Surface forces and colloidal behaviour

The landmark DLVO theory of colloid stability sought to explain the stability of lyophobic colloids in terms of the interplay between attractive dispersion forces, and repulsive electrical (Coulombic) forces between particle surfaces. The net interaction energy between two particles (resulting from these so-called surface forces) as a function of surface separation can exhibit a maximum, a deep (primary) minimum and/or a shallow (secondary) minimum, giving stable, unstable or weakly flocculated dispersions. Other surface forces include steric forces arising from grafted or adsorbed polymer chains on the surfaces. Unadsorbed polymer can result in attractive depletion forces between particles, and polymer molecules that bridge particles can cause flocculation. Other forces mentioned are oscillatory structural forces, attractive hydrophobic forces and repulsive hydration forces between surfaces in water. Direct measurement of surface forces between both solid/liquid interfaces and between liquid/liquid interfaces is discussed at the end of the chapter.

Film Elasticity and Film Stabilization in Firefighting Foam Stabilized by Solid Particles

The structure of the thin liquid films determines the stability of foams and emulsions. In this work the bubbles stretched length with different hollow SiO2 particles concentration is measured when the foam has been stilled for different time. The results show that the bubbles stretched length is longer than that of bubbles when the foam is free of hollow SiO2 particles even when the foam has been stilled for 500mins. The bubbles stretched length increases with increasing the concentration of hollow SiO2 particles. A strong hydration effect leaves a large volume of hydration layers on the solid particles surfaces in aqueous solutions. The water in hydration layers can help the film keep a certain thickness. The existence of hydration forces leads that two particles cannot be too close each other. The high concentration surfactant limited in the fixed area helps the film keep good elasticity. Therefore the film has a long life time with compatible thickness and elasticity and the three-phrase foam is upper stable.

Ice-like water supports hydration forces and eases sliding friction

The nature of interfacial water is critical in several natural processes, including the aggregation of lipids into the bilayer, protein folding, lubrication of synovial joints, and underwater gecko adhesion. The nanometer-thin water layer trapped between two surfaces has been identified to have properties that are very different from those of bulk water, but the molecular cause of such discrepancy is often undetermined. Using surface-sensitive sum frequency generation (SFG) spectroscopy, we discover a strongly coordinated water layer confined between two charged surfaces, formed by the adsorption of a cationic surfactant on the hydrophobic surfaces. By varying the adsorbed surfactant coverage and hence the surface charge density, we observe a progressively evolving water structure that minimizes the sliding friction only beyond the surfactant concentration needed for monolayer formation. At complete surfactant coverage, the strongly coordinated confined water results in hydration forces, sustains confinement and sliding pressures, and reduces dynamic friction. Observing SFG signals requires breakdown in centrosymmetry, and the SFG signal from two oppositely oriented surfactant monolayers cancels out due to symmetry. Surprisingly, we observe the SFG signal for the water confined between the two charged surfactant monolayers, suggesting that this interfacial water layer is noncentrosymmetric. The structure of molecules under confinement and its macroscopic manifestation on adhesion and friction have significance in many complicated interfacial processes prevalent in biology, chemistry, and engineering.