Figure 1. Evaporation complexity of colloidal droplets.
(a) Hydrodynamic flows during evaporation: the evaporative vapor flux, JE, from the droplet surface to the atmosphere and J0 at the center; the outward coffee-ring flow, JC, induced by JE; and the fluid flow, JF, through the compact region with a width, w, as marked by the gray part. (b) Three evaporation stages with respect to the growth of the compaction region with width w up to a final width, wf: w ≈ 0 at the initial (pinning) stage, 0 < w < wf at the intermediate (packing) stage, and w ≈ wf at the final (percolation) stage. Through evaporation, the particle volume fraction ϕ reached the random close packing, ϕrcp. (c) Confocal microscopic observations for each stage, determined by differences in fluorescence intensities among the air, solvent, and colloids.