Experimental and numerical analysis of shear-driven droplet coalescence on surfaces with various wettabilities

Abstract

The goal of this study is to explore and analyze the concurrent shear-driven droplet shedding and coalescence under the effect of various parameters, such as droplet size and distance, as well as airflow velocity and surface wettability. To investigate and capture different aspects of droplet dynamics, both experimental modeling and numerical modeling are conducted. The volume of fluid coupled with the large-eddy simulation turbulent model in conjunction with the dynamic contact angle is implemented to model droplet shedding on different surface wettabilities. Analysis revealed a great match between the numerical and experimental outcomes. It is shown that in addition to surface wettability and airflow speed, droplet sizes, and the distance between them are crucial factors in controlling droplet dynamics during the shedding and coalescence. It is illustrated that on the aluminum (hydrophilic) surface, the second droplet (the one further from the airflow inlet) tends to move toward the first droplet (the one closer to the airflow inlet) more significantly when the distance between droplets is larger as well as the cases where the first droplet is also the larger one. It is revealed that if the first droplet is larger, after coalescence the resulting droplet will break up into smaller droplets known as satellites. On the superhydrophobic surfaces, on the other hand, droplets behaved differently, which is mainly related to initial droplet shape and dynamic contact angles. For the cases of the larger distance between the droplets, the first droplet is lifted off from the surface after a few milliseconds, and consequently, the second droplet is not prone to move toward the first one. When the first droplet is larger between the two, the second droplet tends to move toward the first one in contrast to the case where the first droplet is the smaller one. To better interpret the droplet dynamics, and the effect of different parameters on their behavior, further details on aerodynamic forces including the drag and lift forces before and after the coalescence are presented in this work.