Foam, properties, production and destruction, application
Foam is a disperse system with a gas dispersed phase and liquid or solid dispersion medium.
Foam is one of the varieties of coarsely dispersed systems. Its feature is the composition: as a dispersed phase of bubble of air or vapor, and the disperse medium can be liquid (most often water), and solids.
By its nature, foam is much more sort of concentrated disperse systems. Examining in detail their structure is easy to determine that the important role plays not only the composition of the solution, but the volume ratio of the phase and environment. This factor has a direct impact on the ratio of foam cells and their shape: the cells are in the form of spheres or polyhedra (polyhedral shape). There is also a transitional form – the stage when the spherical cell is transformed into a multi-faceted, this form has the name “honeycomb“, which is given to her in connection with the similarity with honeycombs.
The spherical shape of the cells of the foam occurs in disperse systems, where the difference of concentration between the gas phase and the liquid medium is not more than 10-20 times, i.e., the vapour or gas is not increased, and dissolved in sufficient volume of fluid. A similar composition is reflected in the thickness of the film covering the bubbles – it is quite dense.
It is known that, the smaller the ratio of the volume phase medium, the higher the film thickness, but such density cannot be maintained for an unlimited period of time. So, bubbles are aging and there comes a time when due to the thinning of films covering bubbles form spheres becomes polyhedral. The life expectancy of polyhedral cells is quite large, because in such a situation, the condition of the foam almost in equilibrium, and she is fairly stable.
The characteristics of the foam due to a set of properties that describe their main aspects. These properties, in most cases, are determined by such factors as conditions of receiving pins or their destruction, and can be:
– structural-mechanical (rheological);
Structural and mechanical properties of foam:
The main ones include:
– foam-forming ability of solutions;
– stability (sustainability);
– critical shear stress;
Like any disperse system, the foam has such an important property as aggregate stability. On the contrary, it has an excess surface energy. Such closed systems are in unstable equilibrium, which contributes to the decrease in free energy, its reduction to a critical value and, consequently, the appearance of balance. Foaming in dispersed systems, this is possible only in a situation when all the foam will be completely separated into liquid medium and a gaseous phase.
Foaming properties foam:
This term determine the final amount of foamobtained from a certain pre-defined amount of blowing agent dispersed system in the designated standard foaming conditions for a specified period of time. This files most often measured this parameter as:
– the ratio of volume of foam (cubic centimeters), or height of foam column (in meters) by the time its complete destruction;
– change the height of the column or volume in the time interval, are represented graphically.
There are other types of criteria, but a single universal standard for evaluating all types of foam disperse systems, no.
Foam-forming ability depends on the presence and type of surfactants in the system, their quantitative characteristics. Not the last role played by impurities which may be contained in the liquid dispersion medium – salts, suspended mineral particles of oil. Also takes into account the temperature of the foam and the pressure in it.
Thus, the greatest foaming ability are solutions, including the anion-active substances: the more, the stronger their surface tension, the higher the initial option, and also the dispersion and the stability of the composition. Oil and salt, on the contrary, significantly reduce foaming ability, with the greatest lowering effect of divalent cations.
Different acts on the solutions of clay: when it is added to anion-active substances, foam-forming ability increases to the nonionic decreases. If you increase the temperature of the mixture parameter is also increased, but not the nonionic compounds: after the critical point of 100 degrees the ability of such a disperse system to form foam completely disappears and returns as it cools down.
Depends on foam-forming ability and structural features of the device, whereby is obtained a foam (an aerating device), the mode of its operation – the ratio of air and disperse systems.
Stability (stability) of the foam:
Stability (stability) of the foam is its ability to preserve the received quantity, dispersion and the ability to prevent fluid loss (or syneresis). The most common measurement object acts as a single bubble or a small volume of the resulting foam.
Depends on the setting of several conditions – the presence and type of pressure, the inclusion of surfactants. Thus, the foamobtained from non-ionic solutions, at least as stable as those formed by exposure to natural atmospheric pressure. Obtained under the pressure of the foam, and also the one that has a mixture of anion-active surface-active substances that retain their state for much longer.
