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A dispersion is a system in which particles are dispersed in a continuous phase of a different composition (or state). See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not precipitation occurs, and the presence of Brownian motion.
There are three main types of dispersions:
Types of dispersions 
|Dissolved or dispersed phase||Continuous medium||Solution: Homogeneous mixture: Dissolved phase < 1 nanometer||Colloid: Dispersed phase between 1 nanometer and 1 micrometer||Coarse dispersion (Suspension): Heterogeneous mixture: Dispersed phase > 1 micrometer|
|Gas||Gas||Gas mixture: air (oxygen and other gases in nitrogen)||None||None|
|Liquid||Gas||None||Aerosol: fog, mist, vapor, hair sprays||Aerosol|
|Solid||Gas||None||Solid aerosol: smoke, cloud, air particulates||Solid aerosol: dust|
|Gas||Liquid||Solution: oxygen in water||Foam: whipped cream, shaving cream||Foam|
|Liquid||Liquid||Solution: alcoholic beverages||Emulsion: miniemulsion, microemulsion||Emulsion: milk, mayonnaise, hand cream|
|Solid||Liquid||Solution: sugar in water||Sol: pigmented ink, blood||Suspension: mud (soil, clay or silt particles are suspended in water), chalk powder suspended in water|
|Gas||Solid||Solution: hydrogen in metals||Solid foam: aerogel, styrofoam, pumice||Foam: dry sponge|
|Liquid||Solid||Solution: amalgam (mercury in gold), hexane in paraffin wax||Gel: agar, gelatin, silicagel, opal||Wet sponge|
|Solid||Solid||Solution: alloys, plasticizers in plastics||Solid sol: cranberry glass||Gravel, granite|
Structure and Properties of Dispersions 
It is still common belief, that dispersions basically do not display any structure, i.e., the particles (or in case of emulsions: droplets) dispersed in the liquid or solid matrix (the "dispersion medium") are assumed to be statistically distributed. Therefore, for dispersions usually percolation theory is assumed to appropriately describe their properties.
However, percolation theory can only be applied if the system it should describe is in or close to thermodynamic equilibrium. There are only very few studies about the structure of dispersions (emulsions), although they are plentiful in type and in use all over the world in innumerable applications (see below).
In the following, only such dispersions will be discussed with a dispersed phase diameter of less than 1 µm. To understand the formation and properties of such dispersions (incl emulsions), it must be considered, that the dispersed phase exhibits a "surface", which is covered ("wet") by a different "surface" which hence are forming an interface (chemistry). Both surfaces have to be created (which requires a huge amount of energy), and the interfacial tension (difference of surface tension) is not compensating the energy input, if at all.
A review article in  introduces into various attempts to describe dispersions / emulsions. Dispersion is a process by which (in the case of solids becoming dispersed in a liquid) agglomerated particles are separated from each other and a new interface, between an inner surface of the liquid dispersion medium and the surface of the particles to be dispersed, is generated. Dispersion is a much more complicated (and less well understood) process than most people believe.
The above cited review article also displays experimental evidence for that dispersions have a structure very much different from any kind of statistical distribution (which would be characteristics for a system in thermodynamic equilibrium, but in contrast very much showing structures similar to self-organisation which can be described by non-equilibrium thermodynamics. This is the reason why some liquid dispersions turn to become gels or even solid at a concentration of a dispersed phase above a certain critical concentration (which is dependant on particle size and interfacial tension). Also the sudden appearance of conductivity in a system of a dispersed conductive phase in an insulating matrix has been explained. The above cited review article also introduces into some first complete non-equilibrium thermodynamics theory of dispersions (http://www2.organic-nanometal.de/Research/wisslit/nonequ2.html).
- ^ Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H.S., Ed.; Academic Press: New York, NY, USA, 2000; Volume 5, pp. 501-575
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