Spreading and freezing of supercooled water droplets impacting an ice surface

Abstract

The present study conducts supercooled droplet impact experiments on flat ice surfaces with varied impact velocities, initial diameters, droplet supercoolings, and ice surface temperatures. Droplet spreading at room temperature can be described by equating the inertial force to capillary and viscous forces. A universal scaling on maximum spreading can be obtained by interpolating between the laws in the capillary and viscous regimes where the dominant forces are capillary and viscous, respectively. During supercooled droplet spreading on ice surfaces, dendritic solidification occurs, forming a moving ice–water interface and leading to new droplet spreading mechanisms. The maximum spreading factor is sensitive to droplet supercooling, whereas surface temperature has a minor influence on spreading. A new dimensionless number, the icing number, is proposed to scale icing effects on a flow field, refining the law in the viscous regime for supercooled droplets. Nevertheless, icing does not affect the scaling in the capillary regime. The total influence of icing is related to the magnitude of the icing number and the competition between capillary and viscous forces. The icing number can then form a simple criterion for determining whether icing is negligible. The newly derived scaling performs well when modeling the present experimental results.