Research on evaporation and deposition characteristics of suspended droplets containing nanoparticles

Evaporation of functional nanoparticle-containing droplets on solid surfaces is a key process in several fields, such as air conditioning, refrigeration, and electronics cooling. In this paper, we experimentally investigate nanofluid droplets' evaporation and particle deposition process on solid surfaces, photograph the deposition patterns, and perform microscopic characterization. The results show that the droplets always evaporate in the mode of constant contact radius, the changes in substrate temperature and droplet volume have little influence on the evaporation mode and morphology of the droplets, and the contact angle changes linearly with time. The surfactant can significantly regulate the kinetic behavior of droplet spreading, and the addition of only 0.25 wt % of surfactant sodium dodecyl sulfate (SDS) increases the droplet spreading radius from 0.71 mm to 1.12 mm, the initial contact angle decreases from 83° to 54°, and the area of spreading increases by 89%. Substrate temperature and droplet volume significantly affect the deposition pattern after droplet evaporation. The higher the substrate temperature, the larger the droplet volume, and the more obvious coffee ring pattern is formed after evaporation. SDS significantly increases the coffee ring width, which reaches 230 μm when the concentration of SDS reaches 1%, and the particles have been widely distributed throughout the entire evaporation area, suggesting that the coffee ring effect has been effectively suppressed. By introducing the Ma number, the influence of the Marangoni effect guided by temperature, volume, and concentration changes on the internal flow of droplets and the mechanism of coffee ring formation is explained.