We wash with them. We shave with them. We shape our hair with them. We sit on them. We put fires out with them. We drink from them. We even eat them. What are they? Foams! They are of both scientific, and mythological, interest. According to legend, Aphrodite, the ancient Greek goddess of love, was born from the white foam (“aphros,” in Greek) produced when the severed private parts of the god Uranus were tossed into the sea. Uranus is central to the Greek creation myth, but this god apparently had a real character flaw. He hated his children and hid them from view once they were born. One of them, Cronus, objected to this treatment and sought revenge by castrating his father with a sickle. He then triumphantly flung the severed parts into the ocean! And it was from this bloody foam, as the story goes, that Aphrodite was created. I don’t know about that, but the sea certainly does foam. We’ll get back to that after we find out what foams are all about.
Simply stated, a foam is nothing more than a stable mass of bubbles. The most common variety forms when a gas is dispersed in a liquid. Shaving cream and hair mousse are typical foams. They consist of tiny pockets of gas surrounded by a thin film of water. Critical to the formation of a foam is the ability of water to form a stretchable film. Pure water will not foam because water molecules are attracted to each other very strongly. This attraction is called surface tension, and must be reduced in order to form a foam. Dissolved proteins can do this by getting in between the water molecules. That’s why egg whites, which contain plenty of proteins, can be foamed. Furthermore, as these proteins come into contact with air during whipping, their long molecules unfold and bond to each other, strengthening the bubbles and preventing them from collapsing. That’s the basis of meringue.
You can even make a meringue without an eggbeater. Just dilute some egg white with three times as much water, and add baking soda. Then add some citric acid. Carbon dioxide will form as the bicarbonate reacts with the acid. The gas rises to the surface, but does not break through and escape into the air like it would in a pure liquid because the surface layer of water can stretch to accommodate the gas and form a bubble. As it stretches, the protein molecules coagulate and help form a tough stable film or, in other words, a bubble. As more gas rises to the surface, more bubbles form, and pretty soon we have a foam or, in French, a “mousse.”
One of the great advantages of foams is that they allow for the dispersal of small amounts of chemicals over a large area, as in the case of a hair mousse. When we mousse, we actually coat the hair with a thin layer of plastic. We certainly don’t want a lot of plastic in our hair, just enough to hold it in place. Remember that a mousse is mostly gas, with a little liquid stretched around each bubble. The active ingredient is dissolved in this little water. Basically, a mousse makes a small amount of liquid, and whatever is dissolved in it, go a long way. Shaving cream is the same idea. This time, the substance dissolved in water is not a plastic; it’s soap. But again, a little soap goes a long way. And here’s an interesting way you can make use of this notion. Since shaving cream contains little water, it is ideal for cleaning jobs where water is undesirable, such as on upholstery.
Another practical use of foams is in fire extinguishers. A fire is sustained only when there is ample fuel, oxygen, and heat, and it can be extinguished by removing any one of these components. Water removes heat as it vaporizes. But water can actually spread an oil fire, since oil floats on top of water. It cannot be used on electrical fires, either, because water conducts electricity and can cause electrocution. Even when the use of water is appropriate, it can cause extensive damage. One way to minimize the amount of water is through the use of carbon dioxide foam fire extinguishers. Such foam acts as a wet blanket on a fire, preventing the burning material from contacting oxygen. A carbon dioxide foam is made by combining solutions of sodium bicarbonate and aluminum sulfate, which are stored in separate compartments in the fire extinguisher. When mixed, these chemicals produce carbon dioxide as well as aluminum hydroxide, which is a great foaming agent.
A foam does not necessarily have to contain water; it is just a stable mass of bubbles. A foam rubber pillow, or foamed polyurethane insulation, or a Styrofoam cup all consist of bubbles surrounded by various flexible materials. To make foam rubber, for example, ammonium carbonate can be mixed with the rubber latex. When this is heated, carbon dioxide and ammonia are produced, and become entrapped in the complex rubber structure.
The type of substance added to reduce the surface tension of water in order to make a foam is critical. If it is too effective at reducing surface tension, the bubbles will burst because the attraction between water molecules is too greatly weakened. When this happens, we have an anti-foaming effect. Anti-foaming agents, typically silicones, are often added to oils to prevent bubbling. They are also used to prevent large bubble formation in our digestive tract. Simethicone is a typical anti-flatulent given to gassy people. This ensures that intestinal gases escape in a slow steady stream instead of in large bursts.
Foam is not welcome in our digestive tract, but it is welcome in beer. Indeed, foaming agents are sometimes added on purpose. Propylene glycol alginate is an example. It can be added to light beer, which has fewer carbohydrates capable of stabilizing the foam. And where does one find the alginate used in beer? In seaweed. And now you understand why the sea foams. It has absolutely nothing to do with Uranus’ private parts.