Introduction

There are a number of methods for generating foams, and these have been reasonably well documented throughout the literature. They may be classified into two groups; the first involves the entrapment of air bubbles from the atmosphere and this can be achieved by relatively simple techniques such as shaking, pouring, circulation, sparging (introduction of gas using frits), etc. The second method involves artificially producing gas bubbles by physical methods (e.g. by nucleation or electrolysis) or chemical methods, which are commonly exploited in the production of polymer foams and involve the use of so-called blowing agents. These are chemical compounds that decompose or react to produce gas bubbles. While it is difficult to control the bubble size using physical methods of bubble formation, with chemical methods it is much easier to achieve a narrow-size distribution along with a high generation rate. Many different types of gases are used for foam generation, but it is important to recognize that foams generated with less soluble gases, such as N2 or air, will coarsen more slowly than foams produced with more soluble gases such as CO2 since gas diffusion through the soap films is largely determined by the gas solubility and the diffusion coefficient. In traditional foaming processes such as froth flotation, mechanical air entrapment methods are frequently used since they are relatively inexpensive, whereas in the production of material foams more sophisticated chemical processes have been developed.

The adsorption of surfactant on the freshly generated bubbles

The initial step in the generation of bubbles and foams involves the formation of a gas/liquid interface. This process involves work which can be quantified as the product of the interfacial tension and the increase in area of the interface; it be expressed by the equation

where ΔA is the created interfacial area and γ is the surface tension of the freshly produced bubbles. In water, bubbles have a high interfacial energy and become instantly unstable. Therefore, it is essential that surfactant adsorbs at the interface and reduces the surface tension and stabilizes the bubble. The adsorption kinetics plays an important role in the stabilization of the bubble, and the surfactant molecules need to rapidly diffuse from the bulk solution to the bubble interface.