Dendritic ice crystals with faceted tip growing from the vapor phase Title Dendritic ice crystals with faceted tip growing from the vapor phase T.Gonda & S.Nakahara

Purpose Investigate experimentally the morphology and the formation mechanism of dendritic ice crystals growing in air of 1.0 x 105 Pa and just above and below water saturation.

Experimental method The growth chamber was cooled down to a desired temperature by circulating isopropyl alcohol cooled by a thermostatic refrigerated bath. The temperature of an ice plate for water vapor supplier and a sapphire substrate in the chamber was precisely controlled by adjusting an electric current which passed through the thermoelectric modules. After the air in chamber was evacuated to about 10 Pa, water vapor was supplied by keeping the ice plate at a slightly higher temperature than that of the growth substrate.

A small amount of dilluted silver iodide smoke was inserted into the chamber together with clean air of 1.0 x 105 Pa. Minute ice crystals nucleated in air fell on the sapphire substrate and only an ice crystal was grown on the substrate in air of 1.0 x 105 Pa at -150 C and at controlled supersaturation. Dendritic ice crystals were observed in situ using a transmission microscope and recorded by a video tape recorder through a TV camera.

Experimental results Fig. 1. Sector-like ice crystal grown in air of 1.0 x 105 Pa at -15.20 C and at constant bulk supersaturation of 18%. (a) 0,(b) 13,(c) 28, (d) 36, (e) 38 and (f) 42 s.

Under this growth condition, the tip width of six primary branches is not so broad. In this case, the concaves do not develop into the side branches.

Fig. 2. Further growth of the ice crystal shown in Fig Fig. 2. Further growth of the ice crystal shown in Fig. 1: (a) 0, (b) 5, (c) 34, (d) 48, (e) 67 and (f) 142 s When bulk supersaturation is just above water saturation and is kept at constant, the concaves do not develop into the side branches.

Fig. 3. Dendritic ice crystal with side branches grown in air of 1 Fig. 3. Dendritic ice crystal with side branches grown in air of 1.0 x l05 Pa at - 150 C and at constant bulk supersaturation of 15%: (a) 0, (b) 26, (c) 29, (d) 41, (e) 57 and (f) 67 s.

When bulk supersaturation is just below water saturation and is kept constant, the tip width of primary branches becomes broad. Side branches are formed by the bunching of giant steps and then by the formation of large concaves on two prismatic facets at the tip of primary branches

Fig. 4. Further growth of the ice crystal shown in Fig Fig. 4. Further growth of the ice crystal shown in Fig. 3: (a) 0, (b) 9, (c) 20, (d) 23, (e) 32 and (f) 74 s. When the bulk supersaturation is just below water saturation and is kept constant, a dendritic ice crystal with side branches is formed.

Discussion When bulk supersaturation is very high and is kept constant, the side branches are not formed When bulk supersaturation is very high and varies with time, the side branches are formed A dendritic ice crystal with side branches is formed when the bulk supersaturation is just below water saturation and is kept constant. This means that not only the volume diffusion of water molecules onto ice crystal but also the surface kinetics of water molecules on the ice crystal surfaces is the rate determining process for the growth of dendritic ice crystal with faceted tip.

Conclusions Dendritic ice crystals with faceted tip are formed in air of 1.0 x 105 Pa at -15°C and just below water saturation. That is, when large concaves are formed on two prismatic facets at the tip of primary branches, dendritic ice crystals with side branches are formed.

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