Watching droplets deform in a solid

Fluid droplets form spheres to minimize their surface energy. But when electrified—in a thunderstorm cloud, say, or an inkjet printer—they can become unstable and deform into a cone that emits a stream of smaller droplets into the surrounding gas or liquid. G. I. Taylor explained the phenomenon in 1964, and it has been extensively explored ever since. Now Qiming Wang, Xuanhe Zhao (both at Duke University), and Zhigang Suo (Harvard University) have made the first observations of electrified droplets trapped in a solid. To do so, they sandwiched a millimeter layer of water-laden silicone rubber between two rigid insulating plates and monitored the embedded water droplets’ behavior using optical microscopy. As the voltage across the layer was ramped up to a few kilovolts, the team noticed each droplet elongate along the field direction and, at a critical field strength, undergo abrupt instabilities manifest as the sharp tips shown in the figure. Further increases in field strength compressed the droplet into a long tube that appeared to crack the material. The researchers found that the solid’s elasticity, which resists a droplet’s deformation, competes with electrostatic and surface energies to influence a droplet’s shape. The stiffer the solid, the greater the field required to make a droplet unstable, and the more resistant the solid to dielectric breakdown induced by the instability. The result may help engineers synthesize, for example, more robust dielectric capacitors and power-line insulators. (Q. Wang, Z. Suo, X. Zhao, Nat. Commun. 3, 1157, 2012 .)—R. Mark Wilson