Freezing homogeneous nucleation

Abstract A simplified quintuple model for the description of freezing and thawing processes in gas and liquid saturated porous materials is investigated by using a continuum mechanical approach based on the Theory of Porous Media (TPM). The porous solid consists of two phases, namely a granular or structured porous matrix and an ice phase. The liquid phase is divided in bulk water in the macro pores and gel water in the micro pores. In contrast to the bulk water the gel water is substantially affected by the surface of the solid. This phenomenon is already apparent by the fact that this water is frozen by homogeneous nucleation. 

Early work by Vonnegut and Schaefer demonstrated that water could be undercooled by close to 40 degrees below the equilibrium freezing point. Wood and Walton carried out a careful series of experiments in 1970 that were interpreted as due to homogeneous nucleation and from which the surface free energy and its temperature derivative were extracted. These experiments used the droplet emulsion method with the fraction of droplets crystallized as a function of time measured with a camera through a micro-... 

Our results are consistent with ice f nucleating in all (or most) the droplets, and the final fraction of ice Ic being governed by increase in droplet temperature during crystal growth. From this we infer that ice f, rather than ice Ih, is likely to be the crystalline phase that nucleates when water droplets freeze homogeneously at approximately 235 K. 

Water is probably the most important and the most intensely studied substance on Earth. It is the solvent of life and it is also of vital importance in many aspects of our existence, ranging from cloud microphysics to its key role as a solvent in many chemical reactions. The familiar process of water freezing is encountered in many natural and technologically relevant processes. In this contribution, we discuss the applicability of the methods of computational chemistry for the theoretical study of two important phenomena. Namely, we apply the molecular dynamics (MD) simulations to the study of brine rejection from freezing salt solutions and the study of homogeneous nucleation of supercooled water. 

FIGURE 16.6 State diagram of the sucrose-water system. T is temperature, ij/s mass fraction of sucrose. Tf gives the freezing temperature of water and Ts the solubility of sucrose. Tg is the glass transition temperature and Thom the homogeneous nucleation temperature. 

Fig. 4.4. Supercooling obtainable before freezing for pure water droplets as a function of size, as observed by various workers Bigg (A), Carte (B), Chahall and Miller (C), Day (D), Dorsch and Hacker (E), Heverley (F), Hosier (G), Jacobi (H), Kiryukhin and Pevsner (I), Langham and Mason (J), Meyer and Pfaff (K), Mossop (L), Pound (M), and Wylie (N). The broken curve shows the homogeneous nucleation threshold calculated from theory with /r = 22 erg cm . ...

In our experiments we were able to determine the rate J of homogeneous nucleation in water droplets to a high precision by observing the freezing transition of more than 2000 individual droplets. 

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