Liquid-like Layer Research Articles

Atomic simulations for surface-initiated melting of Nb(111)

A modified analytic embedded-atom model(MAEAM) was applied to investigate surface premelting and melting behaviors of Nb(111) plane by molecular dynamics(MD) simulations. First the relaxation of surface interface space at 300 K was studied. Then a number N of the disordered atoms per unit area was determined at the given temperatures to investigate the surface premelting and melting evolution. The obtained results indicated that the premelting phenomena occurred at about 1 100 K and a liquid-like layer emerged on (111) plane simultaneously. As temperature increased up to 2 200 K, the number N grew logarithmically for short-range metallic interactions. Upon 2 350 K surface melting generated originally and the number N increased exponentially with the incremental temperature.

Absence of grain boundary melting in solid helium

Crystals are often expected to start melting at their free surface or at the interface betweengrains. Grain boundary melting corresponds to the situation where the interfacebetween grains is invaded by a thick liquid film at the bulk melting temperatureTm. Insome cases, premelting is predicted, with liquid-like layers appearing between grains at temperaturesbelow Tm. We review this topic, and describe our experiments on solid helium 4.We find that grain boundaries are not wetted by the liquid atTm: they emerge at the liquid–solid interface with a non-zero contact angle. This is consistentwith a general argument which predicts that, although systems with short-range forcesmight show grain boundary melting and premelting, in systems with long-range forces(like helium), grain boundaries can only be wetted incompletely by the liquid atTm.

Molecular dynamics simulation of surface melting behaviours of the V(110) plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V(110) plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V(110) surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V(110) surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.

Anionic microsolvation in helium droplets: OH−(He)N structures from classical and quantum calculations

Diffusion Monte Carlo calculations are carried out for clusters of OH- (1Sigma+) with N 4He atoms, N varying up to 15, while classical configurations from a genetic algorithm optimization are obtained up to N=20. The overall interaction potential is assembled from ab initio data for the partners using the sum-of-potentials scheme. In contrast with the cationic dopants' behavior, the results indicate a very marked spatial delocalization and quantum features of the solvent adatoms surrounding the anionic impurity, thus making classical calculations of solvent's spatial locations of only limited use. In spite of the generally known repulsive interaction of negative charges in He droplets, the calculations show that this polar molecular anion is solvated by a liquidlike solvent layer, reminiscent of what happens in pure helium droplets.

An electron microscopy appraisal of tensile fracture in metallic glasses

Three glass-forming alloy compositions were chosen for ribbon production and subsequent electron microscopy studies. In situ tensile testing with transmission electron microscopy (TEM), followed by ex situ TEM and ex situ scanning electron microscopy (SEM), allowed the deformation processes in tensile fracture of metallic glasses to be analysed. In situ shear band propagation was found to be jump-like, with the jump sites correlating with the formation of secondary shear bands. The effect of structural relaxation by in situ heating is also discussed. Nanocrystallization near the fracture surface was observed; however, no crystallization was also reported in the same sample and the reasons for this are discussed. Both the TEM and the SEM observations confirmed the presence of a liquid-like layer on or near the fracture surface of the ribbons. The formation of a liquid-like layer was characterized by the vein geometries and vein densities on the fracture surfaces and its dependence on shear displacement, δ, is discussed. A simple model is adapted to relate the temperature rise during shear banding to the glass transition and melting temperatures and this is used to explain the variety of fracture surfaces which are developed for macroscopically identical tensile testing of metallic glasses together with features which exhibit local melting.

The influence of van der Waals forces on the state of water in the shallow subsurface of Mars

Microscopic liquid layers of water can evolve via adsorption on grain and mineral surfaces at and in the soil of the surface of Mars. The upper parts of these layers will start to freeze at temperatures clearly below the freezing point of bulk water (freezing point depression). A sandwich structure with layers of ice (top), liquid water (in between) and mineral surface (bottom) can evolve. The properties of the interfacial water (of adsorption water and premelted ice) on grain surfaces are described by a sandwich-model of a layer of liquid-like adsorption water between the adsorbing mineral surface layer and an upper ice layer. It is shown that the thickness or number of mono-layers of the interfacial water (of adsorption water and premelted ice) depends on temperature and atmospheric relative humidity. The derived equations for the sandwich model fit well to a known phenomenological relation between thickness of the liquid layer and relative humidity, and can be a tool to estimate or to determine for appropriate materials Hamaker's constant for van der Waals interactions on grains and in porous media. The curvature of grain surfaces is shown to have no remarkable effects for particles in the μm-range and larger. The application of these equations to thermo-physical conditions on Mars shows that the thickness of frost-layers, which can evolve over several hours on cooling surface parts of Mars, is typically of the order or a few tenths of one millimeter or less. This is in agreement with observations. Furthermore, an equation is derived, which relates the freezing point depression for van der Waals force governed interfacial water to the value of the Hamaker constant, to the latent heat of solidification, to the mass density of water ice, and to the thickness of the liquid-like layer. Again, this equation fits well to a known phenomenological relation between freezing point depression and thickness of the liquid-like layer. The derived equation shows that the lower limiting temperature of the liquid phase can reach about 180 K under martian conditions having an atmospheric water content of around 10 pr μm. An “Equilibrium Moisture Content” (EMC)/“Equilibrium Relative Humidity” (ERH) relation for the water content of martian soil has been derived, which relates, for equilibrium conditions, soil water content and atmospheric relative humidity. This relation indicates that the content of liquid interfacial water in the upper surface of Mars can reach up to 10% by weight and more in course of saturation during night hours, and it can be of about 2% by weight during the dry daytime hours.

