Modelling the Coffee Ring Effect

IAS Open Programme Fellow Dr Benjamin David Goddard delivers a seminar on their research.

Drying of droplets containing colloidal particles (such as coffee) is encountered in a wide number of industrial applications, including ink-jet printing, forensic investigations, and crop-care. Of particular interest is how the suspended particles are deposited on a surface as the droplet evaporates. This often takes the form of a ring-like pattern, known as the coffee ring effect. This is frequently undesired in applications, and we would like to understand how to control the droplet's evaporation to avoid it. However, the underlying mechanisms are not yet fully understood, either experimentally or theoretically.

Two challenges in modelling such dynamics are (i) the large number of particles involved, and (ii) the highly multi-scale nature of the dynamics. What is required is a model, and associated numerical scheme, that can describe systems with many thousands of particles, on scales ranging from the particle size (~10^-8 m) to that of the droplet (~10^-3 m). Dr Goddard will present one such approach from statistical mechanics, known as Dynamic Density Functional Theory (DDFT). After introducing the DDFT model chosen here, he will describe how it is solved numerically and, most importantly, compare the results to state-of-the-art experiments. Finally, he will explain what the model tells us about the experimental results, and how it can be used to inform future work.

Joint work with Job Thijssen and Carmen Morcillo Perez (University of Edinburgh, Physics)