The influence of topography on dynamic wetting

The paramount importance of wetting applications and the significant economic value of controlling wetting-based industrial processes has stimulated a deep interest in wetting science. In many industrial applications the motion of a complex liquid front over nano-textured surfaces controls the fate of the processes. However our knowledge of the impact of nano-heterogeneities on static and dynamic wetting is very limited. In this article, the fundamentals of wetting are briefly reviewed, with a particular focus on hysteresis and roughness issues. Present knowledge and models of dynamic wetting on smooth and rough surfaces are then examined, with particular attention devoted to the case of nano-topographical heterogeneities and solid–fluid–fluid systems.     

The influence of interionic interactions on the dynamic characteristics of metal melts

The dynamic properties of melts of metals of different valences were calculated from the first principles using the approach of collective excitations and the pseudopotential method for determining the characteristics of interionic interaction potentials. Direct interrelation of these characteristics with the properties of calculated compressibility and sound propagation velocity in metallic melts was established. The collective excitation approach was shown to be capable of estimating the entropy and sound velocity. It provides closer agreement with experiment than the use of phonon spectra.     

Mucin-electrolyte interactions at the solid-liquid interface probed by QCM-D

The interaction between mucin and ions has been investigated by employing the quartz crystal microbalance technique with measurement of energy dissipation. The study was partially aimed at understanding the adsorption of mucin on surfaces with different chemistry, and for this purpose, surfaces exposing COOH, OH, and CH(3) groups were prepared. Mucin adsorbed to all three types of functionalized gold surfaces. Adsorption to the hydrophobic surface and to the charged hydrophilic surface (COOH) occured with high affinity despite the fact that in the latter case both mucin and the surface were negatively charged. On the uncharged hydrophilic surface exposing OH groups, the adsorption of mucin was very low. Another aim was to elucidate conformational changes induced by electrolytes on mucin layers adsorbed on hydrophobic surfaces from 30 mM NaNO(3). To this end, we investigated the effect of three electrolytes with increasing cation valance: NaCl, CaCl(2) and LaCl(3). At low NaCl concentrations, the preadsorbed layer expands, whereas at higher concentrations of NaCl the layer becomes more compact. This swelling/compacting of the mucin layer is fully reversible for NaCl. When the mucin layer instead is exposed to CaCl(2) or LaCl(3), compaction is observed at 1 mM. For CaCl(2), this process is only partially reversible, and for LaCl(3), the changes are irreversible within the time frame of the experiment. Finally, mucin interaction with the DTAB cationic surfactant in an aqueous solution of different electrolytes was evaluated with turbidimetry measurements. It is concluded that the electrolytes used in this work screen the association between mucin and DTAB and that the effect increases with increasing cation valency.     

The influence of colloidal interactions on fiber network strength

Various forces govern the fiber-fiber interaction in a flowing suspension, causing fibers to create flocs. The aim with this investigation was to examine the influence of colloidal interactions on the fiber network strength by varying surface charge density, electrolyte concentration, and type of counterion. This was accomplished by comparing surface force measurements, utilizing colloidal probe microscopy (CPM), with the apparent yield stress, using a parallel plate rheometer. Results show that by increasing the charge density by grafting carboxymethyl cellulose (CMC) to the surface, a large electrosteric repulsion is created, which gives weaker network strength. Increasing the electrolyte concentration decreases the repulsion. The network strength was, however, not affected by electrolyte concentration for untreated fibers whereas a high electrolyte concentration increased the yield stress for CMC-treated fibers. The change of counterions affect the repulsion, causing a change in network strength due to differences in the surface swelling of cellulose.     

The influence of urea and trimethylamine-N-oxide on hydrophobic interactions

Molecular dynamics simulations are used to obtain potentials of mean force for pairs of neopentane molecules immersed in aqueous solutions containing urea, trimethylamine-N-oxide (TMAO), or both solutes at once. It is shown that the hydrophobic attraction acting between neopentane pairs in pure water and in water-urea solution is completely destroyed by the addition of TMAO. This strongly suggests that TMAO does not counter the protein denaturing effect of urea by enhancing hydrophobic attraction amongst nonpolar groups.     

