Water jets may break up into droplets thanks to jiggling molecules

Water drops plinking at the bottom of a sink may by spawned by the jiggling of individual molecules.

A stream of liquid, like one from a trickling faucet, separates into droplets as it falls. This phenomenon, found in garden hoses, teapots and water fountains alike, can originate in the vibrations of the liquid’s molecules, physicists Daniel Bonn and colleagues argue in a paper in press in Physical Review Letters.

The researchers wanted to understand these liquid jets on a deep level. “We are the jet whisperers,” says Bonn of the University of Amsterdam. “One of the things that a jet does is it breaks up into droplets and we wanted to know when, how and why.” The breakup into droplets, Bonn notes, is important for a variety of applications, including ink-jet printing and the mist created by asthma inhalers.

A concept known as the Rayleigh-Plateau instability is behind the breakup process. It causes small bumps and wiggles on a jet of water to grow until the stream fragments, due to surface tension. But the origin of the original wiggles was unknown. It’s sometimes ascribed to imperfections in the nozzle or external jostling. “We typically wave our hands around and we say, ‘there’s some noise coming from somewhere; someone sneezed in the office next door,’” says applied mathematician James Sprittles of the University of Warwick in Coventry, England, who was not involved with the research.

To tease out the source of the jet breakup, Bonn and colleagues performed experiments with jets. Lots of them. One hundred and fifty-eight, if you insist upon counting. “One of the most impressive things is the range of experiments they do,” says Sprittles. The team used dozens of different types of nozzles — rough and smooth, ranging in size from microns to millimeters, and a variety of fluids with different viscosities, densities and surface tensions. They also tried isolating the equipment from external jostling. They found that the nozzle quality had no effect on the distance over which the jet broke up. All the data could be explained if the initial disturbances included thermal fluctuations — the random jiggling of molecules due to heat. And extrapolating from that data put the size of the original disturbance to the surface of the jet at about a tenth of a nanometer. That matches the typical size of thermal wiggles of water molecules.

“I was very impressed by this work,” says physicist Jens Eggers of the University of Bristol in England, who was not involved with the research. “It looks at the problem from so many angles and sees whether all of these inputs really give a consistent picture.”

Given that the effect is a result of heat, one might expect that varying the temperature of the liquid should have a big impact on the breakup. But in fact, increasing the temperature enough to significantly alter the expected result would cause water to boil. Tests at such high temperatures would require exotic liquids such as molten metals, which themselves have properties that are not well understood, making conclusions difficult to draw. So instead of fiddling with temperature, the researchers altered other properties, for example using liquids with different surface tensions to make the thermal fluctuations more or less prominent.

The amplification of tiny molecular movements to something that’s evident on the scale of a kitchen sink is surprising, Eggers says. “It’s not so often that you have a direct link between the microscopic world and something macroscopic.”