A foam is a substance that is formed by trapping many gaseous bubbles in a liquid or solid.
A foam is normally an extremely complex system consisting of polydisperse gas bubbles separated by draining films.
The term foam may also refer to anything that is analogous to such a phenomenon, such as quantum foam, polyurethane foam (foam rubber), XPS foam, Polystyrene, phenolic, or many other manufactured foams. Fine foam can be considered a type of colloid.
Structure of foams
Real-life foams are typically disordered and have a variety of bubble sizes. The study of idealized foams is closely linked to the mathematical problems of space-filling and minimal surfaces. The Weaire-Phelan structure is believed to be the best possible (optimal) unit cell of a perfectly ordered foam, while Plateau's laws describe how the soap-films form structures in foams.
The foam makes a network of interconnected films called lamellae. Ideally, the lamellae are connected by three and radiate 120° outward from the connection points, known as Plateau borders. Several conditions are needed to produce foam: there must be mechanical work, surface active components that reduce the surface tension, and the formation of foam faster than its breakdown.
To create foam, work (W) is needed to increase the surface area (ΔA):
W = γ ΔA
where γ is the surface tension.
Stabilisation of foam is caused by van der Waals forces between the molecules in the foam, electrical double layers created by dipolar surfactants, and the Marangoni effect, which acts as a restoring force to the lamellas.
Several destabilising effects can break foam down. Gravitation causes drainage of liquid to the foam base, osmotic pressure causes drainage from the lamellas to the Plateau borders due to internal concentration differences in the foam, and Laplace pressure causes diffusion of gas from small to large bubbles due to pressure difference.
Technique monitoring foam stability
Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, hence identifying and quantifying destabilisation phenomena. It works on any concentrated dispersions without dilution, including foams. When light is send through the sample, it is backscattered by the bubbles. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (drainage, syneresis) and global changes in size (ripening, coalescence) are detected and monitored.
Measurement principle of multiple light scattering coupled with vertical scanning
Liquid foams can be used in fire retardant foam, found use in extinguishing fires, especially oil fires.
In some ways, leavened bread is a foam, as the yeast causes the bread to rise by producing tiny bubbles of gas in the dough.
The unique property of gas-liquid foams having very high specific surface area are exploited in the chemical processes of froth flotation and foam fractionation.
Solid foams form an important class of lightweight cellular engineering materials. These foams can be classified into two types based on their pore structure: open cell structured foams and closed cell foams.
Open cell structured foams contain pores that are connected to each other and form an interconnected network which is relatively soft. Open cell foam which will fill with whatever it is surrounded with. If filled with air this could be a relatively good insulator, but if the open cells fill with water, insulation properties would be reduced. Foam rubber is a type of open cell foam.
Closed cell foams do not have interconnected pores. Normally the closed cell foams have higher compressive strength due to their structures. However, closed cell foams are also generally denser, require more material, and consequentially are more expensive to produce. The closed cells can be filled with a specialized gas to provide improved insulation. The closed cell structure foams have higher dimensional stability, low moisture absorption coefficients and higher strength compared to open cell structured foams. All types of foam are widely used as core material in sandwich structured composite materials.
From the early 20th century, various types of specially manufactured solid foams came into use. The low density of these foams made them excellent as thermal insulators and flotation devices, and their lightness and compressibility made them ideal as packing materials and stuffings. A modern application of foam technology is aerogel, which is a closed-cell foam with very good insulatory properties, that is also very light. It is usually based on alumina, chromia and tin oxide, with carbon aerogels first developed in the late 1980s.
A special class of closed-cell foams is known as syntactic foam, which contains hollow particles embedded in a matrix material. The spheres can be made from several materials, including glass, ceramic, and polymers. The advantage of syntactic foams is that they have a very high strength-to-weight ratio, making them ideal materials for many applications, including deep sea and space applications. One particular syntactic foam employs shape memory polymer as its matrix, enabling the foam to take on the characteristics of shape memory resins and composite materials, i.e., it has the ability to be reshaped repeatedly when heated above a certain temperature and cooled. Shape memory foams have many possible applications, such as dynamic structural support, flexible foam core, and expandable foam fill.
Integral skin foam
Integral skin foam, also known as self-skin foam, is a type of foam with a high-density skin and a low-density core. They can be formed in an open mold process or a closed mold process. In the open mold process two reactive components are mixed and poured into an open mold. The mold is then closed and the mixture is allowed to expand and cure. Examples of items produced using this process include arm rests, baby seats, shoe soles, and mattresses. The closed mold process, more commonly known as reaction injection molding (RIM), injects the mixed components into a closed mold under high pressures.
Foam, in this case meaning "bubbly liquid", is also produced as an often unwanted by-product in the manufacture of various substances. For example, foam is a serious problem in the chemical industry, especially for biochemical processes. Many biological substances, for example proteins, easily create foam on agitation and/or aeration. Foam is a problem because it alters the liquid flow and blocks oxygen transfer from air (therefore preventing microbial respiration in aerobic fermentation processes). For this reason, anti-foaming agents, like silicone oils, are added to prevent these problems. Chemical methods of foam control are not always desired with respect to the problems (i.e. contamination, reduction of mass transfer) they may cause especially in food and pharmaceutical industries where the product quality is of great importance. In order to prevent foam formation in such cases mechanical methods are mostly dominant over chemical ones.
* Ballistic foam
1. ^ Lucassen, J. (1981) Lucassen-Reijnders, E. H. ed. Anionic Surfactants - Physical Chemistry of Surfactant Action NY, USA: Marcel Dekker