Tunable surfaces

In a new experiment, a liquid drop was maneuvered around a specially treated silicon wafer patterned with an etched forest of tiny stalks. The blades of this “nanograss” could be selectively electrified. At low applied voltage, the drop was spherical (as in the photo below) and highly mobile, able to be guided by moving the voltage. At higher voltage, the drop was impaled on the posts and immobilized, and at still higher voltage, it completely infiltrated the stalks and wetted the entire substrate. The so-called electrowetting technique has been used since the 1980s to adjust the energy of liquid–solid interfaces, but this experiment, conducted at Lucent Technologies’ Bell Labs, showed remarkable abrupt transitions from superhydrophobic to hydrophilic behavior. At the March meeting of the American Physical Society in Montreal, Tom Krupenkin, who led the group, said that such tunable behavior could have many potential applications. For example, a liquid could absorb heat from the hottest spots on a microchip and then rapidly carry that heat away. Optical properties of a surface could be switched from one state to another through electronically controlled wetting. Microfluidic devices and microbatteries might also make use of the technique. (T. N. Krupenkin et al. , Langmuir, in press.)