The exponential power law: partial wetting kinetics and dynamic contact angles

An equation for the kinetics of partial drop spreading is proposed. This equation was empirically derived from experimental data for the spreading kinetics of partially wetting liquids in terms of the wet area versus time. The equation has the form of an exponential power law (EPL), and transforms into the well-known power law for complete wetting, when the equilibrium contact angle approaches zero. The EPL fits very well available experimental data. To lend additional support to the validity of this generalized equation, it will be demonstrated that when it is transformed to present the dynamic contact angle (DCA), it fits very well DCA experimental data for other wetting processes, such as capillary flow and tape coating.     

Influence of anisotropy on the dynamic wetting and permeation of paper coatings

A void network model, named Pore-Cor, has been used to study the permeation of an ink solvent into paper coating formulations coated onto a synthetic substrate. The network model generated anisotropic void networks of rectangular cross-sectional pores connected by elliptical cross-sectional throats. These structures had porosities and mercury intrusion properties which closely matched those of the experimental samples. The permeation of hexadecane, used as an analogue for the experimental test oil, was then simulated through these void structures. The simulations were compared to measurements of the permeation of mineral oil into four types of paper coating formulation. The simulations showed that the inertia of the fluid as it enters void features causes a considerable change in wetting over a few milliseconds, a timescale relevant to printing in a modern press. They also showed that in the more anisotropic samples, fast advance wetting occurred through narrow void features. It was found that the match between experimental and simulated wetting could be improved by correcting the simulation for the number of surface throats. The simulations showed a more realistic experimental trend, and much greater preferential flow, than the traditional Lucas-Washburn and effective hydraulic radius approaches.     

Contact line motion and dynamic wetting of nanofluid solutions

The effect that nanoparticles play in the spreading of nanofluids dynamically wetting and dewetting solid substrates is investigated experimentally, using 'drop shape' analysis technique to analyse aluminium-ethanol contact lines advancing and receding over hydrophobic Teflon-AF coated substrates. Results obtained from the advancing/receding contact line analysis show that the nanoparticles in the vicinity of the three-phase contact line enhance the dynamic wetting behaviour of aluminium-ethanol nanofluids for concentrations up to approximately 1% concentration by weight. Two mechanisms were identified as a potential reason for the observed enhancement in spreading of nanofluids: structural disjoining pressure and friction reduction due to nanoparticle adsorption on the solid surface. The observed 'lubricating effect' that the nanoparticles seem to be inducing is similar to the 'superspreading' effect for surfactant solutions spreading on hydrophobic surfaces, up to a concentration (weight) of approximately 1%, could be a result of the predicted enhanced wetting behaviour. Indeed, Trokhymchuk et al. [Langmuir, 2001, 17, 4940] observed a solid-like ordering of nanoparticles in the vicinity of the three-phase contact line, leading to an increased pressure in the fluid 'wedge'. This increased pressure leads to a pressure gradient which causes the nanofluids to exhibit enhanced wetting characteristics. Another possible cause for the observed increase in advancing/receding contact line velocity could be deposition of nanoparticles on the solid surface in the vicinity of the three-phase contact line resulting in the nanofluid effectively advancing over aluminium rather than Teflon-AF, or the contact line 'rolling' over nanoparticles at the three-phase contact line due to sphericity of nanoparticles. For either of these to be the case, the nanoparticle effect at the three-phase contact line would have to be enhanced for the lower concentration in the same way that it would have to be for the increased pressure in the fluid 'wedge'.     

Experimental investigation of the link between static and dynamic wetting by forced wetting of nylon filament

Forced wetting experiments with various liquids were conducted to study the dynamic wetting properties of nylon filament. The molecular-kinetic theory of wetting (MKT) was used to interpret the dynamic contact angle data and evaluate the contact-line friction zeta0 at the microscopic scale. By taking account of the viscosity of the liquid, zeta0 could be related exponentially to the reversible work of adhesion. This clearly establishes an experimental link between the static and dynamic wetting properties of the material. Moreover, statistical analysis of the equilibrium molecular displacement frequency K0 and the length of the displacements lambda reveals that these two fundamental parameters of the MKT are strongly correlated, not only in the linearized form of the theory (valid close to equilibrium) but also when the nonlinear form of the equations has to be considered at higher wetting speeds.     

