Freezing Of Water Droplets Research Articles

On the role of cubic structure in ice nucleation

To clarify the formation mechanism of snow polycrystals the possibility of formation of a cubic ice embryo is discussed on the basis of the homogeneous nucleation theory for supercooled water formed from ambient water molecules in the phase of supersaturated vapour. In this connection, attention is paid to a finding from a model of broken hydrogen bonds that the plane {111} of a cubic ice crystal has a smaller specific interfacial energy than each of the {0001} or {10ovbar|10} planes of a hexagonal ice crystal. Hence, it follows that a critical cubic embryo has a smaller activation energy than a critical hexagonal embryo below a critical temperature; namely, Ostwald's step rule (Stufenregel) holds for a change from cubic ice to hexagonal ice below a critical temperature. This discussion is reinforced by examining, from the viewpoint of this step rule, the observed misorientation of the c-axis of natural snow polycrystals and the results of experiments using frozen water droplets.

Unusual Types of Single Ice Crystals Originating from Frozen Cloud Droplets

Abstract Laboratory experiments were conducted to study the formation process of unusual types of ice crystals, using a cold chamber in the temperature range −15 to −25°C. Ice crystals of the bottle type seem to originate from frozen water droplets with spicules. Ice crystals of twin columns (or twin bullets) with plates were found. By observing the various stages of development of pyramidal faces of ice crystals with plates and ice crystals of “single bullet” type–it was concluded that these pyramidal portions of ice crystals may originate from the spicule of frozen droplets or from hemispherical frozen droplets, and that the pyramidal faces can survive further growth at temperatures colder that −25°C.

Morphology of ice droxtals grown from supercooled water droplets

This study describes the growth modes of polyhedral ice crystals grown from spherical ice crystals formed by freezing of supercooled water droplets. Frozen water droplets were grown on a thin glass plate at -15 and -7°C and at humidities above and below water saturation. In the case of growth at -15°C and at a humidity above water saturation, a frozen water droplet grows into an 8-faced ice crystal through a 20-faced ice crystal, while in the case of growth at -15°C and at a humidity between the ice saturation and the water saturation, it grows into an 8-faced ice crystal with truncated edges and corners through an 8-faced ice crystal with curved and flat surfaces. The growth modes at -7°C and at humidities above and below water saturation are almost the same as those at -15°C, except that the axial ratios c/a of the droxtals at -15°C are about 0.95 but those at -7°C are about 1.04.

凍結水滴の結晶主軸の方向性について

凍結水滴の結晶主軸の方向性について

The production of sub‐micron ice fragments by water droplets freezing in free fall or on accretion upon an ice surface

AbstractIn an attempt to explain the excess concentration of ice crystals over ice nuclei in some maritime cumulus clouds, laboratory experiments were carried out to determine whether sub‐micron ice particles are produced when water droplets freeze in free fall or on riming. The results show that not enough ice fragments are produced when the droplets freeze in free fall but that the numbers of sub‐micron particles ejected during riming may go some way towards explaining the large concentrations of ice crystals in maritime cumulus clouds.

De‐icing

THE ice protection system chosen by Vereinigte Flugtechnische Werke‐Fokker GmbH for the airframe of their VFW 614 aircraft is a T.K.S. fluid system. This system, which is similar to that used, for example on the HS 125, works on the principle of forming a film of Glycol based freezing point depressant fluid over the protected surfaces. This film of ‘de‐icing’ fluid combats ice formation by both melting the ice (or preventing freezing of supercooled water droplets) and by preventing adhesion of any ice that does form. Hence, the system can be operated in a de‐icing or anti‐icing mode.

Pressures and stresses in freezing water drops

Calculations of the pressure in the liquid core of a freezing water droplet and stresses in the surrounding ice shell are made. The theory is developed in terms of two mechanical models for the ice. The first model assumes it is elastic and the second that it is linearly viscoelastic. The calculated pressure development is compared with that measured in drops one centimetre in diameter. It is concluded that the viscoelastic theory is sufficient for describing the pressure growth, but overestimates the magnitudes of the stresses in the shell. It is suggested that a treatment using a non-linear dependence of strain rate on stress would yield a more realistic stress distribution.

