Indiana University-Purdue University Indianapolis

IUPUI School of Engineering and Technology

Hierarchical micro/nano structures

project 1

project 1 

Many natural and man-made materials exhibit hierarchical structures on more than one length scale. This structural hierarchy can play a major part in determining the bulk material properties. In Dr. Likun Zhu’s group, we are developing advanced micro/nano fabrication techniques to fabricate micro/nano hierarchical structures, understanding the material properties of hierarchical structures, and assessing their applications in biomedical, energy, and environmental fields. For instance, we have developed a low-cost and scalable method of fabricating hierarchical micro/nano structures for superhydrophobic surfaces with extremely low hysteresis. The microstructures were fabricated on a silicon substrate using a customized microfabrication process while silicon nanopillars were created on the surface of microstructures including sidewalls by metal-assisted-chemical etching. Hierarchical surfaces reduce the actual contact area between liquid and solid, thereby increasing the contact angle and reducing the sliding angle. By adding nanostructures on top of a microstructure, a static contact angle of 171° and a sliding angle of 0.5° were demonstrated in this study. The figure shows SEM images of micro/nano-structures fabricated using the developed processes. In each subfigure, the insets on the top right are magnified views of the micropillar before and after the nanopillar deposition. The insets on the bottom right show zoomed in images of silicon nanopillars both on top and the sidewalls of the re-entrant microstructures. After the hierarchical structures were fabricated, a 20 nm thick amorphous fluorocarbon film was coated on the surface to decrease surface energy for improving water repulsion. The video shows that a 20 µL water droplet rolls on the surface of the sample that is tilted at an angle of 1° from the horizontal.