By use of the analytic result for the Laplace transform of the radial distribution function for two large hard spheres dispersed in a fluid of the smaller hard spheres, simple equations for the film interaction energy and film disjoining pressure, due to the structural and depletion forces, are derived. The proposed equations satisfy some known exact results and explicitly express the energy and pressure as a function of the film thickness and volume fraction of the small hard spheres. The predicted results for the film energy and film disjoining pressure are compared with existing computer simulation data. The proposed simple analytical expressions can be applied to understand the stability of liquid films containing colloidal particles and the stability of suspensions, foams, and emulsions.

The effective pair interaction (potential of the mean force) between large hard spherical colloidal particles in a dispersion containing small hard sphere particles has been studied theoretically. We calculated the effective interaction from the total correlation function by solving the Ornstein-Zernike equation. For a simple binary mixture of particles, the calculated effective interaction between two large (colloidal) particles is found to be oscillatory in general, including both the Asakura-Oosawa type attractive depletion well and the repulsive energy barrier (which is caused by the formation of small particle layers between these two large particles). A systematic study was conducted to examine the relationship between the effective interaction potential and the controllable parameters like the particle concentration and the size ratio. The existence of the structural energy barrier is expected to have dual effects on the stability of the large particle dispersion in general: stabilizing in a short time scale and destabilizing in a long time scale. The effect of the polydispersity of small particles on both depletion and the structural force between large particles is also addressed in our study, and we found that polydispersity affects the structural energy barrier more than it does the attractive depletion well. In comparison with the depletion/structuring force measured from the force apparatus experiments, we found that our theoretical results are in good agreement with the experiment.

The mechanisms of stabilization of water-in-crude oil emulsions have been investigated by changing the solvent-solute interactions in crude oil. Diluting the original crude oil with varying amounts of heptane, which is a poor solvent for asphaltenes, changes the solvent-solute interactions, leading to flocculation of asphaltenes and thus changing the emulsion stability. The interactions between the water droplets in an emulsion system have been quantified by measuring the radial distribution function and thereby the pair potential using the digitized optical imaging technique. It has been observed that the force of interaction between water droplets is oscillatory. This shows that non-DLVO forces, such as attractive depletion and repulsive structural forces, exist between the droplets. The interaction between the water droplets has been modeled by studying the properties of a thin liquid film sandwiched between the water droplets. Because of the film confinement effect, asphaltene-resin particles form a layered structure inside the thin liquid film. Also, the role of hydrodynamic interactions has been studied by using the film rheometer to measure the dynamic film tension and film elasticity. It has been found that, because of the adsorption of asphaltene at the film interfaces, the film elasticity plays a significant role in stabilizing these emulsions.

The phenomenon of layering of like-charged particles (ionic surfactant micelles or other colloidal particles) between two plane-parallel film surfaces and the evolution of in-film particle structuring are studied using the canonical Monte Carlo method combined with a simulation cell containing both the bulk solution and the film region. The simulations are performed using an effective one-component fluid potential model that incorporates micelle hard-core repulsion and both the screened Coulomb electrostatic interaction and the entropic contribution due to the discrete nature of the solvent and finite size of electrolyte ions. The proposed potential model is applied to mimic the properties of surfactant micelle solutions in the bulk and under film confinement. The effects of surfactant concentration and film thickness on the in-film micellar structuring phenomenon are analyzed. The relation of obtained results to experiments on stratifying sodium dodecyl sulfate micellar films is discussed.

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The spreading of aqueous solutions of trisiloxane surfactants on solid surfaces has been studied extensively. Trisiloxane surfactants are used in pesticide delivery as adjuvants to promote spreading on leaves and provide a larger area for solute transfer. The spreading of a dew-drop on a leaf when a spray of pesticide is delivered is simulated by studying the spreading of a water drop on a hydrophobic surface when a small drop of aqueous trisiloxane surfactant is brought in contact with it. This study reveals many new features that differ from the spreading of an aqueous trisiloxane drop on a solid surface; the spreading of the substrate drop is characterized by an inertial rather than a viscous response, the imposed surface tension gradient dies out rapidly, and the spreading velocity is consistent with a balance of kinetic energy imparted to the substrate drop and the decrease in its surface energy.

This is probably the first study about the drainage of curved liquid films in the presence of colloidal particles. The systems did not contain any surfactant. In the presence of monodispersed colloidal particles, thinning occurs in a stepwise manner (stratification). It has been shown that the size of the film is an important parameter in the stepwise thinning process. This investigation found a critical film size below which at least one layer of particles stays in the film at equilibrium; a "spot" (a thinner section of the film), even if formed, does not expand and is in equilibrium with the film. The area of the spot expands linearly with time. The rate of spot-area expansion increases linearly with the film perimeter and can be increased or decreased merely by changing the film size. The stepwise film thinning and the effects of film size and particle concentration on film stability are discussed on the basis of the diffusive-osmotic mechanism.

The layering of macroions confined to a wedge slit formed by two uncharged hard walls is studied using a canonical Monte Carlo method combined with a simulation cell that contains both wedge-shaped slit and bath regions. The macroion solution is modeled within a one-component fluid approach that in an effective way incorporates the double layer repulsion due to simple electrolyte ions as well as the discrete nature of an aqueous solvent. The layer formation under a wedge confinement is analyzed by carrying out separate simulation runs for a set of consecutive wedge segments designed to represent a single wedge slit. As the wedge thickness progressively increases, the sequence of regions along the wedge film with distinct features of macroion layering has been established. This sequence comprises (i) a wedge region of the thickness smaller than the macroion diameter that is free of macroions; (ii) a region with a one-dimensional macroion chain along the wedge corner at a wedge thickness of a one macroion diameter; (iii) a region comprising a low-ordered macroion monolayer that extends until the wedge thickness slightly above two macroion diameters; (iv) a region comprising a pair of well-defined two-dimensional configurations of macroions segregated on each of the wedge walls; and (v) a free-of-macroions wedge region between two surface monolayers that now originates from an electrostatic repulsion imposed by the surface macroions, which is followed by (vi) a well-defined macroion monolayer film between two surface monolayers, a less defined bilayer film, a three-layer film, and so on up to the bulk solution. Once formed, the macroion surface monolayers persist for all remaining wedge thicknesses up to the bulk, forming in such a way effective charged wedge boundaries. Such a formation of the macroion surface monolayers on the uncharged confining walls is related to the haloing mechanism for regulating the stability in colloidal suspensions [Tohver et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8951] and is discussed as well. Finally, the estimated boundary of the free of macroion region between surface monolayers correlates well with the location of the boundary of the so-called "vacuum" phase that has been observed experimentally in an aqueous suspension of charged polystyrene spheres bounded by electrostatically repulsive glass walls [Pieransky et al. Phys. Rev. Lett. 1983, 50, 900].