(461g) Simulation of Particle Deposition in an Evaporating Sessile Droplet

Particle deposition in evaporating droplets has various applications including tablet coating, bioassays and microelectronic production. In an evaporating droplet laden with particles, the particles first suspend and flow with the fluid and then deposit on the substrate. Two kinds of microflows, the capillary flow and the Marangoni flow, dominate the fluid flow profile in an evaporating drop and determine the particle deposition shape. Here we used Galerkin finite element method to computationally analyze the flows of the fluid and the concentration of the particles, which are coupled in both ways. The thermal Marangoni flow was incorporated in the model by solving the temperature field, which is coupled with the flow field as well. Our simulation was also validated by experimental results from literature. Many previous studies believed that the Marangoni flow changes particle deposition topography by suppressing the “coffee ring” effect which deposits particles near the contact line. However we showed that the Marangoni flow does not completely eliminate the “coffee ring” effect because of the existence of a stagnation point near the contact line. Between the stagnation point and the contact line, particles still deposit into a ring which has a smaller width compared to the “coffee ring” under no influence of the Marangoni flow. The width change comes from the fact that the Marangoni flow keeps more particles suspended.