Huwerl Thornton, Jr.
An emulsion is a special kind of suspension, one in which the dispersed substance is a liquid that can't mix evenly with the containing liquid.
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In lay terms, this means mixing two or more liquids that under normal circumstances are unblendable. They are liquids that don't like to be with one another as well as dissolve in each other. If two liquids are being emulsified, one could be called the container and the other the contained.
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The liquid that is being contained or dispersed, is broken up into smaller droplets. The smaller droplets are then contained within and surrounded by the other liquid. The dispersed liquid takes the form of tiny droplets that are between a ten-thousandth and a tenth of a millimeter across.
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The droplets are large enough to prevent light from traveling through the surrounding liquid. The light is either deflected or absorbed, thus giving emulsions a milky appearance.
If two liquids like oil and water are mixed together, after a period of time they will separate into two distinct layers. It takes a lot of effort to make an emulsion. The two different liquids will spontaneously arrange themselves in a way that minimizes their contact with each other. This tendency of liquids to minimize their surface area is an expression of the force called surface tension.
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It takes a lot of energy through shaking, stirring or homogenizing (a process that reduces another liquid into extremely tiny droplets) to overcome the surface tension. This generally produces an unstable emulsion. Over time the two liquids will separate. To create an emulsion that lasts over an extended period of time a surfactant or emulsifier is needed. A surfactant or emulsifier both work to do the same thing. They lower the surface tension of one liquid so that it is easier to mix the two liquids. The surfactant/emulsifier works as an intermediary between the two incompatible liquids. Thus the surfactant/emulsifier is able to partly dissolve in each of the two competing liquids. The surfactant/emulsifier is partially hydrophobic, it has the fear of water, and partially hydrophilic, it does not have the fear of water.
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Examples of emulsions are the photo-sensitive side of photographic film, magma, and cutting fluid for metal working.
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There are many examples of emulsions in food. Vinaigrette salad dressing, mayonnaise, and hollandaise sauce are some examples of emulsions. Emulsions that occur naturally in nature are milk, cream, and egg yolks.