Study of properties of modified silicones at solid-liquid interface: fabric-silicone interactions

Silicones are special reagents that impart desired surface properties such as softness, bounciness and antiwrinkle properties to fabrics and related materials. Although these finishing processes have been practiced routinely, very little is known about the mechanisms involved in modification so that they could be improved. The current study was undertaken to develop basic understanding of the mechanisms responsible for surface modification of fibers using silicones. PDMS based amino silicone emulsions, quaternized to various degrees using dimethyl sulphate, were used in the present study. The electrokinetic properties of the modified silicones were studied as a function of pH. It was expected that the silicone emulsions would show a steady positive zeta potential throughout the pH range due to the quaternization by dimethyl sulphate. Surprisingly, a sudden drop in the zeta potential was observed around pH 8 with the samples turning hazy in the pH range of 8-10. Turbidimetric studies also showed a sudden increase in the turbidity in the pH range 8-10 where commercial processes also encounter problems. It was concluded that the emulsions were destabilized at pH 8-10 thus rendering them ineffective for surface treatment. In order to identify reason for the improvement in fabric properties, fiber structure was monitored using atomic force microscopy. It was observed that the treated fibers were far smoother, relaxed and uniform as compared to the untreated fibers. Thus the morphology of the fabric is modified in a specific way by treatment with specialty silicones.     

固-液界面上的吸附膜

利用Gibbs吸附公式处理了硅胶自四氯化碳和环己烷中吸附脂肪醇、环己醇、苯甲醛、苯甲醚、乙酸丙酯的实验结果,得到吸附膜的表面压(π)和分子面积(A)的关系曲线,这些曲线均可用描述不溶物液态扩张膜的Smith方程描述。文中对所得结果给出了初步的解释。     

The influence of an electric field on the wetting of an SiO2 film by ionic liquids

The electrowetting of a dielectric SiO₂ film 100 nm thick by drops (D = 2–3 mm) of [C₄mIm][PF₆], [C₆mIm][PF₆], and [C₆mIm][BTI] ionic liquids was studied at |U| ≤ 60 V in a ～10^?8 mbar vacuum. Electrocapillary curves of the dependence of the wetting angle on electric field potential were constructed with an accuracy of ±1 deg. In conformity with the Young-Lippman equation, the wetting angle θ° decreased by the parabolic law from 51° to 43° for [C₄mIm][PF₆], from 48° to 38° for [C₆mIm][PF₆], and from 35° to 27° for [C₆mIm][BTI] as |U| increased at 298 K. The electrocapillary curve branches were situated symmetrically in the (θ°, U) coordinates with respect to the line passing through the point U = 0; that is, zero-charge potential is zero for the electrowetting of the dielectric film by the ionic liquids. The capacitance of the double electrical layer at the ionic liquid-dielectric interface was determined. This value was found to be 4.65, 2.93, and 1.73 μF/m² for the electrowetting of the SiO₂ film at 298 K by the ionic liquids specified, respectively.     

Nanoporous surface wetting behavior: the line tension influence

The aim of this work is to develop a physical model to describe the evolution of the apparent contact angle for four different liquids on nanotextured alumina surfaces with different pore radius. The nanoporous alumina templates were fabricated by anodization of Al foil in a 0.3 M oxalic acid solution. Scanning electron microscopy was used to characterize the morphology of the surfaces. The templates are approximately 400 nm in thickness and consist of a well-ordered hexagonal array of uniform radius pores spaced 105 nm apart with pore radii from 12 to 42 nm. The wettability of nanoporous alumina templates was investigated using contact-angle measurements. We measured the contact angles using four liquids: water, ethylene glycol, aniline, and a mixture of ethylene glycol and aniline. We developed a new theoretical model for the contact angle on nanoporous surfaces as a function of the pore radius. This model is based on energy considerations and involves liquid penetration into the nanopores driven by the capillarity (Laplace's law). Because the air is compressed inside the pores, this model also includes the effect of the line tension. This is important because the three-phase line length is greatly enhanced in our nanoporous structures. For example: for a millimeter-sized droplet, the three-phase line around the perimeter of the droplet is a few millimeters long, whereas the total three-phase line within the pores can reach several tens of meters. Using our model, the line-tension value for our nanopore samples is positive and ranges from 4 to 13 × 10(-9) N, which falls within the wide interval from 10(-11) to 10(-5) N quoted in the literature. Nanoporous surfaces may allow the effect of line tension to be visible for micro- to macrodroplets.     

