Advances in Colloid and Interface Science (v.216, #C)

Influence of surface roughness on dispersion forces by V.B. Svetovoy; G. Palasantzas (1-19).
Surface roughness occurs in a wide variety of processes where it is both difficult to avoid and control. When two bodies are separated by a small distance the roughness starts to play an important role in the interaction between the bodies, their adhesion, and friction. Control of this short-distance interaction is crucial for micro and nanoelectromechanical devices, microfluidics, and for micro and nanotechnology. An important short-distance interaction is the dispersion forces, which are omnipresent due to their quantum origin. These forces between flat bodies can be described by the Lifshitz theory that takes into account the actual optical properties of interacting materials. However, this theory cannot describe rough bodies. The problem is complicated by the nonadditivity of the dispersion forces. Evaluation of the roughness effect becomes extremely difficult when roughness is comparable with the distance between bodies. In this paper we review the current state of the problem. Introduction for non-experts to physical origin of the dispersion forces is given in the paper. Critical experiments demonstrating the nonadditivity of the forces and strong influence of roughness on the interaction between bodies are reviewed. We also describe existing theoretical approaches to the problem. Recent advances in understanding the role of high asperities on the forces at distances close to contact are emphasized. Finally, some opinions about currently unsolved problems are also presented.Display Omitted
Keywords: Dispersion forces; Roughness; Nonadditivity; Contact; Adhesion;

The term “electric birefringence anomaly” is known as the electric birefringence (EB) signal that occurs in solutions of ionically charged anisometric particles in a narrow concentration region. The signal is of opposite sign to the normal birefringence that occurs below and above this narrow concentration region. The normal electric birefringence signals in the dilute and more concentrated regions are due to the orientation of the particles in the direction of the applied electric field. The origin for the anomalous signal was not completely understood until now.The article summarises previous results in which the anomalous results had been observed but not well understood. It shows that the birefringence anomaly occurs in systems as diverse as micellar solutions, polyelectrolytes, solutions of clays, viruses and fibres. In all these systems the anomaly signals are present at the concentration when the length of the colloidal particles including the thickness of the electric double layer are about the same as the mean distance between the colloidal particles. Under these conditions the electric double layers of the particles overlap along the main axis of the particles but not in the direction across the particles.As a consequence of this situation a dipole is built up across the particles by the migration of the counter-ions of the particles in the electric field and this dipole leads to an orientation of the particles perpendicular to the electric field. The anomalous signal can usually be observed simultaneously with the normal signal. The amplitude of the anomalous signal can be larger than the amplitude of the normal signal. As a consequence the total birefringence changes its sign in the anomalous concentration region. The anomaly signal of the clays can also be explained by a fluctuating dipole around the particles, which is due to the fact that the centre of the ionic charges of the particles does not fall on the centre of the ionic charge of the counter-ions. The resulting dipole could also explain the anomaly signal.Fig. 11: Plot of Δnst  / (λE2) vs. concentration for aqueous solutions of C16C8DMABr for increasing field strengths.Display Omitted
Keywords: Electric birefringence anomaly; Micelles; Electrolytes; Clays; Virus; Fibrilles;

Colloidal aphrons are multi-layered stable bubbles (CGAs) or droplets (CLAs), surrounded by a thin surfactant film. The small size of the aphrons creates a system with a high interfacial area which can be pumped like water without collapsing. The high stability of colloidal aphrons due to a thin soapy shell surrounding the core, and high interfacial area make them of interest in many processes such as mineral processing, protein recovery, drilling fluids, separation of organic dyes from waste water, predispersed solvent extraction of dilute streams, clarification and purification of suspensions, soil remediation, material synthesis and immobilization of enzymes. This article aims to provide a comprehensive database in generation, characterization and applications of colloidal gas and liquid aphrons from more than 140 published works so far. The article also reports scale up, industrial applications, technical limitation regarding aphrons application and important future research scopes.Display Omitted
Keywords: Colloidal gas aphron; CGA; Colloidal liquid aphron; CLA; Microbubbles; Polyaphrons;