Enhanced Aqueous Photochemical Reaction Rates after Freezing

Sunlit snow/ice is known to play an important role in the processing of atmospheric species, including photochemical production of NO(x), HONO, molecular halogens, alkyl halides, and carbonyl compounds, among others. It has been shown that a liquid-like (quasi-liquid or disordered) layer exists on the surface of pure ice and that this quasi-liquid layer is also found on the surface of ambient snow crystals and ice at temperatures similar to polar conditions. However, it is unclear what role the liquid-like fractions present in and on frozen water play in potential photochemical reactions, particularly with regard to organic substrates. Here, we report a detailed study of enhanced rates of photochemical nucleophilic substitution of p-nitroanisole (PNA) with pyridine, a well-characterized and commonly used actinometer system. Reaction rates were enhanced by a factor of up to approximately 40 when frozen at temperatures between 236 and 272 K. Reaction rates were dependent on temperature and solute concentration, both variables that control the nature of the liquid-like fraction in frozen water. The results obtained indicate that a major portion of the organic solutes is excluded to the liquid-like layer, significantly impacting the rate of the photochemical nucleophilic substitution reaction studied here. Also, the direct comparison of liquid-phase kinetics to reactions occurring in frozen water systems is drawn into question, indicating that a simple extrapolation of liquid-phase mechanisms to snow/ice may not be valid for certain reactions.

Morphological evidence for surface pre-melting on Si(1 1 1)

We present what we believe to be the first morphological evidence for the occurrence of surface pre-melting on the Si(1 1 1) surface. Our results complement the extensive previous evidence from diffraction and ion scattering techniques for the presence of pre-melted (liquid-like) layers on Si(1 1 1) below the bulk melting temperature and also suggest how atomic steps are involved in the initiation of such layers. Our results are based on atomic force microscopy studies of morphologies that are preserved when surfaces are annealed in a range of high temperatures and then rapidly cooled to room temperature for observation. A unique feature of the experiments is the use of specially prepared atomically flat or very low step density surfaces; this allows us to see how the liquid-like morphologies are associated with the steps and also allows the high temperature structures to survive the cooling process without being absorbed into the steps which normally would exist on a surface vicinal to (1 1 1). Quenched-in structures ascribed to pre-melting also act as sinks for diffusing ‘excess’ adatoms generated by the (1 × 1) to (7 × 7) transition and this leads to the formation of dendritic islands.

Hydration forces and liquid-like layer on the ice/metal interface

A model to describe the phenomenon of the liquid-like layer is proposed. It is based on thetheory of the hydration forces proposed by Gruen and Marcelja (1983 J. Chem. Soc.Faraday Trans. II 79 225), taking into account the influence of ions on the free energy ofwater. The model was applied to experimental data obtained with the quartz-crystalmicrobalance, in a study of the liquid-like layer between metal and frozen aqueouselectrolytes and between metal and ice.

Surface melting of close-packed Mg(0001)

Using molecular dynamics simulations and an analytic embedded-atom method, an investigation of the surface anharmonicity for the close-packed surface of Mg(0001) is presented in the temperature range from 0 K to its bulk melting temperature. The temperature dependences of the surface phonon frequencies, the mean square vibrational amplitudes of surface atoms and the layer structure factor have been calculated. The calculated results are in good agreement with the experimental data available. Surface melting of the closed-packed Mg(0001) is predicted by the calculation of the layer structure factor, and a liquid-like layer is observed from the simulation of its microstructure. The possible mechanism for the anomalous behavior is discussed.

