Foaming, foam films, antifoaming and defoaming

Oils and mixtures of oils with hydrophobic particles are widely used in various technologies and consumer products to control foaminess and foam stability. The aim of this review is to summarize our current understanding of the mechanisms of foam destruction by such substances, which are usually called antifoams or defoamers. The experimental results show that two types of antifoam can be distinguished (called for brevity "fast" and "slow") which differ in the modes of their action. Fast antifoams are able to rupture the foam films at the early stages of film thinning. As a result, fast antifoams destroy completely the foam in less than a minute, in a typical foam-stability test. Microscopic observations have shown that the fast antifoams rupture the foam films by the so-called "bridging" mechanisms, which involve the formation of oil bridges between the two surfaces of the foam film. The stability/instability of these oil bridges is explained by using the theory of capillarity. In contrast, the oily globules of the slow antifoams are unable to enter the surfaces of the foam films and are first expelled into the Plateau borders (PBs). Only after being compressed by the narrowing walls of the PBs (due to water drainage from the foam), are the globules of the slow antifoams able to enter the solution surface and to destroy the adjacent foam films. Typically, the process of foam destruction by slow antifoams requires much longer time, minutes or tens of minutes, and a residual foam of well-defined height is observed in the foam tests. The experiments show that there is no direct relation between the magnitudes of the entry, E, spreading, S, and bridging, B, coefficients, on one side, and the antifoam efficiency, on the other side. The only requirement for having active antifoam, with respect to the bridging mechanisms, is that B should be positive. On the other hand, the barrier preventing the emergence of pre-emulsified antifoam globules on the solution surface (so-called "entry barrier") is of crucial importance for the mode of foam destruction and for the antifoam efficiency. Measurements of the entry barrier with recently developed film trapping technique (FTT) showed that antifoams possessing low entry barriers act as fast antifoams, whereas high barriers correspond to slow or inactive antifoams, although E, S, and B coefficients could be strongly positive in the latter case. A good agreement between the magnitude of the entry barrier, measured by FTT, and the height of the residual foam, in the presence of slow antifoams, was experimentally established and theoretically explained. The importance of various factors, such as the size of antifoam globules, oil spreading, kinetics of surfactant adsorption, hydrophobicity of solid particles in mixed oil-solid antifoams, and the presence of amphiphilic additives (foam boosters), is discussed from the viewpoint of the mechanisms of antifoaming. The main experimental methods, used for studying the modes of antifoam action, are briefly described.

Mechanisms of foam destruction by oil-based antifoams.

The destabilization of froth by solids. I. The mechanism of film rupture

Here, we have reviewed brief ideas of fabrication of superhydrophobic surfaces and their important applications, which were discussed in the concluded national/international workshops and conferences organized by different industries and laboratories of ISRO (Indian Space Research Organization), DRDO (Defence Research and Development Organization) and AERB (Atomic Energy Regulatory Board) in the last four years. A comprehensive overview of wettability, development of fabrication, techniques for artificial superhydrophobic surfaces and their natures and low surface energy materials along with surface texture is presented successfully. The purpose of this review is to assist future development to make robust superhydrophobic surfaces.

Recent progress in fabrication and characterisation of hierarchical biomimetic superhydrophobic structures

5.5 HYDRODYNAMIC INTERACTIONS IN DISPERSIONS 106 5.5.1 Basic Equations. Lubrication Approximation 5.5.2 Interaction Between Particles of Tangentially Immobile Surfaces 5.5.2.1 Taylor and Reynolds Equations, and Influence of the Particle Shape 5.5.2.2 Interactions Among Non-Deformable Particles at Large Distances 5.5.2.3 Stages of Thinning of a Liquid Film 5.5.2.4 Dependence of Emulsion Stability on the Droplet Size 5.5.3 Effect of Surface Mobility 5.5.3.1 Diffusive and Convective Fluxes at an Interface; Marangoni Effect 5.5.3.2 Fluid Particles and Films of Tangentially Mobile Surfaces 5.5.3.3 Bancroft Rule for Emulsions 5.5.3.4 Demulsification and Defoaming 5.5.4 Interactions in Non-Preequilibrated Emulsions 5.5.4.1 Surfactant Transfer from Continuous to Disperse Phase (Cyclic Dimpling) 5.5.4.2 Surfactant Transfer form Disperse to Continuous Phase (Osmotic Swelling) 5.5.4.3 Equilibration of Two Droplets Across a Thin Film 5.5.5 Hydrodynamic Interaction of a Particle with an Interface 5.5.5.1 Particle of Immobile Surface Interacting with a Solid Wall 5.5.5.2 Fluid Particles of Mobile Surfaces 5.5.6 Bulk Rheology of Dispersions

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Chemical Physics of Colloid Systems and Interfaces

Mechanism of Antifoaming: Role of Filler Particle

Antifoam compositions containing high viscosity polydiorganosiloxane oils, finely divided hydrophobic solids, and siloxanepolyoxyalkylene block copolymer surfactants, which can be used per se or in the form of emulsions.

High efficiency antifoam compositions and process for reducing foaming

Short-range interfacial forces are known to play an important role in the performance of antifoams. To be effective in a given foaming medium, an antifoam agent should have a positive spreading co-efficient and form an unstable interfacial film incapable of foaming; i.e., it should be highly surface active and of limited solubility. Although these conditions are sufficient to describe the essentials of defoaming activity, they are inadequate to explain the variable effectiveness of different antifoams. In the present investigation, using silicone antifoams, we have shown that antifoam effectiveness can be strongly influenced by dispersibility and long-range interfacial electrical forces. The origin of such electrical forces is ionic surfactant adsorption which can impart identical surface charges to a foam bubble and the antifoam droplet, resulting in a repulsive interaction between them. Using the Gouy electrical double-layer model, this repulsive force has been estimated and correlated with antifoam effectiveness. It is also shown that in foaming systems containing anionic or cationic surfactants, electrical repulsive forces change significantly near the critical micelle concentrations of the foaming surfactants.

Mechanism of antifoaming action

It has been commonly recognized that a mineral surface must be sufficiently hydrophobic for flotation to occur. Thermodynamic treatments that deal with the role of the collector have centered largely on the solid/solution interface emphasizing, for example, contact angles and ζ potentials. The forces which are involved in the flotation process are usually considered to be short-ranged. Although it is known that the collector species adsorb also at the liquid/gas interface, the importance of this phenomenon in flotation has received relatively little attention. Since many collectors that are used for oxide mineral flotation bear a charge opposite to that of the oxide, it is expected that the liquid/gas interface would correspondingly bear a net charge opposite to that of the solid/liquid interface. It is possible, therefore, that the interaction of these two oppositely charged electrical double layers would produce a long-range force of attraction which would influence the kinetics of flotation. Using the Gouy treatment, we have estimated the magnitude of the potential energy of attraction for selected systems.

Adsorption and critical flotation conditions

Calcium carbonate is rendered receptive to silanation by pretreatment with soluble sodium silicate.

R. D. Kulkarni

Papers

Mechanism of Antifoaming: Role of Filler Particle

High efficiency antifoam compositions and process for reducing foaming

Mechanism of antifoaming action

Adsorption and critical flotation conditions

Silanated calcium carbonate fillers