Wetting Phenomena Research Articles

Modeling of the dynamic wetting behavior in a capillary tube considering the macroscopic–microscopic contact angle relation and generalized Navier boundary condition

In this study, dynamic wetting phenomena in a capillary tube were studied by using numerical simulations based on the front-tracking method employing the generalized Navier boundary condition (GNBC) and by experimental measurements. For the GNBC, based on molecular dynamics simulations, the microscopic dynamic contact angle is estimated from the grid-scale contact angle using Cox’s macroscopic–microscopic relation. The experimentally measured correlation between the apparent dynamic contact angle and the moving velocity of the contact line is well reproduced by the present simulation technique considering Cox’s macroscopic–microscopic relation. Thus, we found that the dynamics of wetting are well described by combining molecular-scale behavior and macroscopic–microscopic relations.

Modelling and simulation of drop spreading on fibrous porous media

Various operations in petroleum processing, textile technology, printing and composite processing involve wetting and spreading phenomena of liquids in porous media. These phenomena are being investigated using various tools, prominent among them being the spreading of a liquid drop on a porous medium. The spreading is governed by the inertial, gravitational, viscous and capillary forces and their relative importance have been studied to understand the underlying phenomena. In this work, drop spreading on heterogeneous porous media, such as composite reinforcing fabrics, has been investigated. The liquid drop spreading has been modelled in two stages: the flow of liquid on the surface of fabric and the imbibition of the liquid into the fabric due to inter-tow and intra-tow flow of the fluid. The model equations are formulated by making a hypothetical equivalent model of the fabric. The resulting equations are solved numerically. The model is used to simulate the spreading of liquid drops on glass fabrics. The drop spreading is characterized by height, contact radius and contact angle. The front of the liquid within the porous media can also be monitored through the model. Initially, a parametric study on the effect of fabric and tow porosities, the tow radius and the droplet volume on the imbibition time is presented. Finally, the model results are compared with the experimental results from literature. It is shown that the model can capture the essential features and provide insight into drop spreading on heterogeneous porous media.

Влияние гравитации на смачивание и капиллярные явления в контактных системах «жидкость — твердое тело»

Влияние гравитации на смачивание и капиллярные явления в контактных системах «жидкость — твердое тело»

Wetting by solid helium, a model system

Abstract This is a review of the wetting properties of solid helium on various solid substrates. Due to its extreme purity and to its very fast growth dynamics, solid helium 4 is often considered as a model system in materials science. Several wetting phenomena have been studied with helium 4 crystals, namely contact angles on solid substrates with variable roughness, wetting on graphite where epitaxial growth takes place, the roughening transition as a function of film thickness, the wetting of grain boundaries by the liquid phase.

Surface energy characterization of sandstone rocks

The fundamental forces of adhesion are responsible for the spreading of fluids such as crude oil/brine on the reservoir rock surface. These physico–chemical interactions determine the surface energetics of a reservoir and thus their wetting phenomena. Inverse Gas Chromatography (IGC) is introduced to characterize the surface energy of sandstones (Ottawa sand and Berea sandstone). The surface chemistry of the sandstone rocks is further elucidated using X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) techniques. The behavior of the polar and non-polar interaction forces was investigated at varying water coverage and at different temperatures. The results indicated that in general as the water coverage increased, the Lifshitz–van der Waals component of surface energy decreased to nearly that of the bulk water, while the acid–base component also showed a decreasing trend. The Lifshitz–van der Waals component of surface energy always decreased with increase in temperature, while the acid–base properties showed contrasting trends in line with changes in surface chemistry of the sandstones, due to the change in temperature. Finally, the wetting properties arising in reservoir sandstones were related to the surface chemistry of the reservoir fluids and their interactions with the reservoir rock surface.

