Freezing nucleation

Prenni, A. J T. B. Onasch, R. T. Tisdale, R. L. Siefert, and M. A. Tolbert, Composition-Dependent Freezing Nucleation Rates for HNO,-H20 Aerosols Resembling Gravity-Wave-Perturbed Stratospheric Particles, J. Geophys. Res., 103, 28439-28450 (1998). 

These sulfuric acid particles become less concentrated as the temperature decreases or the water vapour increases. Under very cold stratospheric conditions, these liquid aerosols may take up water and HNO, forming ternary solutions H,S0/HN0,/H,0, which eventually freeze [19,24,26], Below 192 K, HNO, becomes the dominant condensed acid, and H,S04 drops to below 3 wt %. The thermodynamics and freezing nucleation of ice and H,S04 or HNO, hydrates from such solutions are however not well understood [27,28]. Other types of solid particles, such as the less stable nitric acid dihydrate (NAD, HN0,.2H,0) [29], sulfriric acid tetrahydrate (SAT, H S04.4H,0) [18,30], sulphuric acid hemihexahydrate (SAH, H2S04.6.5H20) [18], nitric acid penta-hydrate (NAP, HN03.5H,0) [31] and more complex sulfuric acid/nitric acid mixed hydrates [32] may also be a key to understanding Type IPSC nucleation and evolution [28],... 

Urey, H. C., L. H. Dawsey and F. O. Rice (1929) The absorption spectrum and decomposition of hydrogen peroxide by light. Journal of the American Chemical Society 51, 1371-83 Uri, N. (1952) Inorganic free radicals in solution. Chemical Reviews 50, 375-454 Vali, G. (2008) Repeatability and randomness in heterogeneous freezing nucleation. Atmospheric Chemistry and Physics 8, 5017-5031... 

P. J. DeMott, D. C. Rogers, S. M. Kreidenweis and Y. Chen. The role of heterogeneous freezing nucleation in upper tropospheric clouds inferences from SUCCESS. Submitted, Geophys. Res. Lett. 1997. 

A glaring deficiency in our current knowledge of PSCs is the description of phase transitions like freezing nucleation or deposition nucleation. Results from laboratory freezing studies using solutions with stratospheric... 

Two important kinds of PSC phase transitions require nucleation freezing nucleation of ice and H2SO4 or HNO3 hydrates from liquid ternary solutions, and deposition nucleation of ice and HNO3 hydrates on preexisting dry nuclei (without participation of the liquid phase). As mentioned above, an understanding of these phase transitions presents a major obstacle to obtaining a clear picture of solid PSC formation. This situation is... 

There is no doubt that water ice forms when temperatures fall 2 K below the frost point due to freezing nucleation in stratospheric ternary solution droplets. At these temperatures the droplets become so dilute that they basically represent water droplets with small amounts of HNO3, and their freezing rates become higher than 10 °cm s which lets even the smallest droplets freeze. Similarly it is expected that at about Tice — 2 K ice would form on pre-existing solid particles (such as NAT) due to deposition nucleation... 

Normally, when a pond of water freezes over, or when a metal casting starts to solidify, nucleation occurs at a temperature only a few degrees below T, . Flow do we explain... 

A hyper-eutectic alloy containing, say, 50% Sb starts to freeze when the temperature reaches the liquidus line (point a in Fig. 20.39). At this temperature pure pro-eutectic Sb nucleates as the temperature continues to fall, more antimony is deposited from the melt, and the composition of the liquid phase moves down the liquidus line to the eutectic point. When this is reached, the remainder of the melt solidifies. The microstructure of alloys of eutectic composition varies somewhat with alloy system, but generally consists of an aggregate of small particles, often platelets, of one of the phases comprising the eutectic in a continuous matrix of the other phase. Finally, the microstructure of the hypereutectic 50% Sb alloy already mentioned... 

There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

Mendez-Villuendas, E. and Bowles, R.K. (2007) Surface Nucleation in the Freezing of Gold Nanoparticles. Physical Review Letters, 98, 185503-1-185503-4. 

A technique utilizing genetically modified bacteria to report the presence of particular compounds in soil has also been developed (37). These bacteria respond to the presence of specific compounds in their environment by producing ice-nucleation proteins that enter into cell membranes, enabling cells to be detected by means of a droplet freezing assay. The presence of trytophan in 1-10 i molar concentrations has been detected using reporter bacteria in a study examining loss of amino acids from roots of Avena barbata into soil (38). 

How does the presence of ice-nucleating protein affect the freezing point of water ... 

Compare the freezing points of distilled water and distilled water containing ice-nucleating bacteria. 

Form a hypothesis about the effect an ice-nucleating protein will have on the freezing temperature of water. Record your hypothesis on page 98. 

In precipitation reactions, powder characteristics depend on the speed of the nucleation of particles and their growth due to the mass flow to the surfaces. In freeze drying and spray drying, powder characteristics primarily depend on the size of droplets, which in turn is determined by the parameters of a nozzle and characteristics of the flow of a carrier gas. Both these methods enable one to obtain powders with very high surface area. 

Supercooling The hot water sample may have a greater tendency to supercool, because it has less dissolved gas which can act as nucleation points for ice to form. Water that does not supercool may form a thin layer of ice at the surface which can insulate the rest of the water from the freezer and delay the freezing process. 

Reid et al. [ 1.12] described the effect of 1 % addition certain polymers on the heterogeneous nucleation rate at-18 °C the rate was 30 times greater than in distilled, microfiltered water and at -15 °C, the factor was still 10 fold hogher. All added polymers (1 %) influenced the nucleation rate in a more or less temperature-dependent manner. However, the authors could not identify a connection between the polymer structure and nucleation rate. None the less it became clear that the growth of dendritic ice crystals depended on to factors (i) the concentration of the solution (5 % to 30 % sucrose) and (ii) the rate at which the phase boundary water - ice crystals moved. However, the growth was found to be independent of the freezing rate. (Note of the author the freezing rate influences the boundary rate). 

---