The stability of the foam, expressed in the time interval, an important option to consider this coarse system as structured, i.e., having the properties of a solid body. However, it depends on the type and presence of stabilizers.
The ratio of the foam:
The multiplicity of foam is the ratio of volume of foam to the amount of the dispersed solution spent to receive it.
Depending on the foam parts to the solution:
– low magnification – up to 20;
– with an average ratio of from 20 to 200;
– high magnification – 200.
Dispersion of foam:
This setting is characterized by different criteria:
– the average size of the bubbles;
– the distribution of bubble size;
surface of the section, “solutiongas” per unit volume of foam.
The limit tension of the shift:
This parameter is usually expressed through the stiffness of the foam, i.e. its ability to absorb and withstand some mechanical stress that affects their deformation or the change in final volume. Even foams whose liquid film is sufficient, a marked rigidity, because if any, even insignificant external influence, increases the minimum of surface energy isresponsible for the equilibrium solution.
The viscosity of the foam:
This option allows you to calculate the data:
– on the spreadability of the foam mass on the surface;
– the existence of conditions for the flow of foam in pipes;
– the ability dripping of foam out of the holes, etc.
The viscosity is calculated on the basis of multiplicity, the limit tension of the shift and the dispersion of the composition.
The electrical conductivity of the foam:
The electrical conductivity of the foam depends on the volume of liquid dispersed phase and its direct conductivity, as the gas component solution is not capable of conducting electricity.
With the passage of light rays through a layer of spherical foam they are absorbed by the solution and dispersion. As for polyhedral solutions that arises here is mainly diffusion, because the proportion of liquid in such disperse systems is very small.
There are two main types of foams – chemical mechanical and air.
Chemical foam is a solution, where the disperse phase is the mixture of alkali with acid, and the dispersed medium is water. To provide higher durability and stability added to the system stabilizers. The formed bubbles are filled with carbon dioxide.
Air-mechanical foam is a solution of water and foam, the bubbles of which are filled with air. As blowing agents are concentrated solutions of surfactants, which can be:
– fluoro synthetical;
Another known type of foam – solid. Its name is due to the inclusion in the composition of the solid dispersion medium, while the phase remains gaseous substance. This category include pumice (a natural substance), foam concrete, foam glass, various insulating materials, sponge materials (foam, microporous elastomer), etc.
To obtain a stable foam without the use of stabilizers (foam) is impossible, and therefore almost all methods of mixing the dispersion liquids include their use. The process itself is quite simple: when connecting the dispersed phase and the medium first, there are several gas bubbles on the surface which begins the adsorption of the molecules of the stabilizer (usually a surfactant). This allows you to form a top layer of small bubbles. Each subsequent pop-up bubble is pressing on this layer from below, but surface-active substances prevent the rupture of the films already formed bubbles, there is a hemispherical dome with a rapidly-increasing surface. Adsorption continues, the tapes hold the gas or air bubbles inside, and they find themselves surrounded by two layers of foam, between which is a film of fluid.
Due to the stability and film strength provided by surfactants, there is a convergence formed in the upper layer of bubbles. Their form suffers change, turning from spherical to polyhedral, the film thickness is reduced, the liquid film. As a result, adding a single layer of bubbles and the subsequent transformation of the whole liquid volume in the foam.
To foam, like other disperse systems, in two ways:
– dispergation methods of particulate systems;
– condensation methods of true solutions.
Disperazione ways and methods of producing foam:
They are based on crushing bubbles of the gas by feeding it into a solution of the stabilizer. The gas enters into the solutions small amounts usually through a tube placed in liquid. As a result, more small bubbles.
Today, use these methods:
1. the passage of streams of gas through the liquid in:
– aeration plants;
– bubbling plants
– devices with “foam layer”;
– steam generators with the grid;
– liquids, irrigated with a solution of foaming agent;
2. the movement of the moving device for the disperse system or moving the fluid over an obstacle. Most often it is:
– the impact of the agitators, etc.;
3. the ejector (ejection) of air moving stream of liquid in the steam generators.
