Packing and sorting colloids at the contact line of a drying drop

In this article, we study the drying kinetics of a sessile droplet containing latex particles. We find that a depletion film is left at the edge of the drops, whose width is controlled by two geometric parameters, the contact angle and the diameter of the particles. We show that this effect can be used to sort colloidal mixtures because nanometric colloids always segregate at the edge of the drop, whereas micrometric colloids are blocked further away from the edge. We also provide a simple method to measure the velocity of a micrometric latex as it flows toward the contact line. We find that the particles strongly accelerate at the end of the drying process. Using Deegan's prediction for the rate of evaporation in the vicinity of the contact line, we quantitatively explain this phenomenon by the fact that the contact angle vanishes at the end of the drying process, therefore inducing a strong increase in the flux of water and particles close to the edge. The decrease in the contact angle also controls the width of the ringlike deposit.