In the Case of Droplets, Opposites Repel

Ever since American polymath Benjamin Franklin laid out the terminology in 1748, and French physicist Charles-Augustin de Coulomb developed the law, it's been axiomatic: Objects with opposite electric charges--positive and negative--attract each other. But a new observation puts a twist on this concept. Oppositely charged droplets of liquid mutually attract, yet those with a whole lot of charge bounce off each other. The findings could cause a rethinking of some important industrial processes, such as the electrostatic separation of water from crude oil.

The odd phenomenon was discovered accidentally. In 2005, chemical engineer William Ristenpart of the University of California, Davis, was studying the effects of electric charge on droplets of water suspended in oil. When two oppositely charged droplets approach one another, they attract. Because the droplets are pliable, the pulling deforms them, resulting in each forming a shape called a Taylor cone on its surface. Usually the cones touch to form a bridge between the droplets, which then merge. But when Ristenpart inadvertently cranked up the charge too high, the droplets not only didn't merge, they bounced off each other. "I thought it was fascinating and very confusing," he says.

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Two droplets bounce back and forth in a high-strength electrical field.
Credit: W. D. Ristenpart et al., 2009

Ristenpart and colleagues Andrew Belmonte, Jacy Bird, and Howard Stone spent 3 years in Stone's lab, then at Harvard University, investigating the mystery. The solution, it turns out, involves the precise shape of the Taylor cones. Using high-speed video, Ristenpart's team discovered that if the droplets are carrying low or moderate charges produced by an electric field of certain strength, then the cones are relatively short and wide, with a large angle at their tops. However, if the droplets are highly charged, then they pull on each other so strongly that the cones become tall and skinny, with a small angle at their tips.

That difference is key, because when two droplets touch, the external electric field becomes negligible at the point of contact. That means that whether the drops merge depends entirely on the shape of the tiny bridge of liquid between them. If the bridge is made of short, wide cones, then the surface tension of the liquid tends to pull the drops together to make one big droplet. But if the bridge consists of two narrow cones, then the surface tension pulls the liquid back toward the individual droplets and causes the bridge to break. With no electric force holding the droplets together, they rebound from each other, the team reports tomorrow in Nature. In fact, the researchers found that there is a "critical angle"--and hence "critical charge"--above which droplets refuse to merge.

The discovery probably explains why the petroleum industry has not been able to achieve greater efficiencies in the electrostatic removal of water from crude oil, Ristenpart says, a process that has been used for nearly a century, though he adds that the process is extremely difficult to observe because of the opaqueness of the oil.

The results are "stunning," says physicist Frieder Mugele of the University of Twente in Enschede, Netherlands. The findings may have implications in a variety of applications, he says, such as painting, producing synthetic fibers, and performing mass spectrometry, to name a few that depend on the precise control of tiny droplets of liquid augmented by electric fields. It's also possible, he says, that this effect plays a role in the formation of rain clouds, in terms of how well their constituent water-vapor droplets cling together, though in that case "the potential impact of the [research] is more difficult to judge."

Posted in Chemistry, Technology