Splashing of molten tin droplets on a rough steel surface

We photographed the impact of molten metal droplets on a flat plate. From these images we measured droplet dimensions during spreading and counted the number of fingers around a splashing drop. Experiments were done using stainless steel substrates with average roughness of 0.06, 0.07, 0.56, and 3.45 μm respectively. The temperature of the substrate was kept at either 25 or 240 °C. Droplet diameter (2.2 mm) and impact velocity (4 m/s) were kept constant, giving a Reynolds number (Re) of 31 135 and Weber number (We) of 463. Raising substrate roughness from 0.06 to 0.56 μm enhanced the tendency of droplet to splash, whereas increasing roughness even further to 3.45 μm suppressed splashing. This behaviour was attributed to changes in droplet solidification rate with surface roughness. A simple model of droplet spreading was used to estimate thermal contact resistance between the droplet and surface. Increasing surface roughness was found to raise thermal contact resistance and reduce heat transfer from the droplet to the substrate, delaying the onset of solidification and reducing splashing. The number of fingers formed around a droplet splashing on a smooth surface could be predicted reasonably well by a model based on Rayleigh–Taylor instability theory. Increasing surface roughness reduced the number of fingers while enlarging their size.