| AVS 54th International Symposium | |
| Surface Science | Thursday Sessions |
| Session SS-ThP |
| Session: | Surface Science Poster Session |
| Presenter: | D. Yang, Arizona State University |
| Authors: | D. Yang, Arizona State University P. Aella, Arizona State University A. Garcia, Arizona State University D. Gust, Arizona State University M.A. Hayes, Arizona State University S.T. Picraux, Los Alamos National Laboratory |
| Correspondent: | Click to Email |
We report the tailoring of surface roughness to significantly amplify light-induced contact angle switching and wettability of surfaces functionalized with the photochromic azobenzene molecules. Potential applications of controlled surface wettability for microfluidic systems include delivering analyses in lab-on-a-chip environments, bio-assay in drug discovery, and chemical analyses. Transporting increasingly smaller volumes of water is an important task in miniaturized microfluidic systems. However traditional mechanical approaches do not scale well to smaller volumes due to the increased importance of interfacial forces relative to inertial forces. Thus alternative methods to control liquid interactions with surfaces and to drive droplet motion are needed. We have previously demonstrated large photo-induced switching of contact angles (10-12 degrees) for water and other fluids on azobenzene-functionalized smooth Si surfaces. With these surfaces we find one can reversibly switch the surface wettability and in cases of sufficiently low hysteresis move liquid droplets by light-induced gradients in the surface tension. In this work, we extend these studies to the design of surface morphologies for increasing the effectiveness of photo-induced switching of surface wetting. We have fabricated ordered arrays of micrometer sized pillars (heights and spacing ~ 5 to 50 µm) on Si substrates. We also have prepared novel surfaces using CVD vapor-liquid-solid growth to fabrication surfaces with various lengths of Si nanowires (~50-100 nm in diameter) and combinations of nanowire-micropillar structures to form hierarchical arrays. By combining surface roughness with photochromic azobenzene monolayers, we demonstrate the amplification of the light-induced switching angle by up to a factor of 2 compared to smooth surfaces. Particularly effective amplification is found for hierarchical nanowire-micropillar designed surfaces. We discuss how such amplification enables increased control of surface wettability and droplet manipulation by optical means.