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 periodicals archive ?
FoamRecycle owns its own line of trucks and trailers able of carrying up to 300cf of foam from the customer's site as well as a large storage facility.
Foam manufacturers may recommend a type of adhesive for each of their products, but unfortunately, in practice there are no "one size fits all" solutions that will work for all applications of a particular foam material, according to the company.
Also in the running are hydrocarbons--isopentane, n-pentane, and cyclopentane--costing 50cents to 90cents/lb, as well as liquid, methyl-formate based Ecomate from Foam Supplies, which is used in spray-foam insulation and integral-skin foams for bicycle seats, golf-cart tires, and automotive headrests.
Preliminary rheological tests showed that the viscoelastic behavior of the foams depended on the range of foam density--higher or lower than the minimum density value.
Insulated concrete form (ICF) construction uses foam blocks to create a form into which concrete is poured.
His ultimate goal--a lightweight, environmentally friendly plastic foam that may one day replace solid plastics in some applications,
Investigating how the foam becomes a piezoelectric impersonator, Bauer and his colleagues showed a year ago that air in a pore breaks down into electrons and positively charged ions that cling to opposite walls.
But the negative publicity that foam received from environmental groups ultimately led to a lot less of the material being used and generated as a recyclable at cafeterias.
The testing, which ends today, was watched by representatives of Foam Pro, which developed the proportioning system for mixing air, water and concentrated liquid detergent into foam; Astaris, which makes the foam concentrate; and Waterous/Pneumax, which makes the air compressing equipment used in Foam Pro's new system.
Foam Fix is a new family of PVC foam products which constitute a layer of PVC foam cast onto a polyester liner which is coated with an aggressive high temperature stable adhesive; furthermore, an additional optional layer of adhesive can be purchased to offer a double sided adhesive PVC foam product (Foam Fix II).
Delta-Therm nonrated and Class I rigid foam systems and Flexipol pour and spray foam systems are for applications such as reach-in coolers and freezers, display cases, flotation, ice machines, construction panels, refrigerated trucks, and picnic coolers.
Automation, improved compaction and better raw materials make it easier to consistently produce high-quality castings in lost foam.