(4e) Marangoni Effect Reverses Coffee-Ring Depositions

We report here that Marangoni effect reverses coffee-ring depositions. Most of particles deposit at the center of the droplet, rather than the edge, due to a Marangoni flow induced during droplet evaporation. We develop a full analytical solution to the temperature and velocity fields in the drying droplet to analyze the particle deposition process. The measurement of the fluid flow in the drying droplet confirms our theoretical analysis. Combining the analytical solution for the flow field with Brownian dynamics simulations, we are able to compare our experimental results for particle deposition with predictions. The good consistent between experiment and theory is obtained.