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Advances in Colloid and Interface Science (v.165, #2)
Wetting and surface forces by Ludmila Boinovich; Alexandre Emelyanenko (pp. 60-69).
In this review we discuss the fundamental role of surface forces, with a particular emphasis on the effect of the disjoining pressure, in establishing the wetting regime in the three phase systems with both plane and curved geometry. The special attention is given to the conditions of the formation of wetting/adsorption liquid films on the surface of poorly wetted substrate and the possibility of their thermodynamic equilibrium with bulk liquid. The calculations of contact angles on the basis of the isotherms of disjoining pressure and the difference in wettability of flat and highly curved surfaces are discussed. Mechanisms of wetting hysteresis, related to the action of surface forces, are considered.
Keywords: Contact angles; Disjoining pressure; Surface forces; Wetting films; Adsorption films; Liquid films; Hysteresis
Theory of non-equilibrium force measurements involving deformable drops and bubbles by Derek Y.C. Chan; Evert Klaseboer; Rogerio Manica (pp. 70-90).
Over the past decade, direct force measurements using the Atomic Force Microscope (AFM) have been extended to study non-equilibrium interactions. Perhaps the more scientifically interesting and technically challenging of such studies involved deformable drops and bubbles in relative motion. The scientific interest stems from the rich complexity that arises from the combination of separation dependent surface forces such as Van der Waals, electrical double layer and steric interactions with velocity dependent forces from hydrodynamic interactions. Moreover the effects of these forces also depend on the deformations of the surfaces of the drops and bubbles that alter local conditions on the nanometer scale, with deformations that can extend over micrometers. Because of incompressibility, effects of such deformations are strongly influenced by small changes of the sizes of the drops and bubbles that may be in the millimeter range. Our focus is on interactions between emulsion drops and bubbles at around 100μm size range. At the typical velocities in dynamic force measurements with the AFM which span the range of Brownian velocities of such emulsions, the ratio of hydrodynamic force to surface tension force, as characterized by the capillary number, is ~10−6 or smaller, which poses challenges to modeling using direct numerical simulations. However, the qualitative and quantitative features of the dynamic forces between interacting drops and bubbles are sensitive to the detailed space and time-dependent deformations. It is this dynamic coupling between forces and deformations that requires a detailed quantitative theoretical framework to help interpret experimental measurements. Theories that do not treat forces and deformations in a consistent way simply will not have much predictive power. The technical challenges of undertaking force measurements are substantial. These range from generating drop and bubble of the appropriate size range to controlling the physicochemical environment to finding the optimal and quantifiable way to place and secure the drops and bubbles in the AFM to make reproducible measurements. It is perhaps no surprise that it is only recently that direct measurements of non-equilibrium forces between two drops or two bubbles colliding in a controlled manner have been possible. This review covers the development of a consistent theory to describe non-equilibrium force measurements involving deformable drops and bubbles. Predictions of this model are also tested on dynamic film drainage experiments involving deformable drops and bubbles that use very different techniques to the AFM to demonstrate that it is capable of providing accurate quantitative predictions of both dynamic forces and dynamic deformations. In the low capillary number regime of interest, we observe that the dynamic behavior of all experimental results reviewed here are consistent with the tangentially immobile hydrodynamic boundary condition at liquid–liquid or liquid–gas interfaces. The most likely explanation for this observation is the presence of trace amounts of surface-active species that are responsible for arresting interfacial flow.Display Omitted► Interplay of hydrodynamics and surface forces control. ► Film drainage due to mechanical and electrical perturbations. ► Quantitative predictions of stability or coalescence. ► All experimental data available indicate the interface is tangentially immobile. ► The Reynolds flat film model lacks quantitative precision.
Keywords: Young–Laplace; Stokes–Reynolds; Coalescence; Film drainage; AFM force measurements; Deformable drops and bubbles
Charge heterogeneity of surfaces: Mapping and effects on surface forces by Jaroslaw Drelich; Yu U. Wang (pp. 91-101).
