Microdroplet impact at very high velocity*

At APS-DFD 2011, we presented preliminary data of water microdroplet impact at velocities up to 100 m/s and droplet diameters from 12 to 100 $\mu$m. Now we place these results into context and use them to improve understanding of droplet spreading. The parameter range covers the transition from capillary-limited to viscosity-limited spreading of the impacting droplet. The maximum spreading radius is compared to several existing models. The model by Pasandideh-Fard et al. (1996) agrees well with the measured data, indicating the importance of a thin boundary layer just above the surface. Here, most of the viscous dissipation of 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.