Bulk Freezing Research Articles

Effect of Confinement on the Phase Transition of Benzene in Nanoporous Silica: A Positron Annihilation Study

We report new results on the solid−liquid phase transition of benzene confined in nanopores of silica using positron annihilation spectroscopy. The pore sizes, ranging from 0.7 to 7.0 nm, were evaluated by positron lifetime and the Brunauer−Emmett−Teller (BET) techniques. Temperature dependent Doppler and lifetime measurements revealed discontinuities at temperatures below the bulk freezing temperature of benzene corresponding to the freezing of benzene confined in pores of different dimensions present in the silica samples. The temperatures corresponding to the freezing of confined liquids were assigned to the pore sizes on the basis of the well-known inverse correlation of depression in the freezing point with pore sizes. However, the magnitude of shift in the freezing point could not be explained on the basis of the classical Gibbs−Thomson relation. On the other hand, the data are seen to be consistent with the molecular cluster theory for microcrystal growth in confined region.

Simulation of molten metal freezing behavior on to a structure

In the severe accident analysis of liquid metal reactors (LMRs), understanding the freezing behavior of molten metal onto the core structure during the core disruptive accidents (CDAs) is of importance for the design of next-generation reactor. CDA can occur only under hypothetical conditions where a serious power-to-cooling mismatch is postulated. Material distribution and relocation of molten metal are the key study areas during CDA. In order to model the freezing behavior of molten metal of the postulated disrupted core in a CDA of an LMR and provide data for the verification of the safety analysis code, SIMMER-III, a series of fundamental experiments was performed to simulate the freezing behavior of molten metal during penetrating onto a metal structure. The numerical simulation was performed by SIMMER-III with a mixed freezing model, which represents both bulk freezing and crust formation. The comparison between SIMMER-III simulation and its corresponding experiment indicates that SIMMER-III can reproduce the freezing behavior observed on different structure materials and under various cooling conditions. SIMMER-III also shows encouraging agreement with experimental results of melt penetration on structures and particle formation.

Freezing Phenomena of Lennard-Jones Fluid Confined in Jungle-Gym Nanospace: A Monte Carlo Study

We performed grand canonical Monte Carlo simulations for a Lennard-Jones fluid confined in a jungle-gym (JG) nanospace of cubic structure modeled on a specific type of metal organic frameworks (MOFs) to investigate freezing phenomena. Our simulations clarified that the JG nanospace with the pore sizes from 5sigma to 11sigma strongly depresses freezing due to a geometrical hindrance effect, resulting in far lower freezing temperature than the bulk freezing point. The fluid-rod interaction is found to give little effect on the freezing temperature in the larger pore sizes. For smaller pores from 2sigma to 3sigma, on the other hand, a dominant factor is a template effect to enhance the localization of molecules into a specific configuration that matches the locations of potential minima, leading to a variety of molecular configurations. In this range of smaller pore sizes, the solidification temperatures are higher than those of the larger pores mainly due to strong influence of the fluid-rod interaction but are still lower than the bulk freezing temperature. In addition, a unique solid-to-solid transition is observed in a specific size of pore of 2.73sigma, which is caused by structural correlation between adjacent cells. On the basis of these results, a phase diagram in the JG nanospace is drawn.

Water in Nanopores: III. Surface Phase Transitions of Water on Hydrophilic Surfaces

Water in hydrophilic pores shows a rich phase diagram due to the appearance of the surface phase transitions. By simulation studies of water in various hydrophilic pores, we have analyzed the evolution of the water phase diagram with the strengthening of a water−wall interaction potential U0. A second-order wetting transition is observed rather close to the liquid−vapor critical temperature when U0 ≈ −2 kcal/mol. The wetting temperature rapidly decreases with the strengthening of a water−wall interaction. A first-order wetting transition, accompanied by a prewetting transition, is observed when the hydrophilicity of the walls is within the range of −4 ≤ U0 ≤ −3 kcal/mol. At U0 ≤ −4 kcal/mol, the temperature of the wetting transition is below the bulk freezing temperature, and one or two layering transitions appear instead of the prewetting transition. Coexistence curves of the first layering transition are well described by the two-dimensional Ising model with critical exponent β = 0.125. The critical tem...

