Different outcomes after inclined impacts of water drops on a cooled surface

The impact of water droplets onto an inclined surface which is cooled down well below the freezing point of water is investigated experimentally. Droplets are generated using a hypodermic needle which is fed by a micropump. To avoid the build-up of condensate and frost on the cooled surface, it is encapsulated within a styrofoam chamber with a controlled environment. The impact process is observed in top view with a high speed video system which can be tilted together with the impact surface to enable the investigation of inclined impacts. Multiple experiments are performed using constant impact parameters and a surface which is cooled down to -17 ^◦C. Additional impacts are performed with a surface near room temperature to serve for comparison purposes. We show that the surface temperature has no influence on the drop dynamics during the main part of the spreading phase, but influences the droplet’s behavior severely after it has reached its maximum spreading. Although the impact parameters are kept constant during the experiments, the outcome can vary significantly, which is explained by a freezing delay after the droplet impact. We show that the freezing probability is influenced by the fluid motion after impact. Velocities of the propagating freezing front are measured and we show that they are constant during the period in which a single droplet freezes and furthermore, are not significantly influenced by the freezing delay. It implies the existence of a typical temperature determining the front velocity, which only depends on the initial temperatures of the surface and the liquid.