Silicon meets the butterfly wing

Also known as the blue mountain swallowtail, the butterfly (shown in the inset) has multilayered structures in its wing that create multiple traps for both air and light. Air trapped on a rough surface is known to significantly reduce the frictional drag on a flowing liquid—a property known as superhydrophobicity (see Physics Today, October 2009, page 16). But artificial superhydrophobic surfaces are usually unstable to capillary flows and other external perturbations and easily lose their trapped air. A multinational group led by Sang-Ho Yun (Royal Institute of Technology, Kista, Sweden) has now used standard microfabrication techniques to drill micron-deep pores in a silicon wafer and build nanocones on the pores' edges as shown in the image. The production process took 10 minutes for a 10-cm wafer. Together with etched grooves and bumps on the walls, the arrays of cones and pores form a hierarchy of air traps that proved to be stable for a full year. Water drops bounce off the surface, and a fine mist bunches up into spherical drops that roll off. In addition, light at wavelengths longer than 750 nm is nearly perfectly absorbed. Together, those properties make the textured surface promising for integrated electro-optical devices, such as solar cells, IR-imaging detectors, cell culture devices, and chemical sensors. (S.–H. Yun et al. Appl. Phys. Lett., in press.)—Stephen G. Benka