1. Surface Engineering on Optically Transparent Materials: Extreme Surface Wetting, Anti-Fogging Behavior, and Enhanced Optical Transmittance Robert A. Fleming 1,2, Nyre Alston 1,*, Samuel Beckford 1,2, and Min Zou 1,2 1 University of Arkansas, Department of Mechanical Engineering 2 Arkansas Institute for Nanoscale Science and Engineering * Saint Louis University, Department of Aerospace and Mechanical Engineering Introduction Surface engineering techniques can be employed to modify the natural wettability of material surfaces Determined by measuring the water contact angle (WCA) Superhydrophilic - WCA < 10°, within 1 s of wetting Superhydrophobic – WCA > 150° WCA is governed by the surface free energy (SFE), with high SFE corresponding to superhydrophilicity The SFE can be changed by modifying 2 surface properties: surface topography and surface chemistry Nanoparticle films present an opportunity to modify surface topography and chemistry simultaneously In addition, surface coatings can reduce the total reflectance of transparent materials Application of nanoparticle films with enhanced surface wetting properties on optically transparent materials used as a final overlayer in solar cell packages can improve both the transmittance and potentially mitigate the effects of environmental factors, such as rain and fog, on overall cell performance Objectives Development of a method of producing superhydrophilic and superhydrophobic surface coatings on glass and polyethylene terephthalate (PET) substrates Characterize surface wetting properties by measuring WCAs Characterize film morphology with SEM and surface profilometry Characterize the effect of the surface coatings on optical transmittance Materials and Methods Glass and PET substrates Cleaned in an ultrasonic bath with acetone and IPA (glass) or only IPA (PET) Dip-coating in 5%- and 2.5%-by-weight colloidal SiO 2 suspensions deposits a nanoparticle film that has superhydrophilic properties Oxygen plasma surface treatments (200 W for 5 min) prior to SiO 2 film deposition creates surface roughness that acts as nucleation sites for particle attachment, leading to better film adhesion Note: For PET substrates, O 2 plasma treatments are required. There is minimal nanoparticle attachment on untreated PET. CVD deposition of a several-nanometer-thick low SFE fluorocarbon film renders the surfaces either superhydrophobic or very hydrophobic (WCA ~ 140°) 5% SiO 2 on O 2 Plasma Treated Glass 2.5% SiO 2 on Glass Surface Wetting Properties Bare Glass (WCA = 18.4°) 5 % SiO 2 on Glass (WCA = 11.3°) 5% SiO 2 on O 2 Plasma Treated Glass (WCA = 5.5°) 2.5% SiO 2 on O 2 Plasma Treated Glass (WCA = 7.1°) Low SFE Film on 5% SiO 2 on Glass (WCA = 134.4°) Low SFE Film on 2.5% SiO 2 on Glass (WCA = 138.2°) Low SFE Film on 5% SiO 2 on O 2 Plasma Treated Glass (WCA = 146.6°) Low SFE Film on 2.5% SiO 2 on O 2 Plasma Treated Glass (WCA = 154.4°) Bare PET (WCA = 76.2°) O 2 Plasma Treated PET (WCA = 46.9°) 5% SiO 2 on O 2 Plasma Treated PET (WCA = 10.8°) 2.5% SiO 2 on O 2 Plasma Treated PET (WCA = 10.4°) Low SFE Film on 5% SiO 2 on O 2 Plasma Treated PET (WCA = 137.5°) Low SFE Film on 2.5% SiO 2 on O 2 Plasma Treated PET (WCA = 136.8°) 2.5% SiO 2 on Glass (WCA = 8.0°) Enhanced Optical Transmittance Film Thickness Measurements on Glass Film thickness increases with increasing SiO 2 concentration O 2 plasma treatments create nucleation sites that increase film adhesion and thus increase film thickness Surface profilometer measurements Surface Morphology on Glass Bare Glass 5% SiO 2 on Glass 2.5% SiO 2 on O 2 Plasma Treated Glass 2.5% SiO 2 concentration produces more continuous films Better film quality is observed for the 5% concentration after O 2 plasma treatment 20,000× magnification Surface Wetting Stability on Glass Superhydrophilic surfaces are initially achieved using all 4 nanoparticle treatments Only the 5% SiO 2 on O 2 plasma treated glass samples stayed superhydrophilic in excess of 30 days; the 2.5% SiO 2 on glass samples stayed superhydrophilic for more than 20 days None of the surface conditions yielded a true consistently superhydrophobic surface Anti-Fogging Behavior All surface treatments improve the optical transmittance as compared to the bare materials across the entire visible wavelength regime For glass, 5% concentration on plain glass results in the best improvement in transmittance at longer wavelengths, while the 2.5% concentration on plain glass has the best performance at shorter wavelengths For PET, the 2.5% SiO 2 concentration results in the best improvement in optical transmittance From optical theory, the transmittance of the nanoparticle films is a function of the film thickness; experiments to determine the optimum thickness are planned. Contact Information Dr. Min Zoumzou@uark.edu Robert “Drew” Flemingdxf04@uark.edu For surfaces displaying superhydrophilic behavior, adsorbed water spreads quickly on the surface, leading to increased evaporation and minimal water retention time on the surface compared to non-treated surfaces. For surfaces displaying superhydrophobic behavior (or even very hydrophobic behavior) water does not readily adsorb, and does not spread at all. Water that does adsorb exists in the form of discrete droplets with small areas of surface contact. Investigations into how these behaviors affect the optical transmittances of wetted surfaces are ongoing. Conclusions A combination of SiO 2 nanoparticle films, O 2 plasma treatments, and a low SFE fluorocarbon film were used to create functional surface coatings with greatly enhanced surface wetting properties on glass and PET substrates. The surface coatings also showed enhanced optical transmittance in the visible wavelength regime as compared to their bare counterparts. Nanoparticle film thickness, which correlates with optical transmittance, can be modified with O 2 plasma treatments and SiO 2 concentration. An optimal surface coating – one that combines the largest optical transmittance enhancement with superhydrophilic/superhydrophobic surface wetting properties – is still under development. NSF EPS-1003970, CMS-0600642, CMS-0645040, DMR-0520550 Arkansas Analytical Lab Electron Optics Facility UA High Density Electronics Center (HiDEC) Arkansas Biosciences Institute Acknowledgements