水の自然凍結温度附近で急凍結する微水滴の荷電について

The electric charge on freezing water droplets with sizes of 20 to 100μ was measured in a state of free fall at temperatures of -25°C to -40°C. As a result of the measurements, it was noted that the number of negatively charged droplets was greater than that of positively charged ones. It was also found that the mean value of the negative charge was much greater than that of the positive charge in a temperature region colder than -35°C, especially at water homogeneous nucleation temperature; -40°C.This result suggests that the freezing water droplets obtained a negative charge by some mecha- nism, when they were frozen very rapidly. However it appears that the electrification mechanism of the freezing water droplets around -40°C was different from that presented by Mason and Maybank, or Stott and Hutchinson.

Freezing temperatures of water droplets in equilibrium with different gases

Many gases react with water at low temperature to form hydrates called clathrates. Among the atmospheric gases, carbon dioxide, argon, methane, and sulfur dioxide behave in this way. The influence of gases forming clathrates at the freezing temperature of water droplets was investigated to study the possible influence of the clathrates on the freezing of cloud droplets. Gases with very low solubility in water have no effect or a very small one. Gases having higher solubilities increase the supercooling, but the increase is greater than the lowering of the freezing point expected because of dissolved gas. It is concluded that the clathrate structure has an inhibiting effect on the freezing of supercooled water droplets. It is expected that this inhibiting effect will not have a detectable influence on the freezing of cloud droplets because of the low concentration of these gases in the atmosphere.

Freezing of Supercooled Water Droplets due to Collision

Freezing of Supercooled Water Droplets due to Collision

Freezing of Water Droplets: Nucleation Efficiency at Temperatures above −5C

Numerical values obtained in laboratory studies of nucleation effectiveness of pyrotechnically produced smokes are strongly dependent on experimental conditions. Contact nucleation is shown to be relatively important at warm temperatures, when the nuclei are produced in a supercooled cloud of high liquid water content. Thus, pyrotechnics designed to produce nuclei in supercooled clouds should be tested under appropriate conditions.

Freezing of Water Droplets in Equilibrium with Different Gases

Abstract Water droplets in thermal equilibrium with gases of different solubilities were frozen at temperatures between −4 and −14C, in order to observe the influence of the volume of dissolved gas on the shattering of the droplet. In air, which has a low solubility in water, no shattering was produced. In methane with a slightly higher solubility, 5% of the droplets shattered. In mixtures of carbon dioxide and air, the frequency of shattering is a function of the temperature and the concentration of carbon dioxide, attaining under the most favorable conditions a value of 40%. However, a large volume of gas is not a sufficient condition to produce shattering. In acetylene, which has nearly the same solubility as carbon dioxide, and in sulfur dioxide, the solubility of which is more than one order higher, shattering was not observed, either in the pure gas or in mixtures with air. When the released gas is readily dissolved in the remaining solution (as for droplets in acetylene or low concentrations of sul...

Freezing and Shattering of Water Droplets in Free Fall

THE shattering of freezing water droplets has been suggested as a source of the large concentrations of ice crystals found in relatively warm supercooled clouds by a number of workers (ref. 1, for example). But recent work by Dye and Hobbs2,3 and Johnson and Hallett4, using droplets approximately 1 mm in diameter suspended on thin insulating fibres, has shown that nearly all the shattering observed in earlier experiments can be attributed to contamination with carbon dioxide or to the droplets not being in thermal and solution equilibrium with the surrounding air. Johnson5 found that suspended droplets ventilated at their terminal velocity shattered very frequently if they were rotated during freezing but not at all if they were not rotated. Because it is difficult to decide if freezing water droplets do rotate as they fall, the experiments reported here were undertaken as a direct test of the shattering of water droplets freezing in free fall.

The Fragmentation of Freezing Water Droplets in Free Fall

Abstract The fragmentation of water droplets during freezing has not been previously established under conditions which resemble those in natural clouds. In laboratory experiments designed to investigate this phenomenon it is important that the droplets be close to the equilibrium state with their environment prior to nucleation. In this paper theoretical expressions are derived which may be used to calculate the degree to which a droplet is in equilibrium with its environment as it falls through a gas which has a vertical gradient of temperature. Experiments are described in which the fragmentation of freezing droplets from 50–100 μ in diameter have been observed under conditions which are close to those in natural clouds. The fragmentation appeared to be independent of the nucleation temperature over the range investigated (−20 to −32C) and it also occurred for droplets nucleated at −8C by silver iodide in supension. Droplets from 20–50 μ in diameter were not observed to fragment during freezing.

