Numerical Study of Finite Element Method Based Solutions for Propagation of Wetting Fronts in Unsaturated Soil

The accurate prediction of the propagation of a wetting front in an unsaturated soil subjected to surficial infiltration is of practical importance to many geotechnical and geoenvironmental problems. The finite element method is the most common solution technique as the hydraulic soil properties are highly nonlinear. Two important issues are often found to create difficulties in such analyses. First, numerical oscillations are usually observed in the calculated pore pressures at the wetting front. Second, when a reasonable mesh size and time step are used, the elevation of the wetting front may be seriously overpredicted. This paper is focused on the second issue. The under-relaxation (UR) technique used in the iterative process within each time step is found to have a serious impact on rate of convergence with refinement in mesh size and time step. Two different techniques are typically used; the first evaluates the hydraulic conductivity using an average of heads calculated from the preceding time node and the most recent iteration of the current time node (UR1), and the second evaluates the hydraulic conductivity using the average of heads calculated from the two most recent iterations of the current time nodes (UR2). The study shows that UR1, which is adopted in programs such as SEEP/W, ensures that the solution converges rapidly to a stable solution within a time step, but may converge to the wrong wetting front at a given elapsed time unless a sufficiently refined mesh is used. UR2 converges much more slowly within a time step, but the error in the wetting front is smaller than that generated by UR1.     

Al-Si3N4材料浸渗工艺研究

用β-Si3N4纳米颗粒浆料浸渍多孔聚合物材料,通过加热烧蚀掉聚合物,制备出三维空间连续网络结构预制块体,再通过无压浸渗将已熔炼好的铝液浸渗到预制体中,成功制备出陶瓷与金属相互贯穿的Si3N4/Al金属基复合材料。利用座滴定法测试了Al在Si3N4基片上的润湿角,探讨了其浸渗机理,分析了润湿角、浸渗力、浸渗温度和浸渗时间对Si3N4/Al金属基复合材料浸渗行为的影响。     

Search for a wetting layer in thin film growth of para-hexaphenyl on KCl(001)

The non-existence of a strongly bound wetting layer for the system para-hexaphenyl (6P)-KCl(001) was verified by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The 6P films were grown by physical vapour deposition under ultrahigh vacuum conditions at 400 K. TDS showed just a single desorption peak even down to a coverage of 0.1 nm mean film thickness. The heat of evaporation for 6P was determined to 2.3 eV. From the change of the XPS K2p substrate signal as a function of the mean film thickness one can conclude that a strong de-wetting exists at the very beginning of the 6P layer growth at 400 K. Additional investigations with atomic force microscopy reveal that at this initial stage the film grows in a form of needle like islands with a high aspect ratio and subsequently terraced mounds of 6P are formed. No dissociation of 6P on KCl was observed.     

Preparation of defined bulk densities for using the capillary rise method in the case of fine powders

The wetting properties of powders are important in many technological processes. Contact angles can be characterized by an effective contact angle of the particle arrangement relative to the wetting liquid, but their quantification is problematic. An exact determination depends on an accurate preparation of the test samples. A defined pre-consolidation by vertical shocks is introduced in this paper.     

Experimental study of ultrasound propagation at a liquid–solid composite interface for inspection of liquid–metal cooled nuclear reactors

The influence of unfilled cavities at a liquid–solid interface on ultrasound propagation is investigated. This kind of interface exists only when the surface is rough and the liquid is non-wetting. Normally incident compression waves are used. Possible modelling approaches are discussed, showing that no model is able to efficiently describe this kind of interface. We demonstrate that wave transmission drops dramatically. It is suggested that the incoming ultrasonic energy induces the growth and the coalescence of the vapour phase contained in the unfilled cavities under ultrasound field. A major result of this paper is to provide proof that difficult experiments in metal–liquid can be replaced by easier experiments in water.     

In situ observation of Ag–Cu–Ti liquid alloy/solid oxide interfaces

In situ investigation methods are a very interesting means for understanding high-temperature interface processes. A method of direct observation of the interactions between transparent materials (Al₂O₃, SiO₂, CaF₂) and metal melts was elaborated. For the Ag–36.65 at.%Cu–8.15 at.%Ti/sapphire system, the formation of a dark compound at the interface was observed to occur at high temperature. This result does not confirm the conclusion of a neutron spectroscopy study which indicated that titanium oxides form at the interface only during solidification of the alloy. Interactions of the same alloy with SiO₂ and CaF₂ were also considered.     

