Published: February 28, 2011 r2011 American Chemical Society 4594 dx.doi.org/10.1021/la104067c | Langmuir 2011, 27, 4594–4602 ARTICLE pubs.acs.org/Langmuir Highly Transparent Superhydrophobic Surfaces from the Coassembly of Nanoparticles (e100 nm) Raghuraman G. Karunakaran, Cheng-Hsin Lu, Zanhe Zhang, and Shu Yang* Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States b S Supporting Information ’ INTRODUCTION A surface that exhibits a water contact angle of 150° or greater with very little flow resistance, such as observed for lotus leaves 1 and the legs of water striders, 2 is considered to be superhydrophobic. Such surfaces are self-cleaned by the enrolling water droplet and therefore are of interest in numerous applications, including self- cleaning coatings, impermeable textiles, microfluidics, and lab-on-a- chip devices. Generally, the wetting behavior is dependent on both the surface chemistry (i.e., surface energy) and surface topography (i.e., physical roughness). Surfaces with the same solid-liquid interfacial area but smaller feature sizes have smaller effective contact areas and less-stable three-phase contact lines and thus higher water droplet mobility. 3 Roughness on two or more length scales has been suggested to be responsible for the observed superhydrophobicity observed on aquatic plant leaves and insect legs and wings. 1,2,4-6 For example, the lotus leaf surface is com- posed of 3D epicuticular wax crystals that have micrometer-sized papillose epidermal cells with sub-micrometer-sized randomly oriented tubules on them. 1 Recent studies suggest that nanoscale roughness has the added benefit of enhancing the roughness, especially when the feature size of the microstructures is large and the asperity is small. 7,8 Fascinated by discoveries pertaining to natural surfaces, many researchers have attempted to fabricate multiscale roughness in synthetic materials together with chemical functionalization to achieve superhydrophobicity. 3,9-20 Sub-micrometer-sized colloidal particles 12,19 and microstruc- tured pillar arrays fabricated by photolithography 3,21 and soft lithography techniques 22,23 are often used to provide a prede- fined microroughness. Other techniques, such as chemical vapor depositio, 24 and electrochemical deposition, 25 have also been employed to generate random microroughness. Nanoparticles (NPs), however, are commonly used to control the surface nanoroughness 12,19,26-28 because they are readily available com- mercially and can be synthesized by St€ ober chemistry 29 with uniform size and tunable surface chemistry. In addition, their deposition on a microstructured surface by spin coating and dip coating is straightforward and inexpensive. Despite these ad- vances, how small the nanoscale roughness should be to achieve superhydrophobicity remains unclear. Meanwhile, little attention has been paid to the optical transparency of the obtained rough surfaces until recent interest in applications such as solar cell panels and window treatments. Surface roughness and transparency are generally competitive properties. The blurring of the surface due to roughness is mainly caused by a Mie scattering effect. Rayleigh scattering is applied when the radius of the scattering particle is much smaller than the wavelength of the incident light. The optical quality of the film increases when the roughness dimension is much less than the wavelength of light. The intensity of the Rayleigh scattering radiation increases rapidly as the ratio of particle size to Received: October 9, 2010 Revised: February 7, 2011 ABSTRACT: We report a simple and versatile approach to creating a highly transparent superhydrophobic surface with dual-scale roughness on the nanoscale. 3-Aminopropyltri- methoxysilane (APTS)-functionalized silica nanoparticles of two different sizes (100 and 20 nm) were sequentially dip coated onto different substrates, followed by thermal annealing. After hydrophobilization of the nanoparticle film with (heptadecafluoro-1,1,2,2-tetrahydrode- cyl)trichlorosilane for 30 min or longer, the surface became superhydrophobic with an advancing water contact angle of greater than 160° and a water droplet (10 μL) roll-off angle of less than 5°. The order of nanoparticles dip coated onto the silicon wafer (i.e., 100 nm first and 20 nm second or vice versa) did not seem to have a significant effect on the resulting apparent water contact angle. In contrast, when the substrate was dip coated with monoscale nanoparticles (20, 50, and 100 nm), a highly hydrophobic surface (with an advancing water contact angle of up to 143°) was obtained, and the degree of hydrophobicity was found to be dependent on the particle size and concentration of the dip-coating solution. UV-vis spectra showed nearly 100% transmission in the visible region from the glass coated with dual-scale nanoparticles, similar to the bare one. The coating strategy was versatile, and superhydrophobicity was obtained on various substrates, including Si, glass, epoxy resin, and fabrics. Thermal annealing enhanced the stability of the nanoparticle coating, and superhydrophobicity was maintained against prolonged exposure to UV light under ambient conditions.