Stabilization of Thin Films, Foams, Emulsions and Bifluid Gels with Surface-Active Solid Particles

This chapter summarizes the current advances in stabilization of thin films, foams, emulsions and bifluid gels by surface-active particles. Small particles of different shape and morphology with partial wettability can adsorb strongly at liquid/liquid and gas/liquid interfaces and are able to stabilize foams and emulsions. Factors such as particles size, shape and hydrophobicity and their effect on the stabilization mechanism are discussed in detail. The energy of attachment of a surface-active particle to the interface depends on the contact angle and interfacial tension and is proportional to the particle radius. Partially hydrophobic and shape anisotropic particles (e.g. rods) are more efficient than spheres in covering and protecting bubbles and emulsion droplets against coalescence. The mechanism of stabilization by surface-active particles is based on contributions from several effects. These particles stabilize dispersions of immiscible fluids by providing steric hindrance to their coalescence, by bridging the droplets or bubbles and by increasing the bending energy of the interface. Additional stability can be achieved by interconnecting the particles on the interface by chemical linking or physical entanglement. In addition, stabilization against gravity-induced separation (creaming and sedimentation) can be achieved by the formation of a network from dispersed phase or from excess particles in the continuous phase. Surface-active particles can be used to stabilize more complex structures such as double emulsions, which was proven to be very difficult with molecular surfactants. Finally, it was shown theoretically that surface-active particles with equal affinity for two fluids could create bicontinuous particle-stabilized fluid gels. Stabilization by surface-active particles has already found many industrial applications; however, a new level of stability and (structure) control in various dispersion systems and the possibility of deriving new materials need to be further explored.