Dynamics of Droplet Shedding and Coalescence under the Effect of Shear Flow

Droplet shedding and coalescence has various industrial applications from ink-jet printing to ice accretion on wind turbine blades, power lines or aircrafts. It is known that the incipience of icing phenomenon in the mentioned applications arises from the shedding and coalescence of the rain droplets. The coalesced droplets then start to form a runback flow and when temperature is below the water freezing point, ice can be accumulated on these structures which alters the performance in the mentioned technologies. Accordingly this work is dedicated to a fundamental study on dynamics of droplet shedding and coalescence as a preliminary stage of ice formation. Sessile droplets are deposited on surfaces where various air flows are introduced to them for analyzing their shedding behavior. As it is believed that using superhydrophobic coatings decreases the amount of ice accumulation on the solid surfaces, the effect of different surface wettabilities ranging from hydrophilic to superhydrophobic is also studied on droplet dynamics. It is shown that on a hydrophilic substrate when the air speed is high enough, rivulets are formed from merging droplets. In contrast, on a superhydrophobic substrate there is no rivulet formation. Instead coalesced droplets roll on the surface and detach from it if the air speed is sufficiently high. In addition, the results indicate a contrast in the mechanism of the coalescence and subsequent detachment between a single and two droplets on a superhydrophobic surface. At low air speeds, the two droplets coalesce by attracting each other before detaching with successive rebounds on the substrate, while at higher speeds the detachment occurs almost instantly after coalescence, with a detachment time decreasing exponentially with the air speed. A wind tunnel experiment was designed to characterize the rivulet dynamics on various surface wettabilities. Smoothed Particle Hydrodynamics method was performed and its results were compared with the ones obtained from the experiments. The results indicate that increasing the air speed results in formation of waves with higher frequency in comparison with the lower air speeds. It was also demonstrated that as on superhydrophobic substrates instead of rivulets series of droplets are formed these substrates can be suitable candidates for anti-icing purposes