Single Phase Transition Leads to the Coffee Ring Deposition of Nanoparticles in an Evaporating Sessile Droplet

The coffee-ring formation upon evaporation of a sessile droplet laden with dispersed particles occurs via capillary flow of the fluid towards the three-phase pinned contact line. In the process, the particles are carried towards the contact line followed by gradual deposition that accelerates towards the end of drying. However, we have observed using surface enhanced Raman spectroscopy (SERS) with gold nanoparticles that the deposition process at the contact line occurred in a short window of time that was akin to a phase transition and that was complete well before the drying of the droplet. Further, the concentration of nanoparticles at the time of deposition was the same irrespective of the initial concentrations studied, the value of which was calculated to have been on the order of 13 M.