Novel model for film growth based on surface temperature developing during magnetron sputtering

New physical phenomenon consisting in development of the surface temperature T surf, which being equal to the substrate temperature T s at the beginning of deposition, steeply increases and becomes several times higher than T s at the end of the process, is revealed by means of IR-camera and new calorimetric method during sputter deposition of metal films. The reason for the phenomenon is the formation of a liquid-like layer on the growth surface with extremely low (∼10 9 times lower than for metals) thermal conductivity. Variation in the film structure along thickness correlates with the variation in T surf. To explain these effects we developed a model according to which film grows by “gas → liquid → solid” rather than “gas → solid” mechanism which is realized provided that the film grows from energetic atoms.

Subsecond sintering of ice

Ice grains bond together in less than 1s. The tensile strength of the resulting bond was measured above −25°C in the contact time range of 10–1000ms as a function of temperature and contact load. The bond strength increases nonlinearly with time and with increasing temperature. The results indicate that the most likely mechanism of ice sintering on this time scale is the freezing of the liquidlike layer present on the surface of the ice. A model based on the plastic behavior of ice and the complete freezing of the entire contact patch well describes the observations.

Viscoelastic nanoscale properties of cuticle contribute to the high-pass properties of spider vibration receptor (Cupiennius saleiKeys)

Atomic force microscopy (AFM) and surface force spectroscopy were applied in live spiders to their joint pad material located distal of the metatarsal lyriform organs, which are highly sensitive vibration sensors. The surface topography of the material is sufficiently smooth to probe the local nanomechanical properties with nanometre elastic deflections. Nanoscale loads were applied in the proximad direction on the distal joint region simulating the natural stimulus situation. The force curves obtained indicate the presence of a soft, liquid-like epicuticular layer (20-40 nm thick) above the pad material, which has much higher stiffness. The Young modulus of the pad material is close to 15 MPa at low frequencies, but increases rapidly with increasing frequencies approximately above 30 Hz to approximately 70 MPa at 112 Hz. The adhesive forces drop sharply by about 40% in the same frequency range. The strong frequency dependence of the elastic modulus indicates the viscoelastic nature of the pad material, its glass transition temperature being close to room temperature (25 +/- 2 degrees C) and, therefore, to its maximized energy absorption from low-frequency mechanical stimuli. These viscoelastic properties of the cuticular pad are suggested to be at least partly responsible for the high-pass characteristics of the vibration sensor's physiological properties demonstrated earlier.

Electrochemical Behavior of Hexacyano Iron (III/II) in Frozen Aqueous Electrolytes

Electrochemical studies in phases that have been solidified by freezing were carried out. A 5 mM [Fe(CN)6]-3/-4 [Hexacyano Iron(III/II)] in either aqueous 0.1M KCl or in Agar gel containing 0.1 M KCl Agar were the two phase systems subject to this study. Formation of pristine ice crystals (salt free) in pure aqueous electrolyte explains smaller ΔEp recorded in frozen agar than that in absence of Agar. Furthermore, greater capacitive current was observed in frozen agar compared to that in frozen aqueous electrolyte. The results also indicates that while a liquid-like layer (≈ 4 time the value of √2Dt) is formed in frozen aqueous electrolyte at electrode/ electrolyte interface, opposite observation were recorded in frozen agar electrolyte. This evident from the thin-layer electrochemical outcome generated in frozen electrolyte, and low diffusion coefficient determined under these conditions. Deviations from the liquid state behavior at room temperature were explained on the assumption that freezing created conditions similar to those caused by application of high pressure on liquid electrochemical systems. Some kinetic data related to these systems were determined and recorded.

Hot isostatic pressing of Cu–Bi polycrystals with liquid-like grain boundary layers

The grain boundary segregation in an Cu–50 at.ppm Bi alloy annealed at two temperatures and under various hydrostatic pressures in a hot isostatic pressing apparatus was investigated by means of Auger electron spectroscopy. It was found that high pressures have only little effect on grain boundary segregation. At a temperature of 973 K the segregation level remained approximately constant at 2 monolayers of Bi for all pressures studied. Some decrease of the grain boundary segregation with increasing pressure was observed at 1173 K. It was also demonstrated that the segregation level in the alloy treated at 0.01 GPa depended on the sample cooling rate after annealing. The observed pressure dependence of Bi segregation to the grain boundaries was interpreted in terms of non-equilibrium segregation during specimen cooling.

Thickness of shear bands in metallic glasses

A review of measurements and atomistic modeling shows that shear bands in metallic glasses have a characteristic thickness of ∼10nm. Such extreme localization of plastic deformation, within a thicker liquidlike layer implied by fracture-surface morphology, cannot have a thermal origin. By analogy with granular materials, the thickness is linked to the local structural rearrangements required to generate dilatation. This analysis suggests that first-coordination-shell clusters may be significant structural units in metallic glasses.

