Langmuir 2008, 24, 10421-10426
10421
Superhydrophobic and Low Light Reflectivity Silicon Surfaces Fabricated by Hierarchical Etching Yonghao Xiu,†,‡ Shu Zhang,‡ Vijay Yelundur,§ Ajeet Rohatgi,§ Dennis W. Hess,*,† and C. P. Wong*,‡ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe, Atlanta, Georgia 30332-0100, School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst DriVe, Atlanta, Georgia 30332-0245, and School of Electrical and Computing Engineering, 777 Atlantic DriVe NW, Georgia Institute of Technology, Atlanta, Georgia 30332-0250 ReceiVed April 16, 2008. ReVised Manuscript ReceiVed July 5, 2008 Silicon is employed in a variety of electronic and optical devices such as integrated circuits, photovoltaics, sensors, and detectors. In this paper, Au-assisted etching of silicon has been used to prepare superhydrophobic surfaces that may add unique properties to such devices. Surfaces were characterized by contact angle and contact angle hysteresis. Superhydrophobic surfaces with reduced hysteresis were prepared by Au-assisted etching of pyramid-structured silicon surfaces to generate hierarchical surfaces. Consideration of the Laplace pressure on hydrophobized hierarchical surfaces gives insight into the manner by which contact is established at the liquid/composite surface interface. Light reflectivity from the etched surfaces was also investigated to assess application of these structures to photovoltaic devices.
Introduction Artificial superhydrophobic surfaces have attracted considerable attention, since they allow the design of self-cleaning and water-repellent surfaces with high water contact angles and low contact angle hysteresis. Requirements for realization of superhydrophobicity are often viewed in two different ways. One view suggests that achievement of a high water droplet contact angle may be insufficient to establish superhydrophobicity.1 That is, in self-cleaning applications, the ease with which a droplet rolls off the surface is critical. Therefore, contact angle hysteresis/ roll off angle/sliding angle must be considered when evaluating superhydrophobicity. Alternatively, it is sometimes suggested that superhydrophobicity implies only a high water droplet contact angle.2 Although a contact angle g150° is the accepted demarcation for considering a surface to be superhydrophobic, a wide range of contact angle hysteresis values have been reported for such surfaces. Generally, superhydrophobic surfaces with contact angle hysteresis >10° are termed “sticky” superhydrophobic and are considered most likely to be in the Wenzel wetting regime, while superhydrophobic surfaces with contact angle hysteresis
