Describing non-Fickian water-vapour sorption in wood

Moisture transport and sorption in wood may not be accurately described by Fick's law of diffusion. The problem of making a model of non-Fickian behaviour (NFB) for wood is discussed. Some measurements in which NFB in wood is clearly seen are also reviewed. Four criteria, which must be satisfied by a model describing sorption in wood cell walls, are presented: (1) the model should not only describe the response to step changes in vapour pressure; (2) it should be able to predict sorption with more than one time scale; (3) the sorption rate should not depend on the thickness of the cell wall; (4) small rapid changes in vapour pressure should give slower fractional weight change than large rapid changes. A review of models of NFB in synthetic polymers indicates that there is presently no model of NFB which fulfills the above criteria. More measurements of the sorption behaviour of the cell wall are needed to construct such a model for wood. This model can then probably be used, together with a Fickian diffusion model, to model the sorption behaviour of whole wood.Nomenclature c concentration in a material kg m^–3 - D _c diffusivity with c as potential m² s^–1 - D _p diffusivity with p as potential kg/(m s Pa) - F flux kgm^–2–1 - p partial vapour pressure Pa - t time s - x distance m     

吸湿法测试纺织品透湿性影响因素分析

为减小纺织品透湿率检验工作的误差,按照吸湿法的测试原理对比分析干燥剂粒径、风速和试验时间对纺织品透湿率的影响。结果表明:干燥剂粒径、风速、平衡时间和测试时间对透湿率较小的试样无明显影响;干燥剂粒径越大,平衡时间和测试时间越长,透湿率较大的试样的测试结果越小;当风速保持在0.3 m/s时试样的透湿率测试结果较为稳定。采用线性回归分析方法测试透湿率更为精准,有利于减小实际检测误差。     

The analysis of residual stress in glass-to-metal seals for solar receiver tube

The solar receiver tube is a key component in the parabolic trough solar thermal power system. Breakage of glass-to-metal sealing is main cause for damages of receivers in existing power plants. The residual stresses that are generated during the cooling process of the seal can decrease the seal strength and induce the breakage of the glass-to-metal sealing. Residual stresses were simulated by finite element software ANSYS and measured experimentally by photoelastic techniques. In order to lower the seal failure probability, the effects of the component dimensions were analyzed using finite element method. The simulation results agreed with the measurement results. The dangerous tensile stresses occur not only at the glass–metal interface but also on the outer surface of glass tube near the sealing area. When the depth of metal embedded into glass increases, the magnitude of residual stress decreases and the sealing strength will increase. The results of this study have important implications on the optimization of seal configuration in the solar receiver tubes.     

A versatile plasma technique to improve plastic materials against gas and water-vapour permeation

Plasma-enhanced chemical deposition processes have been studied by comparing the performance of flexible diffusion barrier layers on plastic films produced in the same reactor. Under similar experimental conditions, a higher deposition rate is achieved by microwave discharges than by bipolar, pulsed d.c. magnetron sputtering processes. However, with both discharge modes, dense hydrocarbon coatings were produced, exhibiting a barrier improvement factor up to 120 and a flexibility ranging from 1.1%–8.8% before formation of microcracks started to dominate permeation characteristics. The density of the coatings is 1.0–1.6 g cm–3 and their hydrogen content varies from 23%–33%.     

Wetting transparency of graphene

We report that graphene coatings do not significantly disrupt the intrinsic wetting behaviour of surfaces for which surface-water interactions are dominated by van?der?Waals forces. Our contact angle measurements indicate that a graphene monolayer is wetting-transparent to copper, gold or silicon, but not glass, for which the wettability is dominated by short-range chemical bonding. With increasing number of graphene layers, the contact angle of water on copper gradually transitions towards the bulk graphite value, which is reached for ~6 graphene layers. Molecular dynamics simulations and theoretical predictions confirm our measurements and indicate that graphene's wetting transparency is related to its extreme thinness. We also show a 30-40% increase in condensation heat transfer on copper, as a result of the ability of the graphene coating to suppress copper oxidation without disrupting the intrinsic wettability of the surface. Such an ability to independently tune the properties of surfaces without disrupting their wetting response could have important implications in the design of conducting, conformal and impermeable surface coatings.     

