Potassium chloride dewetting on micro-pyramid substrate surface

Abstract

The study of evaporation dynamics and wettability of droplet is vital to several important processes in the science and technology sector such as ink-jet printing, selfcleaning, condensation heat transfer and water harvesting. The initial wettability and evaporation characteristic of droplets with dissolved potassium chloride (KCl) on micro-pyramid substrate with pillars are studied by varying the KCl concentration. Octagonal initial wetting areas are formed when the droplets are placed on the substrate. The b/a side ratio of the initial wetting octagon decreases from 0.98 at 0% KCl concentration to 0.94 at 20% KCl concentration. During evaporation, the droplets experience a pinning-depinning transition. The deionized (DI) water undergoes three evaporation stages from the constant contact line (CCL) stage, to the constant contact angle (CCA) stage, and then the mixed stage. However, KCL droplets demonstrate only two stages which are the CCL and CCA stages. The total pinning time and the contact angle at the start of CCA stage increases as the KCl concentration increases. Also, the evaporation rate decreases as the concentration increases. The actual precipitation time and the predicted precipitation time decrease as the KCl concentration increases. Furthermore, the actual precipitation time is longer than the predicted precipitation time which shows that the droplets have reached supersaturation prior to precipitation. For the crystallization process, random crystal appears near the contact lines during the process of evaporation regardless of the concentration, and moves towards the centre as it grows due to the increase in crystal size which is confined between the solid substrate and the droplet free surface. Future studies on the evaporating characteristic on heated and cooled micro-pyramid substrate can be conducted and the precise movement during crystallization can be examined with better imaging techniques. Additionally, the experiment can be carried out with a better deposition technique and in a controlled environment to increase the accuracy.