable of reproducing both coffee ring and surface capture phenomena in their respective asymptotic limits.