Superfast Responsive Ionic Hydrogels: Effect of the Monomer Concentration

A series of strong polyelectrolyte gels were prepared in aqueous solution, using the sodium salt of 2‐acrylamido‐2‐methylpropane sulfonic acid (AMPS) as the monomer and N,N'‐methylene(bis)acrylamide (BAAm) as a crosslinker. The gels were both prepared below (−22°C) and above (25°C) the bulk freezing temperature of the water, producing cryogels and hydrogels, respectively. The crosslinker (BAAm) content was set at 17 mol%, while the initial monomer concentration Co was varied over a wide range. It was found that, at −22°C, a macroscopic network starts to form at an initial monomer concentration of as low as 0.1 w/v%. In contrast to the conventional hydrogels formed at 25°C, the cryogels have a discontinuous morphology consisting of polyhedral pores of sizes 100–102 μm. The cryogels exhibit superfast swelling properties, as well as reversible swelling–deswelling cycles in water and acetone. An increase in the initial monomer concentration from 2.5 to 10% further increases the response rate of the cryogels due to the simultaneous increase of the porosity of the networks.

Swelling–deswelling kinetics of ionic poly(acrylamide) hydrogels and cryogels

AbstractA series of ionic poly(acrylamide) (PAAm) gels was prepared by free‐radical crosslinking copolymerization of acrylamide and N,N′‐methylenebisacrylamide in aqueous solutions. The gels were prepared both below and above the bulk freezing temperature of the polymerization solvent water, which are called as the cryogels and the hydrogels, respectively. The deswelling behavior of swollen gels in acetone as well as the reswelling behavior of the collapsed gels in water were investigated. It was shown that the cryogels respond against the external stimuli much faster than the hydrogels. The interior morphology of the cryogel networks exhibits a discontinuity and a two‐phase structure, compared to the continuous morphology of the hydrogel networks. Introduction of the ionic units in the network chains further increased the response rate of the cryogels. In contrast to these advantages of cryogels, they exhibit lower swelling capacities than the conventional hydrogels. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 319–325, 2006

Faraday Instability in a Surface-Frozen Liquid

Faraday surface instability measurements of the critical acceleration, a(c), and wave number, k(c), for standing surface waves on a tetracosanol (C24H50) melt exhibit abrupt changes at T(s)=54 degrees C, approximately 4 degrees C above the bulk freezing temperature. The measured variations of a(c) and k(c) vs temperature and driving frequency are accounted for quantitatively by a hydrodynamic model, revealing a change from a free-slip surface flow, generic for a free liquid surface (T>T(s)), to a surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall (T<T(s)). The change at T(s) is traced to the onset of surface freezing, where the steep velocity gradient in the surface-pinned flow significantly increases the viscous dissipation near the surface.

Surface freezing of chain molecules at the liquid–liquid and liquid–air interfaces