Crack propagation in gaseous atmospheres

Environmentally-assisted cracking is studied for an Al-Zn-Mg alloy in different gaseous atmospheres. A sophisticated experimental installation with an ultra-high-vacuum chamber (10^-5) is used to maintain and monitor a high-purity atmosphere surrounding the specimen. The experimental data show that water vapor is the main factor responsible for the enhancement of the crack growth rates as compared with those in the high vacuum. Embrittlement is attributed to the influence of hydrogen produced as a result of the surface reaction of water vapor with the fresh metal surface near the crack tip. Departamento de Ciencia de Materiales, Escuela de Ingenieros de Caminos; Universidad Politecnica de Madrid. Madrid, Spain. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 65–70, July–August, 1998.     

Wetting, Prewetting and Superfluidity

Experiments on adsorption and wetting of quantum fluids (⁴He and ³He) on weakly binding alkali metal substrates are reviewed. Helium on weak substrates can undergo a variety of phase transitions including wetting, prewetting, layering, and liquid-vapor transitions. Another characteristic feature of weak substrates is the absence of an immobile quasi solid layer which is present on all conventional strong substrates. Both the absence of the immobile layer and the interaction with surface phase transitions can strongly affect superfluid onset. The Kosterlitz-Thouless superfluid transition can terminate in either a critical endpoint where it meets a first order structural transition or a tricritical point where it meets critical point such as the prewetting critical point or the 2D liquid-vapor critical point.     

Wetting of Rough Walls

Quenched geometric disorder of a wall delimiting a spectator phase can have dramatic effects on the nature of critical wetting transitions. We consider self-affine walls in 2D with roughness exponent _W. Transfer matrix results for directed interfacial models with short-range interactions suggest that wetting turns first-order as soon as _W exceeds ₀, the anisotropy index of interface fluctuations in the bulk. Discontinuous interface depinning is best identified by a peculiar two-peak structure in the statistical distributions of wall–interface contacts obtained by sampling over disorder. On the other hand, for _W<₀ wetting remains continuous, most plausibly in the same universality class as with flat walls. This occurs both with ordered (₀ = 1/2) and with bond-disordered (₀ = 2/3) bulk. A precise location of the thresholds at _W = ₀ can be argued on the basis of an analysis of different terms in the interfacial free energy. This analysis elucidates the peculiar role played by the intrinsic interfacial roughness and suggests extensions of the results to 3D and to long-range substrate forces.     

Wetting Phenomena in bcc Binary Alloys

We study the influence of the surface orientation on the wetting behavior of bcc binary alloys, using a semiinfinite lattice model equivalent to a nearest-neighbor Ising antiferromagnet in an external magnetic field. This model describes alloys that exhibit a continuous B2–A2 order–disorder transition, such as FeAl or FeCo. For symmetry-breaking surfaces like (100), an effective ordering surface field g ₁0 emerges. Such a field not only crucially affects the surface critical behavior at bulk criticality, but also gives rise to wetting transitions below the critical temperature T _c. Starting from the mean-field theory for the lattice model and making a continuum approximation, a suitable Ginzburg–Landau model is derived. Explicit results for the dependence of its parameters (e.g., of g ₁) on the microscopic interaction constants are obtained. Utilizing these in conjunction with Landau theory, the wetting phase diagram is calculated.     

Wetting of octadecylsilylated silica in methanol-water eluents

The wetting of an octadecylsilylated silica in methanol-water mixtures was studied by optical transmittance, visual observations, and measurements of the retention of model compounds. The octadecylsilylated silica particles remain wetted as the methanol content is decreased from 100% (v/v) to 20% (v/v). With the methanol content in the range of 20% (v/v) to 10% (v/v), the octadecylsilylated silica particles are still wetted, but the degree of solvation of the C(18) chains decreases with decreasing methanol concentration. The octadecylsilylated silica particles are not wetted when the methanol content in the mobile phase is lower than 10% (v/v). After equilibration with water, the octadecylsilylated silica particles remain nonwetted until the methanol content reaches 65% (v/v). The nonwetted phase showed significantly reduced chromatographic retention. A much longer equilibration time is required when the octadecylsilylated silica particles are not well solvated or nonwetted. The retentive behavior of the column will depend on the history of mobile-phase exposure.     

