If you’ve ever luxuriated in a bubble bath, you might have wondered how the bubbly foam could last so long—after all, when you shake pure water, the air bubbles you create coalesce rapidly. The bubbles in pure water come together because the system is seeking its lowest-energy state; due to the surface tension of the air–water interface, creating bubbles costs energy. What keeps the foam in a bubble bath from breaking up is surfactant (surface-tension reducing) molecules absorbed onto the bubbles’ air–water interfaces. (For details, see the Quick Study by Doug Durian and Srini Raghavan, Physics Today, May 2010, page 62.) It is not just in foams that surfactants are used to stabilize—that is, inhibit coalescence; emulsions such as mayonnaise also include surfactants.

Though not as familiar as the detergents in bubble baths, colloidal particles can likewise act as surfactants and stabilize emulsions and foams. Moreover, as with...

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