Surface freezing (SF) is the formation of a crystalline monolayer at the free surface of a melt at a temperature Ts, a few degrees above the bulk freezing temperature, Tb. This effect, i.e. Ts > Tb, common to many chain molecules, is in a marked contrast with the surface melting effect, i.e. Ts < or = Tb, shown by almost all other materials. Depending on chain length, n, the SF layer shows a variety of phases, in some cases tuneable by bulk additives. The SF behaviour of binary mixtures of different-length alkanes and alcohols is governed by the relative chain length mismatch, /delta n/n/2, yielding a quasi-"universal" behaviour for the freezing of both bulk and surface. While SF at the liquid air interface was studied rather extensively, Lei and Bain (Phys. Rev. Lett., 2004, 94, 176103) have shown only very recently that interfacial freezing (IF) can be induced also at the water: tetradecane interface by adding the ionic surfactant CTAB to the water phase. We present measurements of the interfacial tension of the water: hexadecane interface, as a function of temperature and the ionic surfactant STAB, revealing IF at a STAB-concentration-dependent temperature Ti > Tb. The measurements indicate that a single frozen monolayer is formed, with a temperature-existence range of up to 10 degrees C, much larger than the 1.2 degrees C found for SF at the free surface of the melt. We also find a new effect, where the IF allows tuning of the interfacial tension between the two bulk phases to zero for a range of temperatures, deltaT = Tmix - Tb < or = Ti - Tb by cooling the system below Ti. We discuss qualitatively the factors stabilizing the frozen layer and their variation from the liquid-air to the liquid-liquid interfaces. The surfactant concentration dependence of Ti is also discussed and a tentative theoretical explanation is suggested.

Surface layering of liquids: The role of surface tension

Recent measurements show that the free surfaces of liquid metals and alloys are always layered, regardless of composition and surface tension; a result supported by three decades of simulations and theory. Recent theoretical work claims, however, that at low enough temperatures the free surfaces of all liquids should become layered, unless preempted by bulk freezing. Using x-ray reflectivity and diffuse scattering measurements we show that there is no observable surface-induced layering in water at T=298 K, thus highlighting a fundamental difference between dielectric and metallic liquids. The implications of this result for the question in the title are discussed.

Experimental and Analytical Studies on the Penetration Integrity of the Reactor Vessel Under External Vessel Cooling

Experimental and analytical studies on the penetration integrity of the reactor vessel have been performed to investigate the potential for reactor vessel failure during a severe accident in the Advanced Power Reactor 1400. Six tests have been performed to analyze the effects of the annulus water between the in-core instrumentation nozzle and the thimble tube, external vessel cooling, in-vessel pressure, melt mass, and melt flow for the maintenance of penetration integrity using alumina (Al2O3) melt as a simulant. The experimental results have been evaluated using the Lower head IntegraL Analysis computer Code (LILAC) and the Modified Bulk Freezing (MBF) model. The test results have shown that the water inside the annulus is very effective in the maintenance of the reactor vessel’s penetration integrity because the water prevents the melt from ejection through penetration. The penetration in the no external vessel cooling case has more damage than that in the external vessel cooling case. An increase in in-vessel pressure from 1.0 to 1.5 MPa did not create penetration damage, but an increase in melt mass from 40 to 60 kg and melt flow due to the vessel geometry significantly increased the amount of penetration damage. The analytical results using the LILAC computer code and the MBF model are very similar to the experimental results for the ablation depth of the weld and the melt penetration distance through the annulus, respectively.

Surface freezing in binary mixtures of chain molecules. II. Dry and hydrated alcohol mixtures.

Surface freezing is studied in dry and hydrated alcohol mixtures by surface x-ray scattering and surface tension measurements. A crystalline bilayer is formed at the surface a few degrees above the bulk freezing temperature. The packing is hexagonal, with molecules aligned along the surface normal in all cases. The in-plane lattice constant reveals a qualitatively different behavior with composition for hydrated and dry mixtures. The simple theoretical approach used successfully for alkane and deuterated alkane mixtures accounts well also for the alcohol mixtures. The repulsive length-mismatch term opposing the mixing entropy term in the free energy of the mixtures is shown to have a universal behavior for all mixtures studied: protonated alkanes, deuterated alkanes, and dry and wet alcohols. This universality is somewhat counterintuitive in view of the different interactions (e.g., hydrogen bonding in alcohols) in the different mixtures.

Surface freezing in binary mixtures of chain molecules. I. Alkane mixtures.

