The Effects of Wetting and Surface Roughness on Liquid Metal Droplet Bouncing

In droplet-based manufacturing processes, such as dropwise rapid prototyping, solder bumping, and spray forming, the quality of the deposit is adversely affected by bouncing of liquid droplets off the target surfaces. This study investigates the effects of wetting and surface roughness on the bouncing phenomenon. An analytical model, based on the conservation of energy during deposition, was developed to correlate wetting and surface roughness to a dimensionless droplet bouncing potential. In addition, experiments were conducted to image the deposition behavior of Sn-37 wt % Pb solder droplets, averaging 280 mu m in diameter, on prepared substrates with a wide range of wetting properties and roughness levels. The high-speed image data correlate well with the model prediction that droplets are likely to bounce as a target surface becomes less wetting or is roughened.