Foam

Foam (from Latin spuma ) are gaseous bubbles, which are enclosed by solid or liquid walls. For fire fighting foam used is called foam.

Liquid foam walls

Liquid foam consists of small gas bubbles separated by liquid walls which are formed of surfactants and mostly water. Surfactants have two differently structured ends.

• One end is hydrophilic, which means "water loving". In the following figures, these ends are indicated by a minus sign or a red dot.
• The other end is hydrophobic ( water repellent ) and lipophilic ( fat loving )

With this structure, the molecules are surface active, ie they try to be so arranged that the hydrophobic part does not come into contact with water. This can be done in three major ways:

• The surfactants accumulate at the interface between water and air. They thus reduce the interfacial tension. The surface tension is referred to in the case of an interface between liquid and gas phase and surface tension.
• The surfactants can " solve " usually in water by coalesce and form small balls. The hydrophobic ends point towards the center. They form so-called micelles. This provides surfactants " in reserve " available to fill in an increase in the interfacial area with the new molecules.
• With surfactants may form a slight film of water with two surface layers. The film is, for example, at a prior bubble. The hydrophilic ends stick here in the aqueous phase.

Reducing the surface tension of the aqueous phase by the addition of surfactants at the interface of water / air has the result that, by breaking, bubbling, or like methods air in such a solution can be introduced, and the air bubbles thus generated are partially stabilized through the formation of a surface layer can not coalesce quickly. Foam is thus a dispersion of air in a surfactant-containing solution, wherein a three-dimensional network includes the air liquid lamellae as continuous phase, thereby forming polyhedra. Therefore, we call such a foam also polyhedral.

Depending on the structural and electrostatic properties of the surfactant molecules are formed foam bubbles of different size, wall thickness and durability. In principle, the lifetime of a liquid foam is limited. Foams are thermodynamically metastable systems, since the entire system will seek to reduce the very large interfacial water / air to a minimum value. Due to gravity, the interlamellar fluid between the foam bubbles flowing down slowly. Thus, the wall in the upper part becomes thinner and thinner until it tears there.

Anti-foaming agents and defoamers accelerate the decay of the foam. The Marangoni effect contributes, however, to stabilizing of foams under dynamic conditions.

In addition to the above-described Polyederschäumen which are formed only in the presence of surfactants, there are ball foams. They consist of independent bubbles, optionally unite in contact with each other. The lifetime of these foams is dependent on the strength ( viscosity) of the liquid. In low-viscosity liquids such as water, the foam breaks down in seconds. (Example: Opening a bottle of mineral water containing CO2. )

Problems due to liquid foams

Foam formation in liquids caused in process plants often have problems because the foam penetrates into plant areas where the liquid is not attain unto. This is for example in the distillative removal of solvents in the manufacture of synthetic resins, the case where the now gaseous solvent causes foaming of the resin. Usually here defoamers.

Two-dimensional foam

As a two-dimensional foam is called Polyederschäume, where the polyhedra are just next to each other. To the foam between two glass plates is compressed, the bubbles are thus limited to two dimensions. The plate distance must be smaller than the diameter of the smallest bubbles. Since no bubbles are superimposed, can be easily observed a two-dimensional foam.

Between two adjacent cells with the same number of corners of the blade is straight. If two adjacent cells different corner numbers, the slat is curved in between, and in the direction of the cell with the higher number of corners. In cells with a higher number of corners thus there is a lower air pressure. The Von Neumann 's law describes how the size of the foam cells varies over time.

Fixed foam walls

Solid foam consisting of small gas bubbles, which are separated by solid walls.

Examples:

• Pumice, a porous glassy volcanic rock whose specific gravity is less than that of water.
• Foam, foam rubber, polystyrene as insulation and packaging material
• Mounting foam for installation in building (eg windows, doors)
• Rigid foam panels for insulation in construction and as a partition in Stand
• Situ
• Foam glass
• Aerogels
• Metal foams, such as Aluminum foam for high-strength, yet lightweight metal structures
• Foam concrete
• Sandwich foam with fiber composite materials (eg PVC foam )