Peter A. Kralchevsky

Capillary forces and structuring in layers of colloid particles

Abstract ‘Capillary forces’ are interactions between particles mediated by fluid interfaces. Recent advances in this field have been achieved by experiments and theory on lateral capillary forces, which are due to the overlap of menisci formed around separate particles attached to an interface. In particular, we should mention the cases of ‘finite menisci’ and ‘capillary multipoles’. The capillary-bridge forces were investigated in relation to capillary condensation and cavitation, surface-force measurements and antifoaming by oily drops. The studies on colloidal self-assembly mediated by capillary forces developed in several promising directions. The obtained structures of particles have found numerous applications.

A possible mechanism of stabilization of emulsions by solid particles

Abstract Possible explanations of some experimental findings with emulsions stabilized by small adsorbed solid particles are proposed. Films consisting of a particle monolayer are considered and the stability of the liquid menisci between the particles is investigated theoretically. The effect of contact angle hysteresis on the effective disjoining pressure isotherms is also taken into account.

Capillary meniscus interaction between colloidal particles attached to a liquid—fluid interface

Abstract General expressions for the energy of capillary meniscus forces acting between particles attached to a liquid—fluid interface are derived. These expressions are specified for the cases of two vertical cylinders and two similar spheres partially immersed in a liquid layer on a horizontal solid substrate. The shape of the meniscus around the particles is determined from the Laplace equation by using the method of the matched asymptotic expansions. The derived asymptotic expressions are valid with a very good accuracy when the contact line radius and the interparticle distance are smaller than 100 μm (liquid—gas interface). The range of validity can be wider for emulsion-type interfaces. The capillary meniscus forces turn out to be attractive and very long-ranged. The results can be important for interpreting the surface coagulation phenomena accompanying flotation processes as well as two-dimensional ordering of colloidal particles and protein molecules.

Stability of emulsions under equilibrium and dynamic conditions

In the present article we analyse the influence of various factors, both thermodynamic and hydrodynamic, on the stability of emulsion systems. The effect of the droplet size on the droplet lifetime in an emulsion cream is quantified. The comparative importance of kinetic factors such as surface and bulk diffusion fluxes, or viscous and elastic surface stresses, is investigated. The fact that the emulsion films drain much more slowly when the surfactant is dissolved in the continuous phase (rather than in the disperse phase) provides a new understanding of the Bancroft rule and the process of chemical demulsification. New thermodynamic aspects of emulsion stability are also discussed. One of them is the relatively high surface electric potential of pure oil-water interfaces and adsorption monolayers of non-ionic surfactants. Another aspect is the role of non-DLVO surface forces, such as the hydration repulsion, oscillatory structural and depletion forces due to the presence of surfactant micelles. A criterion of emulsion stability is formulated synthesizing the effects of the major factors. Finally we consider the importance of the kinetic phenomena in emulsions formed from non-pre-equilibrated phases.

Flocculation and coalescence of micron-size emulsion droplets

Abstract We analyze the relative importance of droplet deformation, surfactant transfer and interfacial rheology for the properties and stability of emulsions. The appearance of deformation (flattening or film) in the zone of contact of two interacting droplets has the following consequences. It enhances the importance of the surface forces of intermolecular origin and gives rise to contributions from the interfacial dilatation and the bending energy. The flattening increases the viscous dissipation in the gap between two colliding drops and thus prolongs the lifetime of the doublet of two such drops. The critical thickness of the gap also depends on whether the drops are deformed or non-deformed. The factors which facilitate the flattening in the zone of contact between two emulsion drops are the increase in droplet size, the decrease in interfacial tension, the bending energy for water-in-oil emulsions, the increase in droplet–droplet attraction and the suppression of droplet–droplet repulsion. The presence of surfactant strongly affects the interfacial tension, the bending moment, and influences all kinds of DLVO and non-DLVO surface forces operative in the gap between two droplets. The rheological and dynamic properties of the surfactant adsorption monolayers (Gibbs elasticity, surface diffusivity, surface viscosity, and adsorption relaxation time) are major factors for the stability of emulsions under dynamic conditions. The solubility of the surfactant in one of the two phases can determine whether oil-in-water or water-in-oil emulsion will be formed. A criterion for emulsion stability accounting for the interplay of all thermodynamic and hydrodynamic factors mentioned above is obtained. It provides an interpretation and generalization of the Bancroft rule.

Ordered micelle structuring in thin films formed from anionic surfactant solutions: II. Model development

Abstract We observed the phenomenon of the stratification of thinning liquid films with both micellar solutions of anionic surfactants and solutions containing latex particles. To explain this phenomenon, we suggest that the stratification is a layer-by-layer decrease of the thickness of an ordered micellar (or latex) structure inside the film. To interpret the available experimental data for stratifying films from micellar solutions of sodium dodecyl sulfate (NaDS), a simple cell model is suggested. It permits calculation of the disjoining pressure contribution which is due to the presence of micellar structure inside the film. The micelles interact via screened electrostatic repulsion forming an ordered structure due to the restricted volume of the film. The calculated excess energy per unit area of the film exhibits a number of minima corresponding to the metastable states with micellar layers inside the film. The values of the film thickness at the metastable states were predicted by the model and agreed with the experiment.

