The two keys to effective protein skimming (or foam fractionating) are air bubbles and surfactants. DOC are surfactants--compounds whose surface is defined as "active." This means that when a surfactant compound is in water, its non-polar end, labeled hydrophobic or "water hating," seeks the surface, the air.
Normally, the only "air surface" in the aquarium is the surface of the water. However, if bubbles are added to the water, more air surface is created. More air surface means more surfactants (DOC) are attracted and removed.
Smaller bubbles have more surface area than larger ones. Also, the longer the bubble stays in the water, the longer its contact time with the surfactant. Bubble size and contact time determine how effective and how fast a skimmer will work.
Protein skimmers take advantage of these physical properties by producing a large amount of bubbles in a controlled space--the contact column. This serves to concentrate the bubbles and the DOC. As the bubbles in the column rise, the surfactants (DOC) attach to the surface of the bubble so that its hydrophobic end is "inside " and in contact with the air. The bubble carries the DOC to the water surface, where it bursts to form foam. The air-preferring DOC stay at the surface rather than redissolve into the aquarium water.
This process is repeated thousands of times a minute and a large amount of foam can be generated. The foam grows over time, is collected in a cup or other such vessel, and is removed by the aquarist at regular intervals. While there is considerably more physics involved in trying to produce the right bubble size and in the determination of other factors as well, this is the basic operative mechanism involved in protein skimming. The goal of any good skimmer? To produce a great number of small bubbles in the contact column.
foam fractionation will in fact work in fresh water but not very well. It is much more difficult for a skimmer to generate small bubbles in freshwater due to the lack of surface tension and resulting bubble tenacity and it cannot do it as quickly or efficiently.