Sleep and Wakefulness Correction in Different Seasons From Avicenna's Perspective

Sleep and Wakefulness Correction in Different Seasons From Avicenna's Perspective

Rapid, conformal gas-phase formation of silica (SiO2) nanotubes from water condensates

An innovative atomic layer deposition (ALD) concept, with which nanostructures of water condensates with high aspect ratio at equilibrium in cylindrical nanopores can be transformed uniformly into silica (SiO2) at near room temperature and ambient pressure, has been demonstrated for the first time. As a challenging model system, we first prove the conversion of cylindrical water condensates in porous alumina membranes to silica nanotubes (NTs) by introducing SiCl4 as a metal reactant without involving any catalytic reaction. Surprisingly, the water NTs reproducibly transformed into silica NTs, where the wall thickness of the silica NTs deposited per cycle was found to be limited by the amount of condensed water, and it was on the orders of ten nanometers per cycle (i.e., over 50 times faster than that of conventional ALD). More remarkably, the reactions only took place for 10-20 minutes or less without vacuum-related equipment. The thickness of initially adsorbed water layers in cylindrical nanopores was indirectly estimated from the thickness of formed SiO2 layers. With systematic experimental designs, we tackle the classical Kelvin equation in the nanosized pores, and the role of van der Waals forces in the nanoscale wetting phenomena, which is a long-standing issue lacking experimental insight. Moreover, we show that the present strategy is likely generalized to other oxide systems such as TiO2. Our approach opens up a new avenue for ultra-simple preparation of porous oxides and allows for the room temperature formation of dielectric layers toward organic electronic and photovoltaic applications.

Reactive wetting phenomena of MgO–C refractories in contact with CaO–SiO2 slag

Wetting phenomena between MgO–C and CaO–SiO2 slags were investigated by varying carbon content. A sessile drop technique was adopted to study the wetting phenomena in conjunction with a high speed camera for the observation of intrinsic wetting phenomena. The results show that the high content of SiO2 and the presence of Al2O3 in slags enhance the diffusion of Mg2+, leading to the promotion of reactive wetting. The carbon in MgO–C refractory impedes the penetration of slags by repelling the slag and slowing the diffusion of Mg2+. This accounts for the non-wetting behavior of the slag on MgO–C refractory with 17% (mass fraction) carbon similar to that of graphite.

Direct measurement of macro contact angles through atomic force microscopy

The information of liquid film profile near a three-phase contact line is critical for a comprehensive understanding of various wetting and phase-change phenomena. Despite numerous theoretical and simulation studies, an accurate measurement on the thin film profile is difficult due to its very small scale. In the present work, a state-of-the-art atomic force microscopy (AFM) under tapping mode (TM) was employed to achieve a high-power scanning across the contact line. Within a scale of several to tens of microns, a highly linear film profile is observed near the contact line, based on which the contact angle is extracted. Comparing to the macro contact angle measured by the traditional optical method, the AFM result shows good agreement but achieves much higher precision. Moreover, the sub-micron thin film that is beyond the capability of the optical method was observed, in which the film profile is not linear and the concept of macro contact angle is not valid.

Autophobing on Liquid Subphases Driven by the Interfacial Transport of Amphiphilic Molecules

We investigated the phenomenon of incomplete wetting of a high-energy liquid subphase by drops of pure amphiphilic molecules as well as drops of amphiphile solutions that are immiscible with the subphase. We show that amphiphiles escape across the contact line of the drop, move on the subphase/vapor interface, and form a submonolayer or full monolayer external to the drop. If this monolayer is sufficiently dense, then it can reduce the surface tension of the subphase, raise the contact angle of the drop, and prevent the drop from fully wetting the subphase. This phenomenon is called autophobing and has been extensively studied on solid substrates. For the liquid subphase studied here, we measure the surface tensions of the three relevant interfaces before and after the drop is deposited. The measured surface tension external to the drop shows that amphiphiles can move across the contact line and form a monolayer outside of the drop. In some cases, at equilibrium, the monolayer is in a sufficiently packed state to create the nonwetting condition. In other cases, at equilibrium the monolayer density is insufficient to lower the surface tension enough to achieve the nonwetting condition. Unlike on solid substrates where the formation of the monolayer external to the drop is kinetically hindered, the amphiphiles can move rapidly across the liquid subphase by Marangoni-driven surface transport, and local equilibrium is achieved. However, because the amphiphile inventory and subphase area are limited, the achievement of autophobing on a liquid subphase depends not only on the instrinsic subphase/amphiphile interaction but also on the total amphiphile inventory and area of the liquid subphase.

