Qiang Wang

Liquid−Solid Transition of Confined Water in Silica-Based Mesopores

Cooling and heating curves of water confined in partially filled Vycor porous glass were measured for both adsorption and desorption processes. One endothermic and two exothermic peaks were observed for almost all cases. The peak temperature and the enthalpy of the exothermic peak located below 232 K increased initially and then decreased with further increases in the filling factor. These abnormal changes were analyzed based on the liquid-solid transition of nanoconfined water using a core/shell model, and the initial adsorption process of water in this typical mesoporous material with disordered pores is discussed. In addition, an interesting observation is that different peak temperatures for the endothermic peak and an almost constant peak temperature for the exothermic peak were observed at the same filling factor obtained under different sample preparation conditions, that is, adsorption and desorption processes. To compare with the liquid-solid transition temperatures of confined water in fully filled silica-based mesopores of different pore radius, a parameter of the ratio of pore inner surface area to confined liquid volume is proposed in this paper. Referring to this parameter, the core part of confined water in silica-based nanopores has the same liquid-solid transition temperatures. This suggestion is valid for the freezing process of water confined in either fully filled ordered or fully or partially filled disordered pores. For the melting process, different linear changes of melting temperature with the ratio of pore inner surface area to liquid volume were observed for water in disordered and ordered pores.

Glass transition of aqueous solutions involving annealing-induced ice recrystallization resolves liquid-liquid transition puzzle of water.

Liquid-liquid transition of water is an important concept in condensed-matter physics. Recently, it was claimed to have been confirmed in aqueous solutions based on annealing-induced upshift of glass-liquid transition temperature, . Here we report a universal water-content, , dependence of for aqueous solutions. Solutions with vitrify/devitrify at a constant temperature, , referring to freeze-concentrated phase with left behind ice crystallization. Those solutions with totally vitrify at under conventional cooling/heating process though, of the samples annealed at temperaturesu2009u2009 to effectively evoke ice recrystallization is stabilized at . Experiments on aqueous glycerol and 1,2,4-butanetriol solutions in literature were repeated, and the same samples subject to other annealing treatments equally reproduce the result. The upshift of by annealing is attributable to freeze-concentrated phase of solutions instead of ‘liquid II phase of water’. Our work also provides a reliable method to determine hydration formula and to scrutinize solute-solvent interaction in solution.

The decisive role of free water in determining homogenous ice nucleation behavior of aqueous solutions

It is a challenging issue to quantitatively characterize how the solute and pressure affect the homogeneous ice nucleation in a supercooled solution. By measuring the glass transition behavior of solutions, a universal feature of water-content dependence of glass transition temperature is recognized, which can be used to quantify hydration water in solutions. The amount of free water can then be determined for water-rich solutions, whose mass fraction, Xf, is found to serve as a universal relevant parameter for characterizing the homogeneous ice nucleation temperature, the meting temperature of primary ice, and even the water activity of solutions of electrolytes and smaller organic molecules. Moreover, the effects of hydrated solute and pressure on ice nucleation is comparable, and the pressure, when properly scaled, can be incorporated into the universal parameter Xf. These results help establish the decisive role of free water in determining ice nucleation and other relevant properties of aqueous solutions.

Recrystallization of freezable bound water in aqueous solutions of medium concentration

For aqueous solutions with freezable bound water, vitrification and recrystallization are mingled, which brings difficulty to application and misleads the interpretation of relevant experiments. Here, we report a quantification scheme for the freezable bound water based on the water-content dependence of glass transition temperature, by which also the concentration range for the solutions that may undergo recrystallization finds a clear definition. Furthermore, we find that depending on the amount of the freezable bound water, different temperature protocols should be devised to achieve a complete recrystallization. Our results may be helpful for understanding the dynamics of supercooled aqueous solutions and for improving their manipulation in various industries.

