Imaging the drying of a colloidal suspension: Velocity field

We investigate the drying kinetics of a sessile droplet containing nanoparticles. Using a fast two-color confocal imaging technique, we probe the concentration field of nanoparticles along with the velocity and mobility fields of microparticles that act as flow tracers. The flow patterns evolve according to a complex kinetics which is related to recirculating Marangoni flow coupled to the time-evolving rheology of the suspension. The drop is roughly divided in two parts: a fluid zone in which persistent recirculations flow always in the same direction, and a gelled part at the level of the corner which grows with time and also creeps under the capillary pressure.