Temporal and spatial evolution of the thin film near triple line during droplet evaporation

The evaporation process of the thin film region near the triple line for a liquid suspended with nanoparticles involves important interfacial phenomena such as contact line movement, deposition, and evaporation. In this paper, droplets that are seeded with fluorescent nanoparticles of different diameters and of the same particle numbers were investigated for their evaporation rates, deposition patterns, and in plane average velocities within the field-of-view. The results present different modes of deposition patterns for the fluorescent nanoparticles, and stronger evaporation can be obtained using nanoparticles of small diameter. The temporal and spatial evolution of velocities and thicknesses in the thin films near the triple lines during the droplet evaporation were obtained by using a proposed sub-region method, which is developed from an evanescent wave based nanoparticle image velocimetry technique. It shows that, depending on the nanoparticle size, the spatial variation of the local thin film thickness can be linear or nonlinear. The fluorescent nanoparticles can affect the evaporation modes, the internal flow, and the temporal and spatial evolution of the thin film during the droplet evaporation through the interaction between the particles and the interface near the contact line and their influence on the change of the microscopic contact angle during the pinning process of the contact line. This work can help capture insights on how nanosized particles affect the droplet evaporation near the triple line. (C) 2017 Elsevier Ltd. All rights reserved.