Initial rapid wetting in metallic systems

The initial rapid wetting of a solid surface by a liquid phase is an important step in many industrial processes. Liquid-phase sintering of powder metallurgical steels is one such industrial process, where metallic powders of micrometer size are used. Investigating the dynamic wetting of a high-temperature metallic drop of micrometer size experimentally is very challenging. Here, a phase-field-based numerical model is first implemented and verified by accurately capturing the initial dynamic wetting of millimeter-sized metal drops and then the model is extended to predict the dynamic wetting of a micrometer-sized metal drop. We found, in accordance with recent observations, that contact line friction is required for accurate simulation of dynamic wetting. Our results predict the wetting time for a micrometer-sized metal drop and also indicate that the dynamic wetting patterns at the micro- and millimeter length scales are qualitatively similar. We also found that the wetting process is much faster for a micrometer-sized metal drop compared to a millimeter-sized metal drop.     

Theoretical study of wetting behavior of nanoparticles at fluid interfaces

In this work, a theoretical model was developed to describe the wetting behavior of nanoparticles at liquid‐vapor interface by the integration of the renormalization group transformation, the cell theory, and the modified fundamental measurement theory with the first‐order mean spherical approximation method. The results show that the new model can be used to investigate the global behavior and surface tensions of nanoparticle/fluid systems. Particularly, the nanoparticle's wetting behavior inside critical region was discussed systematically. More important, this work proposed a methodology for calculating line tension and contact angle, showing that line tension has considerable influence on wetting properties for small nanoparticles, whereas it is negligible for large nanoparticles. Therefore, this work provides a general method for studying the wetting behavior of nanoparticles that may find wide applications in the field of chemical engineering. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Wetting of Al2O3-Based Oxide Ceramics by Molten Aluminum

The temperature dependence has been determined for the wetting by molten aluminum of materials based on aluminum oxide (polycor and corundum). A temperature range has been identified in which the wetting increases significantly. There is no effect on the wetting characteristics of these oxide materials from the addition to the aluminum of up to 25% silicon, 33% copper, or 6% nickel.     

Wetting Transitions on Post-Built and Porous Reliefs

Abstract

A comparative study of wetting transitions on porous and post-built polymer surfaces is reported. The transitions were observed under vibration of droplets of different volumes deposited on the surfaces possessing controlled roughness. Initial wetting states were identified as the Cassie air-trapping (‘Fakir’) state for porous and the Wenzel state for post-built relief, whereas the final wetting state was the Cassie impregnating one for both kinds of surfaces. For both porous and post-built surfaces, wetting transitions occurred under constant force acting on a unit length of the three-phase (triple) line. Wetting transitions for both porous and post-built surfaces start from the triple line.     

Wetting and adhesion of water-borne printing inks on surface-modified polyolefins

Polyolefin films were surface-modified by different methods to improve the wetting and adhesion of water-borne printing inks. Polyethylene (PE) films were treated with corona at various energy levels. Surface-modified PE films were characterized by contact angle measurements and electron spectroscopy for chemical analysis (ESCA). Good wetting was already achieved with treatment at a lower energy level. Various degrees of adhesion were obtained at various degrees of treatment. A hydrophilic monomer, 2-hydroxyethylmethacrylate (HEMA), was polymerized onto the surfaces of polypropylene (PP) with radiation-induced grafting, which was carried out at two different radiation doses. In both cases, a thick, visible layer of polyHEMA was formed on the surface of PP, and satisfactory wetting was already achieved at lower radiation doses. Scanning electron microscopy (SEM) showed that different degrees of roughness were achieved at various radiation doses. Like the case of corona-treated PE, different degrees of adhesion were obtained at different degrees of surface treatment. This study shows that improved wetting alone is not satisfactory for good practical adhesion', regardless of the surface modification method used.     

Simulation of Liquid Diffusion-Controlled Drying of Shrinking Thin Slabs Subjected to Multiple Heat Sources

Abstract

Although a detailed mathematical model incorporating all physical mechanisms of moisture and heat transfer in the material would yield valuable design information, it is not feasible to do this on a routine basis for the design of dryers. A simple liquid diffusion model was developed in the present study to quantitatively assess the influence of various operating parameters of engineering interest in drying of heat-sensitive materials. Heat of wetting, temperature, and moisture dependent effective diffusivity and thermal conductivity, changes in product density and drying-induced ideal shrinkage of the product are considered in this model. The effects of combining convection with conduction, radiation, and volumetric heating using a microwave field are simulated in view of the increasing interest in multimode heat input drying processes. Numerical results are reported on drying of potato slices to demonstrate how the moisture and temperature profiles as well as drying performance are affected by multi-mode heat input. Effects of key parameters e.g., drying air velocity, temperature, relative humidity, and product thickness are computed and discussed.     

THEORETICAL ANALYSIS OF THE DIFFUSION PROCESS INSIDE PROLATE SPHEROIDAL SOLIDS

ABSTRACT

A numerical solution of the diffusion equation to describe solute transport inside ellipsoids, assuming a constant difiusion coefficient and a corrective boundary condition, is presented. The difiusion equation in a prolate spheroidal coordinate system was used for a hidimensional case. The finite volume method was employed to discretize the basic equation, utilizing a uniform grid size. The equation was solved irteratively using the Gauss-Seidel method. The effect of Biot number and the aspect ratio of the body on diffusion rate and concentration during the process is presented. Plots are presented for Biot numbers from 0.05 to infinity and aspect ratios of 1.1, 2.0 and 5.0. To investigate the effect of the aspect ratio, different results changing the aspect ratio for Bi= 1.0 are shown. The results show that the model is consistent and it may be used to solve other cases such as those that include cylinder and sphere geometry, and/ or those with variable diffusion coefficients with small modifications.     

Spreading,wetting, and contact angles

The thermodynamic energies associated with conventional wetting, spreading, adhesion, cohesion, and disjoining pressure, as defined in classical equations, are re-examined for their significance in a force field. They are then converted into dimensionless form such that the equilibrium properties of both wetting and spreading all fall on the same line when the dimcnsionless spreading coefficient is plotted as a function of the dimensionless work of adhesion. The effects of a force field such as gravity are examined and it is further shown that spreading is always thickness-dependent, whether in a force field or in a gravity-free field. Non-equilibrium processes such as autophobicity are shown on the same dimensionless plot and indicate clearly that the speed with which the process approaches equilibrium depends on the difference between the initial and equilibrium spreading coefficients. All these processes are expressed in terms of a dimensionless group P_n, the reduced wetting energy, which, when lying between the values of + 1 and -1, equals the cosine of the contact angle, . The implication of this approach to non-equilibrium processes is discussed.     

Some observations on the surface wetting relaxation of latex rubbers

The relaxation of dynamic contact angles using distilled water was measured for some elastomeric latex rubbers (unlubricated condoms). These measurements indicated that the wetting force increased with time after the forced motion of the meniscus was stopped. The time variation of the wetting force was analyzed with the help of several models. Neither a simple exponential of the Debye type nor an exponential based on diffusion fitted the data adequately. The experimental data are described best by a stretched exponential. One mechanism for this dependence would be the dispersive motion of the meniscus because of the chemical and physical heterogeneity of the latex rubber surface. A time dependent deformation of the thin rubber sheet due to wetting may also explain the time dependence, but cannot explain all results.