Thermodynamic characteristics of surface densified solid Scots pine wood

Wetting phenomena of surface densified solid wood and its surface energetic behavior were investigated. The specimens of three different initial thicknesses (22 mm, 20 mm, and 18 mm) were surface densified to a target thickness of 15 mm in order to obtain three different degrees of surface densification. After surface densification one third of the specimens was heat treated at 200 °C in the presence of steam, and one third of the specimens was oil treated with linseed oil. The sessile drop technique was used to estimate the apparent contact angle of water, diiodomethane, and formamide on control and surface densified wood. The contact angle data were used to determine the surface free energies of the control and surface densified wood by the Owens Wendt Rabel and Kaelble (OWRK) method. The results of contact angle measurements revealed that surface densification process affects surface wettability. The apparent contact angles of test liquids were higher in the case of surface densified wood compared to the control undensified wood. Heat treatment and oil treatment of the surface densified wood additionally increased the contact angle of tested liquids. Furthermore, a larger reduction in the void spaces of the wood due to higher degree of densification did not affect the apparent contact angles of tested liquids and consequently the surface free energy. The surface densification process lowered the surface energetics and the polarity of wood. Furthermore, heat treatment and oil treatment additionally increased the hydrophobic character of the examined surfaces and lowered the dispersive and polar components of surface free energy.

Macroscopic Phase-Field Model of Partial Wetting: Bubbles in a Capillary Tube

Drops and bubbles are nonspreading, local, compactly supported features. They are also equilibrium configurations in partial wetting phenomena. Yet, current macroscopic theories of capillary-dominated flow are unable to describe these systems. We propose a framework to model multiphase flow in porous media with nonspreading equilibrium configurations. We illustrate our approach with a one-dimensional model of two-phase flow in a capillary tube. Our model allows for the presence of compactons: nonspreading steady-state solutions in the absence of external forces. We show that local rate dependency is not needed to explain globally rate-dependent displacement patterns, and we interpret dynamic wetting transitions as the route from equilibrium, capillary-dominated configurations, towards viscous-dominated flow. Mathematically, these transitions are possible due to nonclassical shock solutions and the role of bistability and higher-order terms in our model.

Advances in the theory of superhydrophobic surfaces

The underlying theories interpreting wetting phenomena are still mainly focused on the Young's equation, the Wenzel equation, and the Cassie–Baxter equation, despite the fact that wetting phenomena have been studied over the past decades. Based on these theories, people have understood that both surface chemical composition and its morphology can influence the contact angle of liquid droplets on solid surfaces. However, such equations are not sufficient to thoroughly explain the mechanisms of wetting phenomena, although they are still necessary. In this feature article, we review the theory, from the classical wetting models to the most recent theoretical advances, of superhydrophobic surfaces with regard to the wetting process, and some promising breakthroughs in the advance of the theory are proposed in the final section.

Surface-enhanced absorption by self-organized silver films with aciniform-like nanoaggregates at elevated temperatures

Morphological evolution of silver nanocomposite films prepared by the wet colloidal route and surface-enhanced phenomena on aggregate nanostructures evolved during annealing were investigated. Dramatic changes in morphologies of particles and pores incurred by rearragement, coarsening, premelting, and dewetting of the silver clusters at different concentrations (i.e., mass thicknesses). At a higher mass thickness, the morphological transitions from self-organized nanoaggregates with aciniform pattern at 300 °C to elongated and coarsened particles with circular holes at 400 °C to island clusters at 500 °C occurred in the films. The peculiar absorption with a much redder and broader surface plasmon feature, which gone far beyond the theoretical prediction, induced by the formation of aciniform nanoaggregates embedded in the porous polymer matrix at a critical mass thickness of 9.6 nm during partial degradation of the PVP polymer and rearrangement of silver clusters at 300 °C. The surface-enhanced absorption was dramatically reduced by the elemination of the aggregate nanostructures and the spontaneous formation of the silver nanoisland film at the dewetting temperature of 500 °C.

