Cascades from a dripping faucet.

When water drips from a faucet, it initially bulges into an elongated, hanging drop. As it enlarges and sags, the drop develops a long, narrow neck. Finally, the drop breaks off, leaving behind a quivering chain of smaller, hanging droplets.

From the creation of spray at the seashore to the splitting of atomic nuclei and the dividing of biological cells, the rupture of an object into two or more pieces is a ubiquitous phenomenon. Observing how a liquid drips from a nozzle may provide insights into what happens in more complicated situations when something splits, says physicist Sidney R. Nagel of the University of Chicago.

Using computer simulations and experiments involving high-speed photography Nagel and his Chicago colleagues Michael P. Brenner and Xiangdong Shi studied how the thickness, or viscosity, of a liquid affects the shapes of the drops dripping from a nozzle. By raising the proportion of glycerol mixed with water, they could make their liquids increasingly syrupy The resulting droplet shapes changed dramatically for liquids with higher viscosities.

The researchers observed that near the breakup point, viscous drops -- unlike water droplets -- develop very long, thin necks, which then spawn a series of smaller necks with ever narrower diameters (see photo). Computer simulations indicate that this stretching of a droplet into a cascade of necks can continue indefinitely as long as there is some kind of disturbance present, whether a slight air current, tiny temperature fluctuation, or pressure variation in the nozzle. In the absence of such environmental "noise," only a single, long neck would form. Nagel and his coworkers report their findings in the July 8 Science.

The researchers are now looking into the role played by noise. "We really want to understand the effect of noise on the drop," Nagel says. "How much noise do we need? Can we control the noise and do something to the drop?"