高分子膜的浸润性能及溶胀

选择磷酸三丁酯、正丁醇和２０％７３０１（煤油）３种溶剂，分别研究了不同材料的致密膜，１３０μｍ的微孔聚砚膜及８０μｍ和１００μｍ的ＰＴＦＥ微孔膜的浸润性能和溶胀率。实验结果表明，由接触角θ值的不同可以看出３种溶剂以膜的浸润性能有明显的差异。所选择的膜有机溶剂的作用下都会有溶胀发生，经过溶胀后膜的物理结构将发生变化，在选择的有机溶剂中，膜在ＴＢＰ溶剂中的溶胀率最大，对于致密膜，溶胀膜的孔隙率变大。对     

Weak Binding Potentials and Wetting Transitions

We present ab initio calculations of the adsorption potentials V(Z) of inert gases and hydrogen on the surfaces of various metals. The ratio of the adsorption well depth to that of the adsorbate pair potential is 3.5, 2, 1.5, 1, 0.9 and 0.9 for adsorption on Mg, Li, Na, K, Rb, and Cs, respectively, with some variation between gases (always smallest for Ne). When this ratio is small, a wetting transition occurs; we predict the wetting temperature T _w using a model of Cheng et al. Comparison is made with other calculations and with experiments.     

Wetting, Capillary Condensation and More

Wetting transitions and their closely related cousins of capillary condensation and filling are found all around us. This article provides an overview of the key ideas and issues related to these phenomena as they have emerged in the past few decades. Both continuous and first order wetting transitions and their analogues for condensation and filling are covered, and some new work on two-dimensional wetting associated with prewetting is presented. A glimpse into the promising future of this field ends the presentation.     

Wetting of Grain Boundaries in Cuprate Ceramics

The wetting of grain boundaries and the formation and transport properties of liquid-channel grain-boundary structures are considered using Bi₂CuO₄ ceramics as an example.     

Wetting of nickel by silver

Sessile drop experiments of molten silver and nickel were performed in air and helium at 970 C. A NiO layer formed at the interface in air; silver formed a 90 contact angle. In helium silver formed a 9 contact angle on nickel. The role of solution reactions in forming these angles is discussed.     

Studies of the Wetting of Gaps in Weightlessness

The geometry of a thin sheet metal vane terminating near a wall in a surface tension propellant management device (PMD) is common in devices designed by various people. A research program into the capillary fluid physics of the common vane-wall gap began in 1998 with the arrival of the second author at the School of Aeronautics and Astronautics at Purdue University. Drop tower experiments, Surface Evolver computations, and analysis were combined to explore the details of the fluid behavior in the vane-wall gap geometry. Results of four vane-wall gap experiment topics: critical wetting, advance rates, sensitivity to vane orientation, and effect of imperfect initial conditions, are discussed here. This work led to a desire by Weislogel to incorporate this type of geometry into his “Capillary Fluids Experiment” (CFE) that operated flawlessly on the International Space Station in 2006 and 2007. It is found that the wetting of vane-wall gaps is predicted correctly through use of the critical wetting analysis of Concus and Finn. Furthermore, the dynamics of the wetting flows are found to have scaling of flow rates versus time similar to those known for capillary advances in solid corners. In some cases, a seemingly misaligned vane is found to have more rapid capillary advance than for the same vane and gap but with the vane normal to the tank wall. An initial drop tower study of sensitivity to imperfect initial conditions shows that a critical wetting flow is largely immune to small tilts in the initial test orientation but that larger errors can be seen in cases that lack critical wetting and in the measurements of the time history of the meniscus minimum point.