Multiscale Characteristic in Symmetric/Asymmetric Solar-Driven Nanofluid Droplet Evaporation

Driven by nanoparticle plasmonic heating, sessile droplet evaporation presents challenges on the coupling mechanisms between time-spatial heat source distribution and flow/temperature fields in a droplet. Here, symmetric/asymmetric solar-driven droplet evaporation is investigated. An infrared camera captures droplet surface temperatures in the micrometer scale after correction. An optical three-dimensional profiler quantifies nanoparticle deposition in the nanoscale. We show that droplet surface temperatures do display a nonmonotonic variation trend. Based on measurements, we are able to decouple the droplet into a contact line region (CLR) and a bulk volume region (BVR). The CLR volume is two to three magnitudes smaller than the droplet volume. The temperature gradient is significant in CLR, but flat temperature exists in BVR. Radial flow in BVR transports nanoparticles from the droplet body to the contact line, while Marangoni flow in CLR stabilizes nanoparticles there. Light energy is also decoupled based on its wavelength band. It is found that CLR dominates the visible energy absorption, but BVR has a weak contribution. Top light heating causes symmetry temperatures and a coffee-ring profile along the circumference. However, side heating yields higher temperatures and more nanoparticles deposition on the sunny side than on the night side. The above findings are valid when the initial droplet volume and incident irradiation flux are changed on the hydrophilic wall. The hydrophilic wall and hydrophobic wall maintain the evaporation modes of constant contact diameter and "stick-slip", respectively. The present paper enhances the understanding of light-induced droplet evaporation from the multiscale point of view.