Most of my days end with a long soak in a deliciously hot bubble bath. I don’t need fancy scents, instead I enjoy the heat of the water and playful texture of the bubbles. Bubbles rise up as a mound ringing the stream of water from the faucet. Further away, bubbles slide into irregular shapes reminiscent of fictitious moon bases and futuristic homes. If it’s really quiet, the muffled pops as multiple bubbles end their existence is audible. In addition to a relaxing end to a day, my bubble bath is an example of a foam.
Foams form when billions of tiny bubbles are packed together within a solid or a liquid. Irregular sized bubbles are common in all foams except the most idealized ones. Like what happens for individual bubbles (check out my bubble post), it’s surface tension that helps keep a foam stable. Liquid foams break down eventually – there are even chemicals on the market to make this process go faster. Gas can diffuse from small bubbles into large ones and eventually out of the foam, or gravity can drain liquid out the bottom making bubbles so weak they pop on top.
A Belgian physicist, Joseph Plateau (1801-1883), figured out the basis of what we know about soap films and foams. His diverse interests also included spending time with moving image illusions like the action one sees when using a “flip-book”. Back to soap film, Plateau came up with a series of laws to describe stable foam structures (foams that don’t follow these laws tend to rearrange themselves until they do). They are:
1.Soap film surfaces are smooth.
2.The soap film curvature is continuous and constant along the entire a surface.
3.When three or more bubbles connect together, they will shift around until each line only contains three bubble-wall intersections – called a Plateau Border. With matching surface tensions all three angles are 120 degrees, the most efficient option.
4.Only four Plateau Borders can meet at a point.
Foams form in nature. Examples include: sea and river foams, and the foaming at the mouth of a rabid dog. There are fish, such as gourami and Siamese fighting fish, that blow a mass of bubbles coated in saliva to house their eggs. In the same spirit, some species of frogs make foam nests to lay their eggs in. These nests may be constructed in crevices, on the surface of water, or on forest floors.
Beyond bubble baths, soaps can be whipped to form a lather with bubbles so small they hardly can be seen. This idea was extended into modern shaving foams where a compressed gas is rapidly decompressed to expand a cream into a foam. Foams can also be hardened into permanent structures like insulation and flotation devices. Ceramic can be made into foams useful for acoustic insulation, absorption of environmental pollutants, and the filtration of molten metal alloys among other applications. Cement foams are used as a light-weight building material with good insulating capability. Even metals can be manipulated into becoming a foam. Metal foams are used in exhaust mufflers as they a great at dampening noise. They also make efficient materials for heaters and heat exchangers since they have so much surface area.
We eat a lot of foams: they can add a light texture to an angel food cake, or be tasty in the form of whipped cream and meringues. Breads are often a foam as the yeast produces tiny bubbles of gas which causes the dough to rise. One of the best foams is the head that forms when a beer is poured into a glass; this foam is made by which is made by carbon dioxide bubble rising to the surface. For the best head on your glass of beer, chose a wheat beer instead of a barley beer.
Note: there is an optimum foam combination of wheat beer consumed in a bubble bath – use with caution.