Microdroplet impact at very high velocity

Water microdroplet impact at velocities up to 100 m/s for droplet diameters from 12 to 100 um is studied. This parameter range covers the transition from capillary-limited to viscosity-limited spreading of the impacting droplet. Splashing is absent for all measurements; the droplets always gently spread over the surface. The maximum spreading radius is compared to several existing models. The model by Pasandideh-Fard et al. agrees well with the measured data, indicating the importance of a thin boundary layer just above the surface, in which most of the viscous dissipation in the spreading droplet takes place. As explained by the initial air layer under the impacting droplet, a contact angle of 180 degrees is used as model input.