foam

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foam:

see colloidcolloid
[Gr.,=gluelike], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. The mixture is also called a colloidal system, colloidal solution, or colloidal dispersion.
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Foam

 

a dispersed system with a cellular internal structure. A foam consists of gas or vapor bubbles separated by thin layers of liquid. Owing to the size of the bubbles, which varies from fractions of a millimeter to several centimeters, foams are classified as coarse dispersion systems.

The total volume of gas that is included within foams may exceed the volume of the dispersion medium, that is, the volume of the liquid layers, by a factor of several hundreds (see DISPERSION MEDIUM). The ratio of the volume of a foam to the volume of the liquid phase is the foam’s multiplicity factor. In highly dispersed foams, the bubbles convert into polyhedral cells, and the liquid layers into films that are several hundreds or, in some cases, several tens of nanometers thick. Such films form a framework that is somewhat stable and elastic, and thus, foams have the properties of structured systems (see DISPERSE STRUCTURE and GELS).

One of the major characteristics of foams is time stability, which can be expressed by the time that is required for a 50-percent reduction of the original volume or height of a layer of foam; among other evidences of a foam’s time stability is the change in the degree of dispersion. Foaming takes place either by dispersion of a gas in a liquid medium or by release of a nascent gas phase within the bulk of a liquid. Stable, highly dispersed foams can be obtained using foaming agents—substances that stabilize foams. These substances facilitate foaming and hinder the drainage of liquid from the foam films, thus preventing coalescence of the bubbles. Like stabilizers of emulsions and of lyophobic colloid systems, they reduce surface tension and create an adsorptive surface with positive disjoining pressure. Soaps, soaplike surfactants, and some soluble polymers are especially efficient stabilizers in aqueous mediums, forming layers on the interface of the liquid and gas phases with highly pronounced structural and mechanical properties. An increase in the viscosity of the dispersion medium increases the stability of a foam. Pure liquids with low viscosity do not foam.

Many types of stable foams with carbon dioxide as the gas phase are widely used in fire extinguishers. These foams are produced either directly in the extinguisher or in another type of foam generator. Foam flotation is used to concentrate valuable minerals. Many liquid and semiliquid food products are foamed and subsequently hardened, for example, breads, biscuits, and various types of confectioneries and creams. Solid, structural cellular materials, for example foam glass, foamed slag, expanded plastics, and porous rubbers, are also obtained by foaming originally liquid suspensions, melts, solutions, or polymer mixtures.

Antifoams are used to destroy foams or to prevent foaming, since in several technological processes, especially in the chemical, textile, and food-processing industries, foaming is undesirable. Effective antifoams are surfactants that displace foaming agents from the surface of the liquid but do not themselves stabilize the foam. They include various alcohols, ethers, and alkylamines. Sometimes, foams are removed by high temperatures, by mechanical means, or simply by settling.

L. A. SHITS

foam

[fōm]
(chemistry)
An emulsionlike two-phase system where the dispersed phase is gas or air.
(fluid mechanics)
A collection of bubbles on the surface of a liquid, often stabilized by organic contaminants, as found at sea or along shore. Also known as froth.
(geology)

foam

1. a mass of small bubbles of gas formed on the surface of a liquid, such as the froth produced by agitating a solution of soap or detergent in water
2. frothy saliva sometimes formed in and expelled from the mouth, as in rabies
3. the frothy sweat of a horse or similar animal
4. 
a. any of a number of light cellular solids made by creating bubbles of gas in the liquid material and solidifying it: used as insulators and in packaging
b. (as modifier): foam rubber
5. a colloid consisting of a gas suspended in a liquid
6. a mixture of chemicals sprayed from a fire extinguisher onto a burning substance to create a stable layer of bubbles which smothers the flames
References in classic literature ?
He could not bear the thought of living any longer; so, first flinging his crown and sceptre into the sea (useless baubles that they were to him now), King Aegeus merely stooped forward, and fell headlong over the cliff, and was drowned, poor soul, in the waves that foamed at its base!
Quasimodo was surrounded, seized, garroted; he roared, he foamed at the mouth, he bit; and had it been broad daylight, there is no doubt that his face alone, rendered more hideous by wrath, would have put the entire squad to flight.
The next day it was clear the fits had been given even as he said: their magazine pages were black with hasty photographs, their prose was convulsive they foamed at the headline.
So frightful was the speed with which we displaced the water, that a wave rose up on either side our bow and foamed aft in a series of three stiff, up-standing waves, while astern a great crested billow pursued us hungrily, as though at each moment it would fall aboard and destroy us.
On each shell were painted precipitous and impossible seas through which full-rigged ships foamed with a lack of perspective only equalled by their sharp technical perfection.
There it fell with mighty splash, one jagged end peaking out above the surface, while the waters bubbled and foamed with far-circling eddy.
Then, astounded by the quantity of beer that was lacking, and remembering having seen stale beer made to foam afresh, I took a stick and stirred what was left till it foamed to the brim.
He spoke without enthusiasm; and the first man, limping into the milky stream that foamed over the rocks, vouchsafed no reply.
He held his head rigid, face forward; but his eyes rolled, he kept on lifting and setting down his feet gently, his mouth foamed a little.
He foamed at the mouth while she stared at him, appalled by this sudden fury.
In their front were stretched those broad plains, which extend, with so little diversity of character, to the bases of the Rocky Mountains; and many long and dreary miles in their rear, foamed the swift and turbid waters of La Platte.
A soft-touch foamed skin technology for automotive interior coverstocks was discussed by Dow Chemical.