PHASE TRANSITION OF WATER DROPLETS WITH DIFFERENT STATIC CONTACT ANGLES

The freezing process of water droplets was investigated for an uncoated and two coated aluminum surfaces subjected to the action of a forced convective environment. One of the coatings was a silicon-based conformal polymer and the other was a hydrophobic coating. Static contact angles of the droplets ranged from 74.6 deg for the uncoated aluminum to 127.1 deg for the hydrophobic coating. Air velocities ranged from 0.2 to 7.85 m/s, surface temperatures ranged from -5 to -13 degrees C, air temperatures ranged from 4 to 24 degrees C, and air relative humidities were varied from 24 to 54%. Droplet radii ranged from 0.4 to 1.5 mm. A digital high speed microscopy system was used to visually observe and measure the freezing in individual droplets. Empirical correlations for dimensionless freezing time as a function of three nondimensional variables (Am-infinity,. Re,,, and Ste) were developed. These correlations could be used to estimate dimensionless freezing time over a wide range of experimental conditions. Droplets on the hydrophobic surface showed the slowest nondimensional freezing times while those on the silicon-based conformal polymer coating and uncoated surfaces were comparable. The data for the silicon-based conformal polymer coating surface were also compared to the data from a prior researcher.