Dynamics and structure in the deposition profile of the coffee-ring effect

We investigate the growth dynamics and three dimensional structure of ring stains in evaporating drops of solutions and colloidal suspensions. Drying solutions generally leave behind ring stains. While this “coffee-ring effect” has been widely studied, the microscopic details of how the deposition patterns form have not been completely characterized experimentally. Using fluorescence and laser-confocal microscopy, we measure the growth rate and height profile of the deposit as functions of time, initial particle concentration, and particle size. Starting at the contact line, the particle deposition grows into the interior of the drop. The deposition front accelerates as the drop dries and the front steadily becomes sharper until the thickest part of the deposit has formed. Our experiments with fluorescent microspheres reveal the growth of a film of particles over the entire fluid-air interface. This film contracts and detaches from the contact line during the later stages of evaporation. The final deposits often contain patterns of particles in the interior of the rings resembling fractals. Prior theoretical work does not adequately describe many of the phenomena that we observe.