X-ray surface scattering and surface tension measurements are used to study surface freezing in molten mixtures of alkanes. These binary mixtures consist of protonated and deuterated alkanes, as well as of alkanes of different lengths. As for pure alkanes, a crystalline monolayer is formed at the surface a few degrees above the bulk freezing temperature. The structure of the monolayer has been determined on an angstrom scale. A simple theoretical approach is used to account for the thermodynamical observations at the surface and in the bulk. The model is based on a competition between entropic mixing and a repulsive interaction due to chain-length mismatch. The surface and bulk liquid phases are treated as ideal mixtures, while the solid phases are treated as regular mixtures. The theory is found to account well for all the mixtures studied, both hydrogenated-hydrogenated and hydrogenated-deuterated. The repulsive interaction and its dependence on the chain lengths of the components are determined from fits to the measured data.

Self-Diffusion of Nonfreezing Water in Porous Carbohydrate Polymer Systems Studied with Nuclear Magnetic Resonance

Water is an integral part of the structure in biological porous materials such as wood and starch. A problem often encountered in the preparation of samples for, e.g., electron microscopy is that removal of water leads to a decreasing distance between supermolecular structural elements and a distortion of the structure. It is, therefore, of interest to find methods to investigate these materials in the native water-swollen state. We present a method to study water-swollen biological porous structures using NMR to determine the amount and self-diffusion of water within the porous objects. The contribution of bulk water to the NMR signal is eliminated by performing experiments below the bulk freezing temperature. Further decrease of the temperature leads to a gradual freezing of water within the porous objects. The contribution of the freezing water fraction to the migration of water through the porous network is, thus, estimated. The results are rationalized in terms of the ultrastructure of the samples studied, namely, wood pulp fibers and potato starch granules.

Structural transition and solid-like behavior of alkane films confined in nano-spacing

By carrying out accurate molecular simulation of dodecane fluid confined between mica surfaces using molecular models appropriately describing wall–fluid interaction, we show that the state condition of the confined fluid is strongly influenced by the wall–fluid interaction. At ambient condition, for strongly attractive surfaces, the effective density is significantly higher than the bulk density and increases with the narrowing of the confinement spacing. This increase results in the density crossing over into a density region higher than bulk freezing density when the confined film becomes narrower than about six molecular layers, driving the transition to solid-like structure. At six molecular layers, the confined film forms distinct layers as well as in-plane orientational order and inter-plane packing correlation. The results suggest that the strong interfacial interaction is the driving force for the dramatic effects observed in experiment.

Precrystallization of fluids induced by patternedsubstrates

It is shown that a fluid near a topographically patterned wallexhibits crystallization below the bulk freezing point (so-calledprecrystallization). In detail, a periodic array of fixed hardspheres is considered as a wall pattern. The actual type of thepattern corresponds to a face-centred-cubic (fcc) lattice cut alongthe (111), (100) or (110) orientation, a hexagonal-close-packed(hcp) solid with (110) orientation as well as a rhombic latticedistorted with respect to the triangular one. The fluid isrepresented by mobile hard spheres of the same diameter as the fixedwall spheres. By computer simulation we find complete wetting by acrystalline sheet proceeding via a cascade of layering transitionsas the bulk freezing point is approached for the fcc (111) and hcp(110) cases, provided that the wall crystal lattice exactly matchesthat of the coexisting bulk crystal. On the other hand, there isincomplete wetting for the fcc (100) and (110) cases. The freezingof the first layer starts at lower bulk pressures for alattice with a larger lattice constant as compared to that of thecoexisting bulk crystal. A rhombic pattern either results inincomplete wetting by a solid sheet, which is unstable as a bulkphase, or prevents wetting completely. Using a phenomenologicaltheory we derive scaling relations for the thickness of thecrystalline layer which are confirmed by the simulation data. Wefurthermore show that the Lindemann rule of bulk freezing can beapplied also for interfacial freezing transitions.

