Evaporation and crystallization dynamics in evaporating saline droplets

Abstract

The evaporation of saline droplets on solid surfaces has been a topic of interest, since it plays increasingly important roles in various industrial and scientific applications. Therefore, in this project, the evaporation of NaCl-water droplet on hydrophilic substrates is studied. The effects of three influencing factors, namely the initial concentration of NaCl, presence of fluorescent particles and different types of substrates, are investigated in terms of droplet dynamics, flow fields, and crystallization by experimental approaches. The droplets are deposited onto unheated substrates and evaporate at controlled room temperature and relative humidity, and PIV analysis is used to visualize the flow fields. First of all, the experimental results agree with the common trend that higher NaCl concentration leads to higher contact angle, faster evaporation rate and flow velocity, and the presence of fluorescent particles leads to higher evaporation rate. Meanwhile, the contact line pinning effect of the fluorescent particles and receding effect of NaCl are spotted, and moreover, the slip-stick behaviour of contact line has been noticed and proven to be induced by the opposing effect of pining and receding, followed by analysis showing that the slip-stick behaviour can be suppressed by increasing the initial NaCl concentration. Next, the analysis of crystallization behaviour shows that, when pinning effect occurs, higher initial contact angle/concentration would result in higher contact angle when crystallization just occurs, but this trend is reversed when pinning effect is weak. Furthermore, four clearly defined distinct stages of evaporation are found in the evaporation on the silicon wafer, and each stage has unique droplet dynamics as well as flow patterns and can be easily distinguished by certain phenomena as starting and ending points. Last but not least, a relationship is established between the flow fields and location of NaCl crystallization forming on soda lemon glass, the NaCl crystal is often generated at where asymmetric vortexes locate.