The DLVO theory treats the total interaction force between two surfaces in a liquid medium as an arithmetic sum of two components: Lifshitz–van der Waals and electric double layer forces. Despite the success of the DLVO model developed for homogeneous surfaces, a vast majority of surfaces of particles and materials in technological systems are of a heterogeneous nature with a mosaic structure composed of microscopic and sub-microscopic domains of different surface characteristics. In such systems, the heterogeneity of the surface can be more important than the average surface character. Attractions can be stronger, by orders of magnitude, than would be expected from the classical mean-field DLVO model when area-averaged surface charge or potential is employed. Heterogeneity also introduces anisotropy of interactions into colloidal systems, vastly ignored in the past. To detect surface heterogeneities, analytical tools which provide accurate and spatially resolved information about material surface chemistry and potential — particularly at microscopic and sub-microscopic resolutions — are needed.Atomic force microscopy (AFM) offers the opportunity to locally probe not only changes in material surface characteristic but also charges of heterogeneous surfaces through measurements of force–distance curves in electrolyte solutions. Both diffuse-layer charge densities and potentials can be calculated by fitting the experimental data with a DLVO theoretical model. The surface charge characteristics of the heterogeneous substrate as recorded by AFM allow the charge variation to be mapped. Based on the obtained information, computer modeling and simulation can be performed to study the interactions among an ensemble of heterogeneous particles and their collective motions. In this paper, the diffuse-layer charge mapping by the AFM technique is briefly reviewed, and a new Diffuse Interface Field Approach to colloid modeling and simulation is briefly discussed.Display Omitted► Local surface charge and interaction are important in stability of colloidal systems. ► AFM can measure and map the diffuse-layer charges on heterogeneous surfaces. ► We attempt to link the AFM charge mapping with modeling of surface forces.
Keywords: Colloidal forces; Surface heterogeneity; Diffuse-layer potential; Surface forces
Interaction forces between adsorbed polymer layers by Tharwat Tadros (pp. 102-107).
The interaction forces between adsorbed polymer layers were investigated. Two types of graft copolymers that were adsorbed on hydrophobic surfaces have been investigated: (i) a graft copolymer consisting of polymethylmethacrylate/polymethacrylic acid back bone (the B chain) on which several poly(ethylene oxide) chains are grafted (to be referred to as PMMA/PEOn); and (ii) a graft copolymer consisting of inulin (linear polyfructose with degree of polymerization >23) (the A chain) on which several C12 chains are grafted (INUTEC SP1). In the first case adsorbed layers of the graft copolymer were obtained on mica sheets and the interaction forces were measured using the surface force apparatus. In the second case the interaction forces were measured using Atomic Force Microscopy (AFM). For this purpose a hydrophobically modified glass sphere was attached to the tip of the cantilever of the AFM and the glass plate was also made hydrophobic. Both the sphere and the glass plate contained an adsorbed layer of INUTEC SP1.In the surface forces apparatus one essentially measures the energy E(D)–distance D curves for the graft copolymer of PMMA/PEOn between mica surfaces bearing the graft copolymer and this could be converted to interaction energy between flat surfaces. Using the de Gennes scaling theory, it is possible to calculate the interaction energy between the polymer layers. The same graft copolymer was used in latex dispersions and the high frequency modulus G′∝ was measured as a function of the volume fraction ϕ of the dispersion. This high frequency modulus could be related to the potential of mean force. In this way one could compare the results obtained from rheology and those obtained from direct measurement of interaction forces.In the AFM method, the interaction forces are measured in the contact area between two surfaces, i.e. a spherical glass particle and a glass plate. Both glass spheres and plates were hydrophobized using dichlorodimethylsilane. Results were obtained for adsorbed layers of INUTEC SP1 in water and in the presence of various concentrations of Na2SO4 (0.3, 0.8, 1.0 and 1.5moldm−3). All results showed a rapid increase of force with a decrease of separation distance and the forces were still repulsive up to the highest Na2SO4 concentration. This explains the high stability of dispersions when using INUTEC SP1 as stabilizer.Display Omitted► Interaction Forces. ► Polymeric Layers. ► Rheology of Dispersions.
Keywords: Adsorption; Graft copolymers; Interaction forces; Polymer adsorbed layers; Rheological measurements; High frequency modulus
About mechanism of foam stabilization by solid particles by Pyotr M. Kruglyakov; Svetlana I. Elaneva; Natalya G. Vilkova (pp. 108-116).
The main objective of the research was to study the properties of the foam and foam films stabilized by solid particles. The properties of the foam films were compared with the analogous ones of the emulsion films. The experiment provided the rheological characteristics of the dispersion medium. The research provided the study of the correlation of the contact angle, the shear stress, the surface tension and the radii of aggregates by the solid particles and the modifier concentration. The effect of the foam film stabilization by the solid particles is significant.