Transition of Particle Motions in an Evaporating Droplet

In an evaporating water droplet laden with particles, the liquid flow profile varies in time and space. It has been shown by Hu and Larson (2005) that evaporative cooling results in a temperature gradient on the surface of the droplet, thus driving Marangoni currents near the surface. Marangoni currents along with the internal edgeward flows of the droplet result in the formation of circulation eddies, which keep the particles in suspension initially. Near the end of evaporation, the particles will begin to deposit and accumulate at the contact line. This process has been studied both experimentally and computationally by Devlin (2015). However, the transition of particles from suspension to deposition is not clearly understood. This transition affects the separation of different sized particles in the droplet, which has implications in biomedical applications such as diagnostic testing. Here we modelled the evaporating system and used the finite element method to computationally analyze the flows of the water and the motion of the particles. The simulation indicates the disappearance of circulation eddies of water at the end of evaporation, and can be used to predict the time and the nature of which the particles accumulate.