The effects of laser-induced modification of surface roughness of Al2O3-based ceramics on fluid contact angle

Laser surface treatment of Al₂O₃-based refractory ceramics, by melting and re-solidification, can be successfully applied to producing surfaces that are pore-free, homogeneous and crack-free. Such treated surfaces can lead to an increase in the corrosion and erosion resistances of the materials, due to lower permeability to corrosive species and higher surface hardness, respectively. Furthermore, the corrosion resistance can be influenced by the wetting characteristics of the treated surfaces in service environment. Therefore, it is important to investigate the effect of laser treatment of ceramic materials on the interaction of the surface with the various environmental elements. This work is concerned with an experimental investigation of the effects of laser surface treatment, by melting and re-solidification, on the fluid contact angles of Al₂O₃-based refractory ceramics. These effects are examined by the modification of the surface roughness characteristics induced by laser treatment. Laser-treated surfaces, both containing cracks and crack-free, are compared with untreated surfaces and the results are reported. The untreated surfaces demonstrated considerable non-uniformity in wetting, in contrast to the treated surfaces. The extent of wetting of the laser-treated surfaces containing cracks was proportional to laser power density. This is due to wetting being enhanced, among other factors, by surface roughness, which increased with power density. The crack-free surfaces were the most smooth and, thereby, exhibited the smallest extent of wettability variations. The reduction in wettability after the laser treatment (crack-free) may have an advantage for corrosion resistance.     

Surface modification of indium tin oxide anodes by self-assembly monolayers: Effects on interfacial morphology and charge injection in organic light-emitting diodes

Three silane derivatives including dodecyltrichlorosilane (DDTS), phenyltriethoxysilane (PTES) and 3-aminopropyl-methyl-diethoxysilane (APMDS) were used to modify the indium tin oxide (ITO) surfaces. The effects of various terminal groups of the self-assembled monolayers (SAMs) on the growth behavior and interfacial morphologies of N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) film deposited on the SAM-modified ITO were studied, as well as their effects on the performance of organic light-emitting diodes (OLED) devices. The results show that the growth behavior of NPB film over-deposited on the SAM-modified ITO is mainly determined by the wettability of the surface. The covering ability and thermal stability of NPB film on the SAM-modified ITO decrease in the order: bare ITO > ITO/PTES > ITO/APMDS > ITO/DDTS. However, the covering characteristic of NPB films on these substrates did not show direct relation to the transport of carriers across the anode/NPB interface as evaluated from the cyclic voltammogram and OLED performance. The turn-on voltages for these SMA-modified OLED devices increase in the order: ITO/PTES < ITO/DDTS ≤ bare ITO < ITO/APMDS. The enhancing effect of PTES on the hole injection is ascribed to the similar structure of PTES to NPB. On the contrary, the inhibition effect of APMDS is caused from the interaction of the lone-pair electrons of amine group to the transport carriers. Since these devices are known to be hole dominant, the luminance efficiency increase in a similar order as that for the turn-on voltage: ITO/PTES < ITO/DDTS ≤ bare ITO < ITO/APMDS.     

The Influence and Interpretation of Surface Parameters for Wetting Transitions in Ternary Mixtures

Starting from a microscopic Hamiltonian defined on a semi-infinite cubic lattice, and employing a mean-field approximation, the surface parameters relevant for wetting in confined ternary mixtures are derived. These are found in terms of the microscopic coupling constants, and yield a physical interpretation of their origins. In comparison with the standard expression for the surface free-energy density, several new terms arising from the derivation are identified. The influence of the surface parameters on a predicted unbinding transition in a mixture of oil, water, and amphiphile demonstrate that existing results are robust to the addition of the extra surface terms.     

Suitable properties of ink to create inkjet-printed surface-directed microchannels that utilize the pinning effect of a triple line on a rough surface

If microfluidic devices can be directly produced using printing techniques, the combination of microfluidics and printing techniques for other applications, such as printed electronics, will make all-printed highly-functionalized microfluidic devices possible. Therefore, we have made efforts to develop a technique for producing microfluidic devices using an inkjet printer. The microchannels that could be created using this technique were a kind of surface-directed channels that utilize the pinning effect of a triple line on a rough surface. In this study, we focused on what were the required properties of the printer ink during the wetting and drying processes of the ink. As a result, one of the properties required during the wetting process was that the advancing contact angle of the ink should be smaller than a certain value, which depended on the average volume of the ink drops ejected from the printhead and the number of drops per unit area. The receding contact angle should be smaller than about one third of each advancing angle. In addition, during the drying process, a small amount of surfactant added to the ink played a critical role in order to leave a continuous stain of the ink. As an application of this inkjet-printed channel, we also created a device for mixing aqueous solutions.