The supercooling and freezing of small water droplets falling in air and other gases

The freezing of individual droplets of very pure supercooled water, ranging in diameter from 5 to 120 μ m, has been photographed and measured as they fall through a vertical tempera­ture gradient established in cold, purified helium, air and other gases. From measurements made on a few hundreds of droplets of the same size, it is possible to deduce the median (or mean) freezing temperature of the group and the statistical spread about the mean temperature. When the ‘observed’ freezing temperatures are corrected for the thermal inertia of the falling droplets and adjusted to a standard rate of cooling, they are in good agreement with earlier work in which the droplets were cooled in a liquid supporting medium. The observed variation of freezing temperature with droplet diameter may be satisfactorily interpreted in terms of the theory of homogeneous nucleation of supercooled water if the specific surface free energy of the crystal/liquid interface is taken to be 20 erg cm -2 between —35 and —40°C. The experimental results are also consistent with, and offer additional support for, the recent Némethy-Scheraga statistical thermodynamical treatment of the ‘flickering-cluster’ model of liquid water.

The Influence of Environmental Parameters on the Freezing and Fragmentation of Suspended Water Drops

Abstract The fragmentation of freezing water droplets in natural clouds has been postulated by several workers, and this phenomenon has been observed in numerous laboratory investigations. However, the profound effect that environmental conditions can have on fragmentation has not been fully appreciated. In the first part of this paper the factors that might affect the freezing behavior and fragmentation of a water drop are discussed, and, where possible, are analyzed in detail. In the second part of the paper results are presented of laboratory experiments on the freezing of suspended water drops 1 mm in diameter. Drops nucleated in air under equilibrium conditions were never observed to shatter and only one drop in ten ejected an ice splinter. The shattering and large splinter counts from suspended drops nucleated in air which have been reported by other workers are attributed to the contamination of the drops by carbon dioxide and nucleation under non-equilibrium conditions. Drops frozen in hydrogen sh...

The electrification of freezing water droplets and of colliding ice particles

When supercooled drops of diameter 1·3 mm were nucleated at — 0·2$C and froze in an environment at — 15$C and fragmented, the average charge on positive residues was 1·5 × 10−3 e.s.u., and on negative residues — 2·2 × 10−3 e.s.u. The charges were comparable with those found in similar measurements by Mason and Maybank but were usually far too large to be accounted for by the Latham-Mason temperature-gradient theory. When ice crystals differing in temperature by up to 10$C were brought into momentary contact, any charge produced was less than 10−4 e.s.u. This result is not inconsistent with the Latham-Mason theory and does not confirm the much larger values reported by Reynolds et al.

The electrification of freezing water droplets and of colliding ice particles

AbstractWhen supercooled drops of diameter 1·3 mm were nucleated at — 0·2$C and froze in an environment at — 15$C and fragmented, the average charge on positive residues was 1·5 × 10−3 e.s.u., and on negative residues — 2·2 × 10−3 e.s.u. The charges were comparable with those found in similar measurements by Mason and Maybank but were usually far too large to be accounted for by the Latham‐Mason temperature‐gradient theory.When ice crystals differing in temperature by up to 10$C were brought into momentary contact, any charge produced was less than 10−4 e.s.u. This result is not inconsistent with the Latham‐Mason theory and does not confirm the much larger values reported by Reynolds et al.

電解質水溶液の霧滴の凍結

電解質水溶液の霧滴の凍結

The Freezing of Water Droplets

A microscope method was used to study the freezing of individual water droplets 9 to 33 microns in diameter formed by condensation on various surfaces and protected with a film of silicone oil. The lowest freezing temperatures were found to be independent of the six surfaces used but dependent upon the volume of the water droplets and the rate of cooling. The results were correlated with those of other observers and it was concluded that homogeneous nucleation occurred in the lowest freezing droplets. Values of the surface free energy for a water-ice interface were calculated.