When Liquids Stay Dry

If you are in the habit of watching your drip coffee maker prepare your morning beverage, you'll occasionally witness something interesting: As the coffee drips into the pot, you'll often see one of the drops land on the surface and remain intact for a second or so before coalescing with the bulk liquid. This temporary noncoalescence of two bodies of the same liquid is neither an isolated phenomenon nor one that has been observed only in the era of drip coffee makers. As early as 1879, Lord Rayleigh examined how water jets bounce over each other, and in 1881 Osborne Reynolds observed what he called floating drops. (See the box on page 40.) A century later, “a scientific curiosity” was how Jearl Walker referred to the phenomenon.