| dc.description.abstract | The phenomenon of droplet evaporation is ubiquitous and has wide-ranging applications, such as thin film deposition, DNA chips, inkjet printing, crystal arrays, and spray cooling. The evaporation process of sessile droplets is a transient system, and its behavior is influenced by factors such as the substrate wettability, temperature difference due to droplet or substrate, solute concentration, etc. These factors create varied flow patterns inside the droplets.
This study built numerical model to simulate the evaporation process of colloidal sessile droplets by varying the initial droplet temperature, the evolution of temperature and flow field distribution over time are investigated, providing a reference for the flow pattern of non-volatile component droplets. The results show that during the initial evaporation time, multiple vortices are generated near the liquid-gas interface due to non-uniform evaporation flux, which gradually integrate into a pair of thermal Marangoni vortices. When the initial droplet temperature is above ambient temperature, two flow periods are observed: periodic flow field and transition flow field. The periodic behavior involves the competition between the upper and lower thermal Marangoni vortices, eventually leading to a dominant upward Marangoni flow along the liquid-gas interface, which then splits into a new cycle of vortices. The cycle time shortens as the initial temperature of the droplet increases, and the greater the number of cycles, the later the transition from periodic behavior to the transition period. The evaporation flux becomes more uneven as the initial temperature rises and gradually becomes more uniform as the droplet temperature drops. During the transition period, the droplet temperature approaches to the ambient, and the periodic flow fades, the upper and lower Marangoni vortices compete continuously in the droplet. Eventually, these vortices split into three, with the central vortex gradually dominating and carrying the fluid from the bottom to the top along the liquid-gas interface, forming a steady ambient temperature flow pattern and concluding the transition period. The end of this period is delayed as the initial droplet temperature increases. The PS concentration field in the heated droplets causes multiple local concentration extremes due to uneven evaporation flux. Multiple vortices move part of the PS into the droplets, but are limited by circulation near the liquid-gas interface; then the droplets enter a periodic flow, the PS concentration accumulates at the apex and is restricted by the periodic flow of the thermal Marangoni vortices below. After entering the transition period, the PS concentration produces two higher areas distributed in the two vortices, which last until it finally transforms into a concentration field distribution of ambient temperature flow pattern. | en_US |