Recent advances in studies of bubble-solid interactions and wetting film stability

Research in the area of bubble-solid interactions is reviewed and highlighted, with a focus on studies of wetting film drainage using theoretical approaches and experimental (interferometric) approaches, and also studies probing the stability of wetting films, where the stability has been affected by physical and chemical modification/factors. Significant advances have been made in recent years in the area of interferometry and force measurement of bubble-surface encounters, with multiple light wavelengths used to improve accuracy and certainty with regard to thickness of wetting films, as well as high speed interferometry. These advances have been accompanied with improvements to models to describe nonequilibrium aspects of opposing interfaces. Experimental studies of the influence of air bubbles and surface roughness have highlighted the importance of dissolved gas and surface condition in determining whether wetting films are stable. Finally, many new studies on the influence of polymer layers on wetting film stability and rupture have been published, and these are described in relation to the increase in our understanding of the role of adsorbed polymers in altering surface chemistry and physics of the underlying substrate.     

Influence of the work of adhesion on the dynamic wetting of chemically heterogeneous surfaces

The velocity dependence of the dynamic contact angle for a glycerol-water mixture wetting two different chemically heterogeneous surfaces (mixed thiols on gold and partially methylated titania, 16 samples in all) was studied. The molecular kinetic theory (MKT) of wetting was used to interpret the dynamic contact angle data. The equilibrium displacement frequency ( K 0) was predominantly determined by the viscous contribution from the bulk liquid, with a minor contribution from the surface. The mean distance between surface sites (lambda) decreased with increasing work of adhesion. The contact line friction coefficient zeta 0 was found to vary exponentially with the work of adhesion, enabling the unit flow volume of the liquid to be obtained.     

Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids

The initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene) have been studied with a high-speed camera. For drops of 1–4 μL volume, the increase in contact radius r can be described by a power law r μ t^a r propto {t^{alpha }} , with the spreading exponent α = 0.50 and for the first ≈8 ms. Thereafter, the three-phase contact line was pinned leading to a macroscopic static contact angle of Θ₀ = 12–15°. The insoluble liquids ethanol (α = 0.47, Θ₀ = 0) and water (α = 0.35, Θ₀ = 90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer, like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport of toluene through the vapor phase plays a significant role.     

The influence of cell adsorbent interactions on protein adsorption in expanded beds

Expanded bed adsorption (EBA) is a primary recovery operation allowing the adsorption of proteins directly from unclarified feedstock, e.g. culture suspensions, homogenates or crude extracts. Thus solid-liquid separation is combined with adsorptive purification in a single step. The concept of integration requires that the solid components of the feed solution are regarded as a part of the process, which influences stability, reproducibility, and overall performance. This aspect is investigated here at the example of the influence of presence and concentration of intact yeast cells (S. cerevisiae) on the adsorption of model proteins (hen egg white lysozyme and bovine serum albumin) to various stationary phases (cation and anion-exchange, hydrophobic interaction, immobilised metal affinity). The interaction of the cells with the adsorbents is determined qualitatively and quantitatively by a pulse response method as well as by a finite bath technique under different operating conditions. The consequence of these interactions for the stability of expanded beds in suspensions of varying cell concentration is measured by residence time distributions (RTDs) after tracer pulse injection (NaBr, LiCl). Analysis of the measured RTD by the PDE model allows the calculation of the fraction of perfectly fluidised bed (phi), a parameter which may be regarded as a critical quantity for the estimation of the quality of fluidisation of adsorbents in cell containing suspensions. The correlation between bed stability and performance is made by analysing the breakthrough of model proteins during adsorption from unclarified yeast culture broth. A clear relationship is found between the degree of cell/adsorbent interaction, bed stability in terms of the phi parameter, and the sorption efficiency. Only beds characterised by a phi value larger than 0.8 in the presence of cells will show a conserved performance compared to adsorption from cell free solutions. A drop in phi, which is due to interactions of the fluidised adsorbent particles with cells from the feed, will directly result in a reduced breakthrough efficiency. The data presented highlight the importance of including the potential interaction of solid feedstock components and the expanded adsorbents into the design of EBA processes, as the interrelation found here is a key factor for the overall performance of EBA as a truly integrated operation.     

From rolling ball to complete wetting: the dynamic tuning of liquids on nanostructured surfaces

In this work, for the first time, a dynamic electrical control of the wetting behavior of liquids on nanostructured surfaces, which spans the entire possible range from the superhydrophobic behavior to nearly complete wetting, has been demonstrated. Moreover, this kind of dynamic control was obtained at voltages as low as 22 V. We have demonstrated that the liquid droplet on a nanostructured surface exhibits sharp transitions between three possible wetting states as a function of applied voltage and liquid surface tension. We have examined experimentally and theoretically the nature of these transitions. The reported results provide novel methods of manipulating liquids at the microscale.