The Science of Washing Up - Foam Explored

The physics underlying common everyday foams is poorly understood. An experiment scheduled to fly on the International Space Station will help fill in the gaps.

Put some dish-washing soap in the kitchen sink and fill it with water. You'll create a truly bizarre substance.

Despite being made almost completely of air, the sudsy foam in that sink somehow behaves like a springy solid. Strange.

Douglas Durian, a professor of physics at UCLA suggests the following: "Take some shaving cream and put it in your hand. Touch it. Run your fingers through it. Ask yourself, is it a solid, a liquid, or a gas?"

Ordinary aqueous foams, like shaving cream or the suds in a dishwasher's sink, are mostly gas (95%) and a little bit of liquid (5%). The gas subdivides the liquid into a matrix of tiny bubbles. Good foams usually contain complex molecules that toughen the walls of the bubbles. Milk fat, for instance, serves this purpose in whipped cream. The way the bubbles stick together or slip past one another determines how the foam behaves.

Many of us are so accustomed to foams that we hardly notice how odd they are. Foams are on our legs or faces when we shave, on our dishes as we wash them, atop our glasses of beer. "Yet the physics of foam is poorly understood," says Durian.

Courtesy Junction of Occupational Therapy Function A little soap and water is all it takes to make your own science experiment.

Much of what is known comes from trial and error. No theory currently exists for predicting exactly how stiff or oozy a foam will be based on its traits like the size of its bubbles or the amount of liquid it contains. And the precise stiffness of a foam is crucial for many uses. Just imagine: a fire-retardant foam that must flow quickly through the valve of the extinguisher and then cling tightly where it lands; or a counter-biological weapons agent that expands to fill cracks and crevasses and kills microbes hiding there.

Durian would like to take the guesswork out of foams by learning more about their fundamental physics. That's the goal of an experiment he and colleagues are designing for the International Space Station (ISS). It's called FOAM, short for Foam Optics and Mechanics.