This article is about the substance formed from espuma gas bubbles. This article needs additional citations for verification.
Please help improve this article by adding citations to reliable sources. Foam is an object formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the regions of gas. Soap foams are also known as suds. Solid foams can be closed-cell or open-cell. In closed-cell foam, the gas forms discrete pockets, each completely surrounded by the solid material.
In open-cell foam, gas pockets connect to each other. A bath sponge is an example of an open-cell foam: water easily flows through the entire structure, displacing the air. Foams are examples of dispersed media. A foam is, in many cases, a multi-scale system. One scale is the bubble: material foams are typically disordered and have a variety of bubble sizes.
At lower scale than the bubble is the thickness of the film for metastable foams, which can be considered a network of interconnected films called lamellae. Solid foams, both open-cell and closed-cell, are considered as a sub-class of cellular structures. They often have lower nodal connectivity as compared to other cellular structures like honeycombs and truss lattices, and thus, their failure mechanism is dominated by bending of members. Low nodal connectivity and the resulting failure mechanism ultimately lead to their lower mechanical strength and stiffness compared to honeycombs and truss lattices. One of the ways foam is created is through dispersion, where a large amount of gas is mixed with a liquid. A more specific method of dispersion involves injecting a gas through a hole in a solid into a liquid.
If this process is completed very slowly, then one bubble can be emitted from the orifice at a time as shown in the picture below. 1 is the density of the gas ρ2 is the density of the liquid. As more air is pushed into the bubble, the buoyancy force grows quicker than the surface tension force. Thus, detachment occurs when the buoyancy force is large enough to overcome the surface tension force. R1 and R2 are the radii of curvature and are set as positive.
At the stem of the bubble, R3 and R4 are the radii of curvature also treated as positive. Here the hydrostatic pressure in the liquid has to take in account z, the distance from the top to the stem of the bubble. R3 or R4 is large while the other radius of curvature is small. As the stem of the bubble grows in length, it becomes more unstable as one of the radius grows and the other shrinks. At a certain point, the vertical length of the stem exceeds the circumference of the stem and due to the buoyancy forces the bubble separates and the process repeats. The stabilization of a 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 lamellae. The Marangoni effect depends on the liquid that is foaming being impure.
Generally, surfactants in the solution decrease the surface tension. The surfactants also clump together on the surface and form a layer as shown below. For the Marangoni effect to occur, the foam must be indented as shown in the first picture. This indentation increases local surface area.
Surfactants have a larger diffusion time than the bulk of the solution—so the surfactants are less concentrated in the indentation. Also, surface stretching makes the surface tension of the indented spot greater than the surrounding area. Consequentially—since diffusion time for the surfactants is large—the Marangoni effect has time to take place. The difference in surface tension creates a gradient, which instigates fluid flow from areas of lower surface tension to areas of higher surface tension. The second picture shows the film at equilibrium after the Marangoni effect has taken place.
Deviations are due to the Marangoni effect and capillary pressure, which affect the assumption that the bubbles are spherical. For laplace pressure of a curved gas liquid interface, the two principal radii of curvature at a point are R1 and R2. A designates the top of the bubble while point B designates the bottom of the bubble. At the top of the bubble at point A, the pressure in the liquid is assumed to be p0 as well as in the gas. 1 and ρ2 is the density for a gas and liquid respectively.