The Journal of Physical Chemistry, Vol. 87, No. 21, 1983 4333
no frost formed on the target. We cannot explain yet why these experiments do not agree.
Dan’s groups have verified that the decay time condtant, r, is proportional to the square of the scattering wave vector.
C. A . Knight: To Takahashi and Illingworth. What field measurements are needed to confirm which mechanism is operating within thunderstorms? Zllingworth: I would agree with Dr. Takahashi that the inductive mechanism is most unlikely to operate within thunderstorms. Although the details of the charge transfer mechanism are still unclear, and we can argue about them, most workers in the lab measure tens of femtocoulombsof negative charge acquired by a hailstone when it collides with a hailstone. Measurements in thunderstorms indicate charges on hail pellets are tens of picocoulombs, so we might estimate it had collided with a thousand crystals. If yo do the sums you find you need crystal concentrations of tens per litre rather than the one per litre found ion average at -10 “C. Simultaneous measurement of crystal size and concentration and precipitation charges should confirm whether the regions of intense electrification coincide with high crystal concentration at the -10 to -20 “C level.
G . W. Gross: How does the convection at the interface (unavoidable due to the 4 “C density maximum) affect the scattering you observe? Brown: The dependence on convection is not known. (See response to Dr. Hammer’s question.) We should note, however, that similar scattering effects are seen in other substances (Salol) which do not have a density instability as in water. This work was done by the Zurich group.
Evidence of Biogenic Nuclei Involvement in Antarctic Coastal Clouds (V. K. Saxena) G. W. Gross: Could the organic materials you discussed have been due to contamination introduced by the aircraft itself? Saxena: No. Samples from the aircraft skin and from the cabin did not show the presence of potential matter as indicated by the infrared absorption spectra. Presence of KC1 salt could not be established in these samples either. The possibility of such a contamination was also avoided due to careful sample collection and preservation procedures.
Enhanced Light Scattering at the Ice-Water Interface during Freezing (R. A. Brown) K . Itagaki: How can you keep the water gas free? Unless you keep the water surface under vacuum air and C02will dissolve and diffuse into ice very quickly. Brown: It is not possible for us to evacuate our system. Therefore we do expect to have some gases trapped in the ice. It has been suggested that the scattering is due to the collection of bubbles on the growing crystal surface. This seems unlikely to us for the following reasons. First, such bubbles would grow as the experiment progresses, causing a change in the decay time constant. This is not observed. Second, the bubbles could be expected to diffuse only in the plane of the interface. The k2 dependence of r, however, indicates that the scattering mechanism must have diffusive properties in all three dimensions. Third, it is observed that, when the growth rate of the crystal is reduced to zero, the scattering vanishes. This implies that the gas in the bubbles must immediately diffuse back into the water at a rate much faster than expected. C. Hammer: In your experiment, the Carr energy should give you some convection. What is the influence of this in your opinion? Brown: Convection in the melt above the interface is unavoidable. Recognizing that convective flow patterns may play an important part in producing the scattering, we intend to perform additional experiments using sample cells of different diameters. Different diameters should produce different flow patterns, so we should be able to discern any dependence on convection. Currently the role of convection is not known.
S. Warren: How do you know that the scattering comes only from the interface and not also from below the interface? Brown: We do not. One proposed mechanism is that the scattering is due to dendrite-like structures on the surface of the crystal. Since the bulk crystal produced during an experiment is a single crystal, such structures would have to vanish in some sort of annealing process. In the section of ice which was not yet annealed, scattering would occur. J . M . Greenberg: Do you have any measure of the angular distribution of the scattering? Brown: The Zurich group measured the angular dependence of intensity and found it to be isotropic. Both the Zurich and
W.L. Brown: Is it possible that the enhanced scattering is due to the development of large growth steps of a size -800 A (in your growth geometry) and -200 8, (in the earlier work) which require a critical growth velocity to exist? At lower velocities the growth could be more nearly planar. Brown: The mechanism you propose does not account for the i2dependence of the decay time constant. Recall that this dependence indicates a three-dimensional diffusion process, not merely two-dimensional surface diffusion.
Growth Rate of Recrystallization in Ice (G. Wakahama) K . Itagaki: Could you estimate the relative volume of “liquid water”? Wakahama: No. NMR studies on mobile water molecules in ice are qualitative at this moment. W.L. Brown: In your annealing experiments is the system truly isothermal or could there be a thermal gradient aiding the growth? Wakahama: Annealing was made under an isothermal condition in our experiments. A. J . Zllingworth: I would like to ask Dr. Wakahama how his results compare with Kvlividze et al. (Surf.Sci. 1974,44,60-8)? This is the only other work I have seen which detects the surface liquidlike layer using NMR. They used an ice sample with a very high surface area and found some liquidlike properties above 260 K. Wakahama: We obtained signals of mobile water molecules only a t -1.7 “C and none at the lower temperatures such as -4 “C in our NMR studies. We suspect that the relative volume of “liquid water” in the grain boundaries of ice decreases rapidly with a decrease in temperature, so that we could not detect it in our experiment at lower temperatures as Kvlivioze et al. found in their powder sample of ice.
Growth Rate and Habits of Ice Crystals Grown from the Vapor Phase (W. Beckmann) C. A . Knight: In view of the fact that most of the faces whose growth rate were measured intersected the substrate, and the substrate is the heat sink, how can you be sure that the growth mechanism is 2-D nucleation rather than by layer growth initiated a t the junction line between face and substrate? Beckmann: We observed some few faces separated from the substrate. For the basal face, we found no difference to rates of faces intersecting the substrate. However, the prism face separated from the substrate were slower by a factor of 5. As the critical supersaturation necessary for 2-D surface nucleation is drastically reduced by the surface layer, this mechanism gives relatively high rates. Furthermore, the initiation of layers at the interface of the substrate and crystal is likely to be retarded by the “quasiliquid layer” existing a t this interface.
E . Hindman: Do your Damkohler numbers achieved in the laboratory represent those in the atmosphere when you produce “atmosphere-like” crystals in the laboratory? Beckmunn: The Damkohler number is universely proportional to the pressure. We worked only up to pressures of 450 mbar. If we extrapolate our data to 1 bar, we obtain qualitatively comparable values to cloud chamber measurements. We thus have comparable Da numbers, if the crystal dimensions are the same. But then, it is difficult to exactly estimate the number for “atmospheric” crystals. What we wanted to show is that, if for the calculation of Da the interface, kinetic rates under pure water