Freezing and Melting Behavior of CO2 in Nanometer Geometry

We present a comprehensive adsorption and desorption phase diagram for CO2 confined in 4 nm diameter pores of mesoporous VYCOR studied using positron/positronium annihilation spectroscopy. We concentrate on special features that occur at the freezing and melting part of the phase diagram. Both the freezing and melting of the confined CO2 occur at temperatures well below that of the bulk freezing phase transition, with pronounced hysteresis between the confined transitions. Freezing creates open voids within the pores. At the early stages of melting, we observe features that are similar to those seen during liquefaction, suggesting that gas−liquid interfaces are formed at this point. At pressures close to the adsorption “triple point” of the confined material, the freezing temperature increases with decreasing pressure, leading to a curvature of the freezing line. We observe that this is related to incomplete pore filling with liquid prior to the freezing transition. Such curvature is not observed during t...

The behaviour of liquid alkanes near interfaces

Simulations of thick films of liquid alkanes supported on a wax-like substrate were carried out at a number of temperatures in order to investigate the structure and dynamics of molecules near the solid-liquid and the liquid-vapour interfaces. Films of butane, octane and a mixture were investigated. Near the solid surface the liquids were found to be structured and molecular diffusion slowed. However, there was no evidence of a frozen layer at this interface even near the bulk freezing temperature. The mixed liquid showed considerable segregation at both interfaces with preferential absorption of butane at the liquid-vapour interface and octane at the liquid-solid interface.

Entrainment at cold glacier beds

Here we present measurements of the gas content and isotopic composition of debris-rich basal layers of a polar glacier, Meserve Glacier, Antarctica, which has a basal temperature of −17 °C. These measurements show that debris entrainment has occurred without alteration of the glacial ice, and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers, without bulk freezing of water. Entrainment at subfreezing temperatures may have formed the U-shaped trough containing Meserve Glacier. In addition to possibly allowing some cold-based glaciers to be important geomorphic agents, entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets, which are rheologically distinct and can limit the time span of ice-core analyses. Furthermore, accumulating evidence suggests that geomorphologists should abandon the assumption that cold-based glaciers do not slide and abrade their beds.

The surface-ordered phase of liquid heptadecane: a simulation study

The unusual surface-ordered state of liquid n-alkanes was investigated by simulations of a slab of liquid heptadecane with two free surfaces. The results confirm that the surface-ordered state consists of a monolayer of molecules aligned perpendicular to the surface which are mostly in an all trans conformation. The local arrangement of molecules within the monolayer is hexagonal and the density is higher than in the bulk phase. Good agreement was found between the simulation results and X-ray diffraction and reflection data. The simulations showed that there is lateral diffusion within the surface ordered layer, albeit reduced compared to the bulk diffusion, and that there is considerable freedom of motion in the direction perpendicular to the surface. The surface-ordered layer is more like a monolayer of a smectic liquid crystal than a monolayer of solid. A disordered liquid surface at a higher temperature shows no preferred molecular orientation near the surface. The transition between the disordered and ordered phases differs from classical surface freezing in that it is apparently first-order with hysteresis and the thickness does not diverge as the bulk freezing temperature is approached. The simulation results are related to various theories of this transition.

The surface-ordered phase of liquid heptadecane: a simulation study

The unusual surface-ordered state of liquid n-alkanes was investigated by simulations of a slab of liquid heptadecane with two free surfaces. The results confirm that the surface-ordered state consists of a monolayer of molecules aligned perpendicular to the surface which are mostly in an all trans conformation. The local arrangement of molecules within the monolayer is hexagonal and the density is higher than in the bulk phase. Good agreement was found between the simulation results and X-ray diffraction and reflection data. The simulations showed that there is lateral diffusion within the surface ordered layer, albeit reduced compared to the bulk diffusion, and that there is considerable freedom of motion in the direction perpendicular to the surface. The surface-ordered layer is more like a monolayer of a smectic liquid crystal than a monolayer of solid. A disordered liquid surface at a higher temperature shows no preferred molecular orientation near the surface. The transition between the disordered and ordered phases differs from classical surface freezing in that it is apparently first-order with hysteresis and the thickness does not diverge as the bulk freezing temperature is approached. The simulation results are related to various theories of this transition.