Experimental investigation of sub-millimetre droplet impingement on to spherical surfaces

This study reports an experimental investigation of the phenomena which occur when discrete, monodisperse droplets of a water–ethanol–glycerol solution in the size and velocity ranges of 160 < D < 230 μm and 6 < U < 13 m/s, respectively, impinge upon a spherical surface with diameter of the order of 1 mm. By altering the concentrations of the water, ethanol and glycerine, the surface tension and viscosity were varied in the ranges 58 < σ < 73 mN/m and 0.0010 < μ < 0.0024 kg/ms, respectively. The boundary between droplets that deposit and droplets that reatomise has been quantified and an empirical correlation in terms of kinematic and liquid properties is reported. The target surface diameter was varied in the range 800–1300 μm and compared with a plane surface of equivalent non dimensional roughness; increasing surface curvature was seen to promote the onset of reatomisation. Stroboscopic images of the impingement process showed that the impinged droplet formed a crown which was influenced by both surface roughness, varied in the range 35 nm–40 μm, and droplet kinematic and liquid properties. The similarity in the trajectories of reatomised droplets produced from consecutive crowns implied that the reatomisation mechanism was repeatable and influenced by surface morphology.