Glass dynamics, theory

It has been noted that although non-equilibrium situations exist in semi-moist food products, observed a values may predict microbial and biochemical activities fairly well (Chirife and Buera, 1994 Chirife and Buera, 1995 Chirife and Buera, 1996 Chirife et al., 1996 Cardona, 1997). The glass transition theory, on the other hand, remains inconclusive as more counter challenging data have been published (Chirife et al., 1996 Cardona, 1997 Buera et al., 1998). In this report, the investigation into water mobility has provided additional insights on the molecular dynamics of water (mobility) since water is the key solvent carrying nutrients and oxygen to cells. 

It will be recalled that in the dynamic theory of solidification [6.90, 91], the liquid - glass transition is related to the appearance of infinite relaxation time of the liquid. It must be noted that in our case the infinite relaxation times appear near Tt (here slow transformations of the metastable liquid into a stable one take place) and near T1, where the relaxation processes are connected with cluster melting and reclusterizations which occur with overcoming large energetic barriers. 

Fig. 4.1.4. Reflexion coefficient at normal incidence versus wavelength for a non-absorbing cholesteric (a) semi-infinite medium, b) film of thickness 25P, where P is the pitch. Curves are derived from the dynamical theory circles represent values computed from the exact theory ( 4.1.3) assuming that the medium external to the cholesteric (e.g., glass) has a refractive index 1.5. The parameters used in the calculations are n = 1.5, Sn = 0.07, X = nP = 0.5 /on. (After reference 21.)...

In conclusion, one should admit that in the interpretation of relaxation processes the thermod3mamic approach is still dominated despite the fact that there are attempts to reunite the kinetic and dynamical theories. It is important to note that the progress in the study of glass transition during the last decades... 

In the next sections we will focus on analyzing the dynamics of supercooled liquids in more detail and discuss our findings in terms of the modecoupling theory of the glass transition, which is a liquid state theory that predicts the dynamics from the structural properties of the liquid. 

It would be an advantage to have a detailed understanding of the glass transition in order to get an idea of the structural and dynamic features that are important for photophysical deactivation pathways or solid-state photochemical reactions in molecular glasses. Unfortunately, the formation of a glass is one of the least understood problems in solid-state science. At least three different theories have been developed for a description of the glass transition that we can sketch only briefly in this context the free volume theory, a thermodynamic approach, and the mode coupling theory. 

Thereby, the features of the a-relaxation observed by different techniques are different projections of the actual structural a-relaxation. Since the glass transition occurs when this relaxation freezes, the investigation of the dynamics of this process is of crucial interest in order to understand the intriguing phenomenon of the glass transition. The only microscopic theory available to date dealing with this transition is the so-called mode coupling theory (MCT) (see, e.g. [95,96,106] and references therein) recently, landscape models (see, e.g. [107-110]) have also been proposed to account for some of its features. 

The only currently existing theory for the glass transition is the mode coupling theory (MCT) (see, e.g. [95, 96, 106]). MCT is an approach based on a rather microscopic description of the dynamics of density fluctuations and correlations among them. Although the theory was only formulated originally for simple (monatomic) fluids, it is believed to be of much wider applicability. In this review we will only briefly summarize the main basis and predictions of this theory, focusing on those that can be directly checked by NSE measurements. 

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