Nanoparticle motion and deposition near the triple line in evaporating sessile water droplet on a superhydrophilic substrate

The evaporation of a sessile droplet with suspended nonvolatile materials has been extensively studied for its particular characteristics in heat and mass transfer. The deposition patterns of it were believed to be controlled by the contact line movement; however, the mechanisms of the deposition patterns are very complicated, especially due to the nanoparticle movement near the triple line. In this paper, the nanoparticle movement and deposition near the contact line of a drying droplet are experimentally investigated, using fluorescent nanoparticles as direct tracers. It shows that the nanoparticle velocity in the thin liquid film region, obtained by the evanescent wave based multilayer nano-particle image velocimetry (MnPIV) technique, can be much higher than that near the liquid-vapor interface at the later stage of evaporation process in water droplet on a superhydrophilic substrate. On the other hand, the deposition patterns, which exhibits scattered dots, radial spokes and multi-rings due to the constant contact radius (CCR) and constant contact angle (CCA) modes happened recurrently during the evaporation process, can also be altered substantially by the nanoparticle concentration, which is an important parameter influencing the energy barrier. The analysis on the energy barrier indicates that there exists the maximum energy barrier when the deposition transits from lambdoid pattern to multi-ring pattern.