On the mechanism of earthquake

The physical mechanism of earthquake remains a challenging issue to be clarified. Seismologists used to attribute shallow earthquake to the elastic rebound of crustal rocks. The seismic energy calculated following the elastic rebound theory and on the basis of experimental results of rocks, however, shows a large discrepancy with measurement-a fact that has been dubbed the heat flow paradox. For the intermediate-focus and deep-focus earthquakes, both occurring in the region of the mantle, there is not any reasonable explanation yet. The current article will discuss the physical mechanism of earthquake from a new perspective, starting from the fact that both the crust and the mantle are discrete collective systems of matters with slow dynamics, as well as from the basic principles of physics, especially some new concepts of condensed matter physics emerging in recent years. 1. Stress distribution in earths crust: Without taking the tectonic force into account, according to the rheological principle that everything flows, the vertical and the horizontal stresses must be in balance due to the effect of gravitational pressure over a long period of time, thus no differential stress in the original crustal rocks is to be expected. The tectonic force is successively transferred and accumulated via stick-slip motions of rocky blocks to squeeze the fault gouges, and then applied to other rocky blocks. The superposition of such additional horizontal tectonic force and the original stress gives rise to the real-time stress in crustal rocks. The mechanical characteristics of fault gouge are different from rocks as it consists of granular matters. Thus the elastic modulus of the fault gouge is much lower than that of rocks, and will become larger with increasing pressure. This character of the fault gouge leads to a tectonic force that increases with depth in a nonlinear fashion. The distribution and variation of tectonic stress in the crust are then specified. 2. Strength of crust rocks: The gravitational pressure can initiate the transition from elasticity to plasticity in crust rocks. A method for calculating the depth dependence of elasticity-plasticity transition is formulated, and demonstrated by exemplar systems. According to the actual situation analysis the behaviors of crust rocks fall into three typical zones: elastic, partially plastic and fully plastic. As the proportion of plastic parts in the partially plastic zone reaches about 10%, plastic interconnection may occur and the variation of shear strength of rocks is mainly characterized by plastic behavior. The equivalent coefficient of friction for the plastic slip is smaller by an order of magnitude, or even less, than that for brittle fracture, thus the shear strength of the rocks for plastic sliding is much less than that for brittle breaking. Moreover, with increasing depth a number of other factors can further reduce the shear yield strength of rocks. On the other hand, since earthquake is a large-scale damage, the rock breaking must occur along a weakest path. Therefore, the actual fracture strength of rocks in a shallow earthquake is assuredly lower than the normally observed average shear strength of rocks. The typical distributions of averaged strength and actual fracture strength in crustal rocks varying with depth are schematically illustrated in the paper. 3. Conditions and mechanisms of earthquake: An earthquake will lead to large volume expansion, and the expansion must break through the obstacles. The condition for an earthquake to occur may be as follows: the tectonic force should exceed the sum of (a) the fracture strength of rocks, (b) the friction force of fault boundary, and (c) the resistance from obstacles. Therefore, the shallow earthquake is characterized by plastic sliding of rocks that break through the obstacles. Accordingly, four possible patterns for shallow earthquakes are put forward. Deep-focus earthquakes are believed to result from a wide-range rock flow that breaks the jam. Both shallow earthquakes and deep-focus earthquakes are the slip or flow of rocks following a jamming-unjamming transition. 4. Energetics and precursors of earthquake: The energy of earthquake is the kinetic energy released from the jamming-unjamming transition. Calculation shows that the kinetic energy of seismic rock sliding is comparable to the total work for rocksshear failure and for overcoming the frictional resistance. There will be no heat flow paradox. More importantly, some valuable seismic precursors are likely to be identified by observing the accumulation of additional tectonic forces, local geological changes, as well as the effect of rock state changes, etc.

Mutual Effects of Glycerol and Inorganic Salts on Their Hydration Abilities

It is a tough challenge to understand the mutual interactions among various components in aqueous solutions of inorganic mixed with organic solutes. The hydration number, nh, and critical hydration number, ncr, determined by the measurements of glass transition of the solutions, in conjunction with tracing the change in local water structure, can provide some insights into the complicated interplays in such a mixture. Here, the nh and ncr for aqueous solutions of glycerol, various chlorides, and mixtures of glycerol with a chloride are determined. The ratio of ncr/nh measures 4 for glycerol and 1.7 for all the chlorides, and for mixtures of glycerol with all of the chlorides except ZnCl2, it falls within these two extremes. Glycerol content dependence of nh and ncr reveals a rich and interesting scenario of mutual effects therein, in particular, the glycerols replacement and sharing of hydration water with salt. In the case of ZnCl2, at most, one hydration water molecule is replaced by glycerol, and the excess glycerol molecules continuously reduce the number of glycerol molecules that share hydration water with ZnCl2. Our results can help establish a pathway for the investigation of interactions among the organic and inorganic components in aqueous solutions, which is desirable for many applications.