Deformation behavior of two droplets successively impinging obliquely on hot solid surface

We investigated the successive oblique collision of two droplets with a hot solid surface using flash photography. A pair of water droplets at room temperature was vertically dropped to impact a tilted smooth sapphire substrate one after the other. The diameter of the droplets was approximately 0.6 mm and the impact velocity was varied between 1.6 and 2.1 m/s. The spacing between the centers of the two falling droplets was also varied between 0.8 and 1.5 mm. The substrate was titled at ⩽45° relative to the horizontal and its temperature was varied between 170 and 500 °C. The leading droplet impacted the substrate and slid downwards over the surface, and this was followed by the off-centered collision of the trailing droplet relative to the deformed leading droplet. The subsequent motion of the “combined liquid” was observed to be essentially three-dimensional. For substrate temperatures of 200-300 °C, the combined liquid was considerably distorted by the bursting of boiling vapor bubbles at the free surface. At a substrate temperature of 500 °C, the liquid motion was roughly linearly symmetric. Rebounding phenomena were also observed in the liquid. The residence time of the droplets was measured for different conditions, and the results were used to derive an expression for predicting this parameter.