Dispergation methods – the most popular in the industry, but their efficiency is small, so scientists continue to develop new types of equipment for these purposes.
Condensing methods and methods of producing foam:
This methods such that future bubbles initially presented in the form of individual molecules. Preparation of foam it is possible:
1. by changing the parameters of the physical state of a disperse system:
the rise in its temperature;
– decrease in vapor pressure above the liquid.
Churning in this case, occurs almost instantly;
2. the chemical reactions in which there is an allocation of gas;
3. the use of microbiological processes, the occurrence of which is accompanied by release of gases, most commonly carbon dioxide;
4. the application of electrochemical processes.
These methods have found wide application in the food and construction industries use them in household fire extinguishers.
The destruction of the foam are important process in manufacturing, as well as its creation. This process possible by such methods:
– prevention of foaming;
– the destruction of already formed foam.
Prevention of foaming:
The first method is based on the use of chemicals that prevent the transformation of the gas dispersed phase in the foam. The disadvantage of this method is the fact that most of these chemicals (alcohols, organosilicone compounds) pollute as products production and the environment. It is also possible to mix the two types of surfactants, one of which partially neutralizes the effect of another, reducing the final amount of foam.
The second method is to remove from the finished colloidal dispersion stabilizer, responsible for her stability, which will lead to her self destruction.
The latter method – change the parameters of the production process: pressure, temperature, diameter of vessels, etc.
The destruction of the foam:
For this purpose chemical and non-chemical methods.
Chemical refers to the use of defoamers – special substances capable of absorbing foam that meets the following criteria:
– having the ability to destroy the foam even with a small number and a long period of time to prevent its re-appearance;
– do not react with the substances which formed the starting solution, and therefore does not change its key performance indicators, not to influence the process, do not change the parameters of the equipment;
– do not change your own settings if you change the conditions of storage, regardless of its duration, as well as physical parameters (for example, by heating);
for some industries it is important compliance requirements such as sterility, non-toxicity, etc.
Note that the foam should be selected taking into account the specifics of production, because not all of them effective for different particulate compositions.
Non-chemical methods are divided into physical and mechanical.
To the physical methods include:
– increasing the temperature of the mixture, wherein the liquid from the solution evaporates, and the film bubbles collapses;
the impact of ultrasound – acoustic method possible, which is not always reliable and not suited to rapidly formed compounds, but is considered the most common for small industries; it is also possible to use an audible siren;
– the effect of electric current – at the moment is an experimental method, since it allows almost instantly besiege the foam, but it requires high security measures.
Mechanical methods are divided into:
– centrifugal – when driving foam collides with a stationary surface;
– aerodynamic – the destruction of the foam is emitted under pressure from a stream of gas;
hydrodynamic analogue of the previous one, but with the use of liquid jets;
– barometric pressure – change the pressure in a special apparatus.
Used in manufacturing is not often because they usually require complex, bulky equipment, serious amounts of electricity. Also such methods are almost not effective for highly stable dispersion systems with a low magnification, and all others do not destroy the foam completely, but only reduce its number.
Foam used in virtually all sectors of modern life. First and foremost, it’s the food industry – bakery, confectionery, ice cream and more. Pinowska is another kind of production used to produce products and ingredients for them. Thus, the drying of certain products with pre-foaming allows you to get similar components, but with very fine structure, the so – produced dry mashed potatoes, juice and coffee, fodder yeast etc.
Impossible without this disperse system to provide a modern bathroom: shampoos, shower gels, household chemicals – all this kind of pen.
Builders have long appreciated these lightweight, durable materials, such as foam, Styrofoam, polyurethane foam.
they are in the pharmacy – some drugs are recommended to take directly in the form of a pen.
However, the wide application of foam received in the fire it is this dispersion system allows you to quickly and effectively fight fires with even the highest levels of difficulty.
Note: © Photo ,