Experimental investigation of sub-millimetre droplet impingement onto spherical surfaces

This study reports an experimental investigation of the phenomena which occ ur when discrete, monodisperse droplets of a water-ethanol-glycerol solutio n in the size and velocity ranges of 160 < D < 230 mu m and 6 < U < 13 m/s, respectively, impinge upon a spherical surface with diameter of the order of I mm. By altering the concentrations-of the water, ethanol and glycerine , the surface tension and viscosity were varied in the ranges 58 < sigma < 73 mN/m and 0.0010 < mu < 0.0024 kg/ms, respectively. The boundary between droplets that deposit and droplets that reatomise has been quantified and a n empirical correlation in terms of kinematic and liquid properties is repo rted. The target surface diameter was varied in the range 800-1300 mu m and compared with a plane surface of equivalent non dimensional roughness; inc reasing surface curvature was seen to promote the onset of reatomisation. S troboscopic images of the impingement process showed that the impinged drop let formed a crown which was influenced by both surface roughness, Varied i n the range 35 nm-40 mu m, and droplet kinematic and liquid properties. The similarity in the trajectories of reatomised droplets produced from consec utive crowns implied that the reatomisation mechanism was repeatable and in fluenced by surface morphology. (C) 1999 Elsevier Science Inc. All rights r eserved.