STUDY OF CONTACT ANGLE OF LIQUID ON SOLID SURFACE AND SOLID ON LIQUID SURFACE

The paper presents a study on the phenomenon of wetting of minerals in the two cases where a wetted solid is placed on a liquid surface and a liquid is on a flat solid surface. Knowledge of physical and chemical processes occurring between a liquid and solid is an important issue. Contact angle measurement and determination of interfacial tensions are very important in development of new technologies in which the gas-liquid-solid system appears. These problems are important for new solutions in many industries. The study confirms that many factors have an essential influence on the contact angle measurement.

Lipid membranes in contact with aqueous phases of polymer solutions

In this highlight, we describe recent developments in research on lipid vesicles encapsulating aqueous two-phase polymer solutions. Special emphasis is given on the morphological changes of the vesicles and the membrane transformations induced by the phase separation process, and on the interactions of the lipid bilayers with the phases. These interactions lead to a variety of interesting wetting phenomena and to the formation of membrane nanotubes. Future directions and possible developments in this research field are also discussed.

Predication of Wettability of Sn Droplet on Copper Plate by MEAM Method

In this paper, Modified Embedded Atom Method (MEAM) method, one of the molecular dynamic (MD) methods, was utilized to investigate the wettability of Sn droplet on the copper plate to simulate the wetting and spreading phenomena of the solder process. And the experiment of spreading and wetting process of Sn droplet on the copper plate was performed by sessile-drop as well to get the contact angles. The results show that the MEAM method is fit to simulate the soldering process. Keywords: Wettability; Soldering; MEAM; Sessile-drop; Molecular dynamic

Discussion notes on “Universality in the dependence of the drop contact angle on liquid-solid interactions and temperature obtained by the density functional theory”, by G.O. Berim & E. Ruckenstein

Discussion notes on “Universality in the dependence of the drop contact angle on liquid-solid interactions and temperature obtained by the density functional theory”, by G.O. Berim & E. Ruckenstein

Disperse Systems Based on the Structure and Properties of Oligomer Blends

This article demonstrates the potential for both homogeneous and heterogeneous structures to form in oligomer blends. The nature of the dependence of viscosity on the composition of the heterophase blend has been found to be governed by the phenomenon of wetting of the surface of the disperse phase dispersion medium. Selectivity offiller distribution in oligomer blends is shown. Also demonstrates the potential for filler formation during the actual composite matrix curing process.

Finite element framework for describing dynamic wetting phenomena

SUMMARYThe finite element simulation of dynamic wetting phenomena, requiring the computation of flow in a domain confined by intersecting a liquid–fluid free surface and a liquid–solid interface, with the three‐phase contact line moving across the solid, is considered. For this class of flows, different finite element method (FEM) implementations have been used in the literature, and in some cases, these produced apparently contradictory results. In the present paper, a robust framework for the FEM simulation of dynamic wetting flows is developed, which, by consistently adhering to the FEM methodology, leaves no room for ad hoc ‘optional’ variations in the numerical handling of these flows. The developed approach makes it possible to conduct a convergence study, assess the spatial resolution required to achieve a preset accuracy and provide the corresponding benchmark calculations. This analysis allows one to identify numerical artefacts, which had previously been interpreted as physical effects, and demonstrates that suppressing numerical errors using a ‘strong’ implementation of a boundary condition creates bigger and less detectable errors elsewhere in the computational domain. We provide practical recommendations on the spatial resolution required by a numerical scheme for a given set of non‐dimensional similarity parameters and give a user‐friendly step‐by‐step guide specifying the entire implementation, which allows the reader to easily reproduce all presented results including the benchmark calculations. It is also shown how the developed framework accommodates generalizations of the mathematical model accounting for additional physical effects, such as gradients in surface tensions. Copyright © 2011 John Wiley & Sons, Ltd.