Fingering structures inside the coffee-ring pattern

The effect of the fingering instabilities formation inside the spot left after nanofluid droplet drying has been investigated numerically and experimentally. The ring-shaped residual spot, well-known as the “coffee ring”, usually forms during the evaporation of a colloidal sessile droplet due to the convective mass transfer toward the three-phase contact line. The series of experiments with a water suspension of 25 nm copper nanoparticles showed the formation of additional, well-distinguished branched structures inside the coffee ring left after the full drying of a millimeter-sized sessile droplet on a silicon substrate. The kinetic Monte Carlo model is used to simulate the observed process, assuming that the fingered structures form on the latest stage of the drying process when the thin film of remaining solvent evaporates inside the already formed coffee ring. The simulation results indicate that the nucleation and growth process, starting inside the ring, leads to the experimentally observed formation of branched structures connected to the inner side of the rim.