Foam

In physical chemistry, foam or foam spray is defined as a colloidal system (it means a dispersion of particles in the continuous medium), where the particles are given as gas bubbles and the medium as a liquid. The word is often used to describe a lightweight rigid or spongy cellular material.

Types of Foams

Let us look at the types of foam or foam spray in detail.

Solid foams are both open and closed-cell, which are considered as a sub-class of the cellular structures. Whereas the liquid foams, at times, are relatively made long-lasting. For example, for fire fighting, adding a few substances, known as a stabilizer, that either retards or prevents the gas bubbles’ coalescence. 

Some of the great variety of substances, which act as foam stabilizers and best known are soaps, are proteins and detergents. Because proteins are edible, they can find wide use as foaming agents in foodstuffs such as marshmallow (made from gelatin and sugar), meringue (from egg white), and whipped cream.

Foam Structure

In many cases, foam is a multiscale system.

One scale is given as the bubble, where the material foams are typically disordered and contain a wide range of bubble sizes. Whereas, at larger sizes, the idealized foam’s study is closely linked to the three-dimensional tessellations, mathematical problems of the minimal surfaces, and also known as honeycombs. The structure of Weaire-Phelan is considered as the best possible (as optimal) unit cell of a perfectly ordered foam, while the Plateau's laws described how the soap films form the structures in foams.

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The above figure represents the order and disorder of bubbles in a surface foam.

At a lower scale compared to the bubble is the film’s thickness for metastable foams that may be considered a network of interconnected films known as lamellae. The lamellae ideally connect in triads and radiate 120° outward from the connection points, called Plateau borders.

Mechanical Properties of Solid Foams

Often, the solid foams have lower nodal connectivity to that of the other cellular structures such as truss lattices and honeycombs, and therefore, their failure mechanism can be dominated by the bending of members. Ultimately, the low nodal connectivity and the resulting failure mechanism lead to their strength of lower mechanical and stiffness compared to the truss lattices and honeycombs.

Experiments

Foam can be studied using many different techniques, being a multiscale system involving several phenomena and a versatile medium. Considering various scales, the experimental techniques are diffraction ones, primarily the light scattering techniques (static and dynamic light scattering, DWS, neutron, and X rays scattering) at either the sub-micrometre scales or the microscopic ones.

Characterizations

Considering this system as the continuous one, its bulk properties may be characterized by light transmittance but also conductimetry. In particular, the correlation between bulk and structure is evidenced more accurately by acoustics. The organization between the bubbles has been numerically studied using sequential attempts of the evolution of the minimum surface energy either in a deterministic way (surface evolver) or at random (Pott's model). The evolution with time (it means the dynamics) may be simulated using these particular models or the bubble model (Durian) that considers the motion of individual bubbles.

Foam Formation

Many conditions are required to produce foam: there should be mechanical work, surface active components (or the surfactants), which reduce the surface tension, and the foam formation faster than its breakdown. To create the foam, work (W) is required to increase the surface area (ΔA):

W = γΔA

where γ is given as the surface tension.

One of the ways that foam creates is via dispersion, where an excess amount of gas gets mixed with a liquid. A more particular dispersion method involves injecting a gas via a hole in a solid into a liquid. If this particular process is completed very slowly, then one bubble may be emitted from the orifice at a time.

One of the theories to determine the separation time is represented below; but, while this theory produces theoretical data, which matches with the experimental data, detachment because of the capillarity can be accepted as a better explanation.

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Applications

Liquid Foams

Liquid foams may be used in the fire retardant foam, like those that are used in the extinguishing fires, especially the oil fires.

In a few ways, leavened bread is a foam, as the yeast causes bread to rise by forming tiny bubbles of gas in the dough. Traditionally, the dough has been understood as a foam type of closed-cell, where the pores do not connect with each other. Cutting the dough will release the gas in bubbles, which are cut, whereas the gas in the remaining dough cannot escape. When the dough is allowed to rise very far, it becomes an open-cell foam, where the gas pockets are connected.

Solid Foams

Solid foams are explained as a class of lightweight cellular engineering materials. Typically, these particular foams are classified into two types according to their pore structure: open-cell-structured foams (which is also called reticulated foams) and closed-cell foams. At the high enough cell resolutions, any type can be treated either as continuous or "continuum" materials, and they are called cellular solids with predictable mechanical properties.