We photographed high velocity impact of small water droplets (0.55 mm) on a heated stainless steel surface. To achieve high impact velocities the test surface was mounted on the rim of a rotating flywheel, giving linear velocities of up to 50 m/s. Two cartridge heaters were inserted in the substrate and used to vary substrate temperature. A CCD video camera was used to photograph droplets impinging on the substrate. By synchronizing the ejection of a single droplet with the position of the rotating flywheel and triggering of the camera, different stages of droplet impact were photographed. Substrate temperature was varied from 100–240°C and the impact velocity from 10–30 m/s. High-resolution photographs were taken of vapor bubbles nucleating sites inside the thin films produced by spreading droplets. For a given impact velocity, the extent of droplet spreading increased with substrate temperature. The superheat needed to initiate bubble nucleation decreased with impact velocity. We derived an analytical expression for the amount of superheat required for vapor bubble nucleation as a function of impact velocity.

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