High-Speed Measurement Of The Air Layer Thickness During Droplets Impact On Heated Substrates

The impact of droplets plays an important role in industrial processes such as spray cooling,industrial spraying,and spray combustion of internal combustion engines.When a droplet impacts on a solid substrate,a dimple-shaped air layer with a thickness of several micrometers is formed between the droplet and the substrate before the substrate is wetted.The presence of the air layer is significant for the impact dynamics of droplets.In this study,through high-speed imaging and color interferometry,the thickness and morphology of the air layer generated by the impact of droplets on heated substrates are studied.For the impact of droplets on uniform heated substrates,the results reveal that the air layer is profoundly affected by the substrate temperature.The thickness of the air layer increases with increasing the substrate temperature.When the substrate temperature is heated from room temperature to 70 °C,the thickness of the air layer increases by about 0.5 ?m.Considering that the edge of the air layer is only about 1?m,the temperature effect on the impact dynamics can be considerable.Besides,the effect of the Weber number,the fluid viscosity,and the size of the droplet on the air layer are also analyzed.We found that the thickness of the air layer increases with decreasing the Weber number,increases with increasing the droplet size,and increases with decreasing the droplet viscosity.For the impact of droplets on non-uniformly heated substrates,this study finds that droplets can migrate on non-uniformly heated substrates without wetting substrates.The migration of droplets on non-uniformly heated substrates undergoes two stages of droplet oscillation and droplet migration.During the oscillating stage,the thickness of the air layer undergoes from thick to thin and then to thick,and the radius of the air layer undergoes from small to large and then to small.At the beginning of the migrating stage,the initial migration speed is small,and the air layer is oscillating.At the end of the migrating stage,the air layer no longer oscillates,the radius of the air layer is almost stable,the air layer is asymmetrical,and the migration speed is significantly accelerated.Besides,we find that the migration speed of droplets increases with increasing the substrate temperature gradient,increases with decreasing the droplet Weber number,increase with increasing the droplet size,and increases with decreasing the droplet viscosity.