Evaporation of a sessile droplet: inside the coffee stain.

We have investigated experimentally, for the first time at microscopic level, the growth of the deposit left around a drop of colloids drying on a solid surface ("coffee stain effect"). Direct observations show that there are several distinct phases of growth, the later ones exhibiting surprising pattern formations with spatial modulation of the deposit. In addition, fluorescence reveals that the initial growth phase is governed by a single length scale, increasing with time as t(23). We show that this exponent is a direct consequence of the divergence of evaporation near contact line evidenced by Deegan et al. We propose a simple ballistic model that allows us to calculate both this exponent and the prefactor, in agreement with yet available more complex descriptions. This model also opens the possibility to include effects neglected up to now.