Adsorption and structure of argon in activated porous carbons

Molecular simulations are used to investigate the adsorption and structure of argon in ordered and disordered models of porous carbons. The ordered porous carbon (model A) is an assembly of regular slit pores of different sizes, while the disordered porous carbon (model B) is a structural model that reproduces the complex pore shape and pore connectivity of saccharose-based porous carbons. The same pore size distribution is used for models A and B so that we are able to estimate, for similar confinement effects, how the disorder of the porous material affects the adsorption and structure of the confined fluid. Adsorption of argon at 77.4 K in the two models is studied using Grand Canonical Monte Carlo simulations. The structure of the confined fluid is analyzed using crystalline bond order parameters and positional or bond orientational pair correlation functions. The filling pressure for the assembly of slit pores is much lower than that for the disordered porous carbon. It is also found that the isosteric heat of adsorption for the ordered porous model overestimates that for the disordered porous model. The results suggest that the agreement between models A and B would be improved if the same density of carbon atoms were used in these two models. Strong layering of Ar is observed at all pressures for model A. The confined phase is composed of liquid-like layers at low-pressures, which crystallize into well-defined hexagonal 2D crystals after complete filling of the pores. The structure of argon in the disordered porous carbon strongly departs from that in the slit pore model. Although its structure remains liquid-like overall, argon confined in model B is composed of both crystalline clusters and amorphous (solid or liquid) nano-domains.

Migration of phenolic antioxidants from linear and branched polyethylene

Plaques of linear polyethylene (LPE) and branched polyethylene (BPE) were exposed to oxygen-free media (nitrogen or water) at 75, 90 and 95 °C. The polymers were stabilized with one of the following three bifunctional phenolic antioxidants: Santonox R, Irganox 1081 or Lowinox 22M46. The initial concentration of antioxidant in the plaques was ∼0.09 wt.%. After ageing, the oxidation induction time profiles obtained by differential scanning calorimetry often became very flat, which indicated that migration was controlled by the boundary loss process. The unexpected higher migration rate from LPE than from BPE was due to the dominance of the boundary loss process. It is proposed that the low boundary loss rate in BPE was due to the presence of a thin liquid-like (oligomeric) surface layer which developed during ageing of this polymer. A qualitative relationship was found between the boundary loss rate to water and the polarity of the antioxidant. The antioxidant diffusivities in LPE and BPE were approximately equal, a finding which, in view of the morphological analysis estimating the geometrical impedance factor, indicated that the constraining effect of the crystals on the non-crystalline fraction was not sensed by the antioxidant molecules. It is suggested that the large molecular size and the low segmental flexibility of the antioxidant molecules inhibited their ability to penetrate the interfacial component.

Thermally Responsive Interactions between the PEG and PNIPAAm Grafts Attached to the PAAc Backbone and the Corresponding Structural Changes of Polymeric Micelles in Water

Graft copolymers comprising poly(acrylic acid) (PAAc) as the backbone and poly(N-isopropylacrylamide) (PNIPAAm) and monomethoxy poly(ethylene glycol) (mPEG) as the grafts were prepared and characterized. These copolymers with the AAc residues in the negatively charged state underwent self-assembling into large polymeric aggregates in water in the range 32−35 °C due to the formation of the multicore structure (comprising several solidlike hydrophobic PNIPAAm cores surrounded by the liquidlike interfacial layers) and the intercore PEG connections. These rather labile intercore PEG connections were partially destroyed due to the continual dehydration and solidification of the liquidlike interfacial layers with increasing temperature, thereby leading to fragmentation of the original aggregates into smaller particles with the more distinct core/shell structure. The final particle size was virtually governed by the structure of the hydrophobic PNIPAAm microdomains in response to the effects of the PNIPAAm concentration and temperature on the intercore PEG connections.

Water on MgO(1 0 0)—An infrared study at ambient temperatures

Transmission Fourier transform infrared spectroscopy (FTIR) is used to investigate water adsorbed on the (1 0 0) face of MgO under ambient conditions. From these measurements, we find that water reversibly adsorbs to MgO(1 0 0) creating liquid-like layers at temperatures ranging from −10 °C to 40 °C with slight hysteresis seen during a complete adsorption/desorption cycle. The adsorption isotherms of water on MgO(1 0 0), throughout the entire temperature range sampled, resemble a typical BET plot, which coupled with the fact that IR absorption due to the OH stretch of the adsorbed water does not change as a function of coverage, suggests that the layer grows in via three-dimensional island formation. Thermodynamic analysis indicates three distinct regions of water adlayer growth.