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Notes: We propose here a new mechanism of droplet coarsening in phase-separating fluid mixtures. In contrast to the conventional understanding that there are no interactions between droplets in the late stage of spinodal decomposition, we demonstrate the existence of interactions between droplets that is caused by the coupling between diffuse concentration change around droplets. We show the possibility that this mechanism plays an important role in droplet phase separation together with Brownian-coagulation mechanism. We also discuss the coupling between hydrodynamic and diffusion modes, namely, "collision-induced collision" phenomena. © 1997 American Institute of Physics.

Notes: We propose a simple two-state model of water to explain the unusual thermodynamic and dynamic behavior of liquid water. Our model is based on a physical picture that there exist two competing orderings in water, namely, density ordering and bond ordering. Short-range bond ordering leads to the formation of a rather stable locally favored structure (in a ground state) in a sea of disordered normal-liquid structures (in an excited state). Its fraction increases with decreasing temperature, obeying a Boltzmann factor. The concept of a "symmetry (or volume) element" is introduced to specify such locally favored structures in an unambiguous manner. The most probable candidate of such locally favored structures is an "octameric unit," which is an elementary structural unit of ice Ih. According to this picture, the uniqueness of water comes from that below the crossover pressure Pc (∼2 kbar) the short-range bond order can develop into the long-range order (crystallization into ice Ih). Note that in ordinary liquids crystallization is induced only by density ordering, while in water it is induced by bond ordering below Pc, while by density ordering above Pc. Our model predicts that the anomalous parts of density ρ, isothermal compressibility KT, heat capacity at constant pressure CP, and the activation energy of viscosity η should all be proportional to the Boltzmann factor in the temperature region, where "bulk water" is in a liquid state. It is found that this prediction well explains not only the thermodynamic anomaly of ρ, KT, and CP, but also the dynamic anomaly of η, including their pressure dependencies. This demonstrates that the anomaly of "bulk water" is a direct consequence of short-range bond ordering and it is not due to the thermodynamic singularity, at least above −20 to −30° C. Our model indicates a new possibility that the viscosity anomaly of water may also be explained by the same mechanism as that of the thermodynamic anomaly; namely, neither critical anomaly nor slow dynamics associated with a glass transition may be a major cause of the dynamic anomaly of water above −20 °C. We also point out that the water's thermodynamic and dynamic anomaly is not necessarily related to the low-temperature phase behavior of liquid water in an obvious manner, contrary to the common belief. © 2000 American Institute of Physics.

Notes: In our previous paper (paper I), we proposed a simple physical model that may universally describe glass-transition phenomena from the strong to fragile limit. It is based on the idea that in any liquid there always exist two competing orderings, which lead to two types of local structures frustrated with each other: (i) normal-liquid structures and (ii) locally favored structures. Here we demonstrate that this frustration, which causes an extra energy barrier for crystal nucleation, can be an additional physical factor to make vitrification easier. It can be regarded as impurity effects on crystallization. This idea provides us with a simple physical criterion for vitrification, which is consistent with the well-known empirical laws. We also check several main predictions of our model. According to our model, the melting temperature of the corresponding pure system free from disorder effects, Tm*, is a key temperature: Below it, a system starts to have special dynamic features peculiar to the Griffiths phase known in the field of random-spin systems, which is characterized by a complex free-energy landscape. We stress that this prediction is specific in the sense that Tm* is directly related to the real melting point Tm, which is an intrinsic physical property of the material. In our view, a stronger liquid suffers from stronger disorder effects due to a higher concentration of locally favored structures. This leads to a larger distance between Tm* and the Vogel–Fulcher temperature T0 for a stronger liquid, which is consistent with experimental results. Finally, the effect of pressure on the fragility is discussed in the light of our two-order-parameter model of liquids. © 1999 American Institute of Physics.

Notes: Here we propose a simple physical model that may universally describe glass-transition phenomena from the strong to the fragile limit. Our model is based on the idea that there always exist two competing orderings in any liquids, (i) density ordering leading to crystallization and (ii) bond ordering favoring a local symmetry that is usually not consistent with the crystallographic symmetry. The former tries to maximize local density, while the latter tries to maximize the quality of bonds with neighboring molecules. For the phenomenological description of these competing ordering effects [(i) and (ii)] hidden in many-body interactions, we introduce density and bond order parameters, respectively. This leads to the following picture of a liquid structure: Locally favored structures with finite, but long lifetimes are randomly distributed in a sea of normal-liquid structures. Even simple liquids suffer from random disorder effects of thermodynamic origin. We argue that locally favored structures act as impurities and produce the effects of "fluctuating interactions" and "symmetry-breaking random field" against density ordering, in much the same way as magnetic impurities for magnetic ordering in spin systems. Similarly to random-spin systems, thus, we predict the existence of two key temperatures relevant to glass transition, the density ordering (crystallization) point Tm* of the corresponding pure system without frustration and the Vogel–Fulcher temperature T0. Glass transition is then characterized by these two transitions: (A) a transition from an ordinary-liquid state to a Griffiths-phase-like state at Tm*, which is characterized by the appearance of high-density metastable islands with medium-range order, and (B) another transition into a spin-glass-like nonergodic state at T0 and the resulting divergence of the lifetime of metastable islands, namely, the α relaxation time. Between Tm* and T0, a system has a complex free-energy landscape characteristic of the Griffiths-phase-like state, which leads to the non-Arrhenius behavior of α relaxation and dynamic heterogeneity below Tm*. This simple physical picture provides us with a universal view of glass transition covering the strong to fragile limit. For example, our model predicts that stronger random-disorder effects make a liquid "stronger," or "less fragile." © 1999 American Institute of Physics.