Capillary mechanisms in membrane emulsification: oil-in-water emulsions stabilized by Tween 20 and milk proteins

We investigate the process of membrane emulsification in the presence of the nonionic surfactant Tween 20, and the milk proteins Na-caseinate and beta-lactoglobulin (BLG). Our goal is to examine the factors which control the drop-size distribution in the formed emulsions. The drops are produced at the outer surface of a cylindrical microporous glass membrane, so that the process of their formation and detachment can be directly observed by an optical microscope. In the case of 2 wt.% aqueous solution of Tween 20 we obtain a relatively fine and monodisperse oil-in-water emulsion with a mean drop diameter about three times that of the pore. The microscopic observations show that in this case the oil drops intensively pop out of separate pores. In contrast, for the lower concentrations of Tween 20, as well as for the investigated solutions of Na-caseinate and BLG, we observe that the membrane is covered by a layer of growing attached emulsion drops, which are polydisperse, with a relatively large mean drop size. This fact can be explained with a greater dynamic contact angle solid-water-oil. In such a case, after a drop protrudes from an opening, it does not immediately detach, but instead, the contact area drop/membrane expands over several pore openings. The smaller drop size in the emulsions stabilized by BLG, in comparison with those stabilized by Na-caseinate, is related to the circumstance that BLG adsorbs faster at the oil-water interface than Na-caseinate. In the investigated emulsions we did not observe any pronounced coalescence of oil drops. Hence, the generation of larger and polydisperse oil drops in some of the studied solutions is attributed mostly to the effect of expansion of the drop contact line and formation of hydrophobized domains on the membrane surface. Therefore, any factor, which leads to decrease of the dynamic three-phase contact angle, and thus prevents the contact-line expansion, facilitates the production of fine and monodisperse emulsions.

Capillary forces between particles at a liquid interface: General theoretical approach and interactions between capillary multipoles

The liquid interface around an adsorbed colloidal particle can be undulated because of roughness or heterogeneity of the particle surface, or due to the fact that the particle has non-spherical (e.g. ellipsoidal or polyhedral) shape. In such case, the meniscus around the particle can be expanded in Fourier series, which is equivalent to a superposition of capillary multipoles, viz. capillary charges, dipoles, quadrupoles, etc. The capillary multipoles attract a growing interest because their interactions have been found to influence the self-assembly of particles at liquid interfaces, as well as the interfacial rheology and the properties of particle-stabilized emulsions and foams. As a rule, the interfacial deformation in the middle between two adsorbed colloidal particles is small. This fact is utilized for derivation of accurate asymptotic expressions for calculating the capillary forces by integration in the midplane, where the Young-Laplace equation can be linearized and the superposition approximation can be applied. Thus, we derived a general integral expression for the capillary force, which was further applied to obtain convenient asymptotic formulas for the force and energy of interaction between capillary multipoles of arbitrary orders. The new analytical expressions have a wider range of validity in comparison with the previously published ones. They are applicable not only for interparticle distances that are much smaller than the capillary length, but also for distances that are comparable or greater than the capillary length.

Film and line tension effects on the attachment of particles to an interface: I. Conditions for mechanical equilibrium of fluid and solid particles at a fluid interface

Abstract A general variational approach has been applied to derive the conditions for mechanical equilibrium of fluid and solid particles at an interface in the presence of an external field. It is shown that Youngs equation for solid particles does not follow from force balance, which is due to the difference between the surface tension and the specific surface free energy of the solid. The correct definition of the contact angles in the presence of an external field is discussed.

Unique Properties of Bubbles and Foam Films Stabilized by HFBII Hydrophobin

The HFBII hydrophobin is an amphiphilic protein that can irreversibly adsorb at the air/water interface. The formed protein monolayers can reach a state of two-dimensional elastic solid that exhibits a high mechanical strength as compared to adsorption layers of typical amphiphilic proteins. Bubbles formed in HFBII solutions preserve the nonspherical shape they had at the moment of solidification of their surfaces. The stirring of HFBII solutions leads to the formation of many bubbles of micrometer size. Measuring the electrophoretic mobility of such bubbles, the ζ-potential was determined. Upon compression, the HFBII monolayers form periodic wrinkles of wavelength 11.5 μm, which corresponds to bending elasticity k(c) = 1.1 × 10(-19) J. The wrinkled hydrophobin monolayers are close to a tension-free state, which prevents the Ostwald ripening and provides bubble longevity in HFBII stabilized foams. Films formed between two bubbles are studied by experiments in a capillary cell. In the absence of added electrolyte, the films are electrostatically stabilized. The appearance of protein aggregates is enhanced with the increase of the HFBII and electrolyte concentrations and at pH close to the isoelectric point. When the aggregate concentration is not too high (to block the film thinning), the films reach a state with 12 nm uniform thickness, which corresponds to two surface monolayers plus HFBII tetramers sandwiched between them. In water, the HFBII molecules can stick to each other not only by their hydrophobic moieties but also by their hydrophilic parts. The latter leads to the attachment of HFBII aggregates such as dimers, tetramers, and bigger ones to the interfacial adsorption monolayers, which provides additional stabilization of the liquid films.