Distribution, coalescence, and freezing characteristics of water droplets on surfaces with different wettabilities under subfreezing convective flow

This paper investigates the effects of surface wettability and surface initial conditions on early frost formation for hydrophobic, hydrophilic, and bare aluminum flat surfaces. The new data revealed that the average droplet sizes were larger and droplet size distributions were wider for the hydrophilic surface than for the hydrophobic and bare aluminum surfaces. This was due to a coalescence phenomenon occurring before the droplets froze into ice beads. Because of the relative high sub-freezing plate temperatures used in this work, the surface wettability of the plates had a smaller impact on the freezing time. Supersaturation degree and temperature difference between the air stream and the surface controlled freezing times for all different surface wettability types. When testing was conducted without cleaning the surfaces in between individual test runs, a film deposited on the surfaces which smoothed surface imperfections, inhibited surface wettability effects on droplet size and shape, and delayed freezing. A large variation in the frost nucleation behavior was measured in the present work due to intentionally different generated initial surface cleanness conditions. This finding could explain some of the significant deviations observed in data from literature studies when comparing freezing times and droplet diameters for similar coatings and similar tests conditions.