Physicists have observed a phenomenon predicted by Lord Rayleigh in 1882 for the first time. Thomas Leisner and colleagues at Ilmenau Technical University in Germany used high-speed microscopic imaging to observe the “Rayleigh jets” that are produced when a charged droplet of liquid becomes unstable and explodes (D Duft et al. 2003 Nature 421 128).
Electrified droplets are routinely used in various technological applications, including ink-jet printing and some forms of mass spectrometry. The Ilmenau group became interested in Rayleigh jets as a result of its research on thunderstorm clouds, which also contain highly charged droplets.
Lord Rayleigh showed that, for a given charge, droplets are only stable when their radius exceeds the “Rayleigh limit”. This limit relates the maximum charge that a droplet can bear to its surface tension and radius. Rayleigh suggested that when a charged droplet becomes unstable, it ejects a microscopic jet of liquid from each end before returning to equilibrium.
Prior to the Ilmenau work, researchers believed that droplets managed to get rid of their charge without undergoing a “global” deformation of the whole droplet. Physicists have observed that highly charged droplets, which are slowly evaporating, become unstable at a certain limit and lose a large fraction of their charge — but only a small fraction of their mass. It is not certain, however, that the droplets become unstable at or beyond the Rayleigh limit.
To investigate this problem, the Ilmenau team used electric fields to levitate droplets of ethylene glycol. At the beginning of the experiment, the droplet has a radius of 58 microns. As a result of evaporation the droplet becomes smaller and its radius approaches the Rayleigh limit of stability — about 24 microns. The researchers observed that as the radius decreases, the droplets stretch from their original spherical shape to an ellipsoid shape, as predicted by Rayleigh. When the Rayleigh limit is reached, fine jets containing about one hundred droplets each are ejected from both ends of the ellipsoids.
The jets formed for much larger droplets than expected. “In contrast to Rayleigh’s conjecture that jets occur for droplets charged much higher than the stability limit, we observed jets for droplets exactly at the Rayleigh limit,” Leisner told PhysicsWeb. The team now hopes to explore the phenomenon in more detail by using different liquids and temperatures.
