Surface Engineering on Optically Transparent Materials: Extreme Surface Wetting, Anti-Fogging Behavior, and Enhanced Optical Transmittance Robert A. Fleming1,2, Nyre Alston 1,*, Samuel Beckford1,2, and Min Zou1,2 1University of Arkansas, Department of Mechanical Engineering 2Arkansas Institute for Nanoscale Science and Engineering *Saint Louis University, Department of Aerospace and Mechanical Engineering Introduction Surface Wetting Properties Surface Morphology on Glass Enhanced Optical Transmittance • 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 Bare Glass Bare Glass 5% SiO2 on Glass 5% SiO2 on Glass 2.5% SiO2 on Glass corresponding to superhydrophilicity (WCA = 18.4°) (WCA = 11.3°) (WCA = 8.0°) • 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 5% SiO2 on O2 Plasma Treated Low SFE Film on 5% SiO2 on 2.5% SiO2 on Glass 5% SiO2 on O2 Plasma Treated • All surface treatments improve the optical transmittance as 2.5% SiO2 on O2 Plasma Treated Glass properties on optically transparent materials used as a final overlayer Glass Glass Glass compared to the bare materials across the entire visible (WCA = 5.5°) (WCA = 134.4°) in solar cell packages can improve both the transmittance and (WCA = 7.1°) wavelength regime potentially mitigate the effects of environmental factors, such as rain • For glass, 5% concentration on plain glass results in the and fog, on overall cell performance • 2.5% SiO2 concentration best improvement in transmittance at longer wavelengths, produces more continuous films while the 2.5% concentration on plain glass has the best • Better film quality is observed performance at shorter wavelengths Objectives for the 5% concentration after • For PET, the 2.5% SiO2 concentration results in the best • Development of a method of producing superhydrophilic and O2 plasma treatment improvement in optical transmittance superhydrophobic surface coatings on glass and polyethylene Low SFE Film on 2.5% SiO2 on Low SFE Film on 5% SiO2 on O2 Low SFE Film on 2.5% SiO2 on 2.5% SiO2 on O2 Plasma Treated • 20,000× magnification • From optical theory, the transmittance of the nanoparticle Glass Glass Plasma Treated Glass O2 Plasma Treated Glass terephthalate (PET) substrates (WCA = 138.2°) (WCA = 146.6°) (WCA = 154.4°) films is a function of the film thickness; experiments to • Characterize surface wetting properties by measuring WCAs determine the optimum thickness are planned. • Characterize film morphology with SEM and surface profilometry • Characterize the effect of the surface coatings on optical Surface Wetting Stability on Glass transmittance Conclusions • A combination of SiO2 nanoparticle films, O2 plasma treatments, and Materials and Methods a low SFE fluorocarbon film were used to create functional surface • Glass and PET substrates Bare PET O2 Plasma Treated PET 5% SiO2 on O2 Plasma Treated coatings with greatly enhanced surface wetting properties on glass • Cleaned in an ultrasonic bath with acetone and IPA (glass) or only (WCA = 76.2°) (WCA = 46.9°) PET and PET substrates. (WCA = 10.8°) IPA (PET) • The surface coatings also showed enhanced optical transmittance in • Dip-coating in 5%- and 2.5%-by-weight colloidal SiO2 suspensions • Superhydrophilic surfaces are initially achieved using all 4 the visible wavelength regime as compared to their bare counterparts. deposits a nanoparticle film that has superhydrophilic properties nanoparticle treatments • Nanoparticle film thickness, which correlates with optical • Oxygen plasma surface treatments (200 W for 5 min) prior to SiO2 • Only the 5% SiO2 on O2 plasma treated glass samples stayed transmittance, can be modified with O2 plasma treatments and SiO2 film deposition creates surface roughness that acts as nucleation superhydrophilic in excess of 30 days; the 2.5% SiO2 on glass concentration. sites for particle attachment, leading to better film adhesion samples stayed superhydrophilic for more than 20 days • An optimal surface coating – one that combines the largest optical • Note: For PET substrates, O2 plasma treatments are required. • None of the surface conditions yielded a true consistently transmittance enhancement with superhydrophilic/superhydrophobic There is minimal nanoparticle attachment on untreated PET. superhydrophobic surface surface wetting properties – is still under development. 2.5% SiO2 on O2 Plasma Treated Low SFE Film on 5% SiO2 on O2 Low SFE Film on 2.5% SiO2 on • CVD deposition of a several-nanometer-thick low SFE fluorocarbon PET Plasma Treated PET O2 Plasma Treated PET film renders the surfaces either superhydrophobic or very (WCA = 10.4°) (WCA = 137.5°) (WCA = 136.8°) Anti-Fogging Behavior hydrophobic (WCA ~ 140°) For surfaces displaying superhydrophilic behavior, adsorbed water Contact Information Film Thickness Measurements on Glass spreads quickly on the surface, leading to increased evaporation and minimal water retention time on the surface compared to non-treated Dr. Min Zou email@example.com • Film thickness increases with surfaces. Robert “Drew” Fleming firstname.lastname@example.org increasing SiO2 concentration Acknowledgements • O2 plasma treatments create For surfaces displaying superhydrophobic behavior (or even very • NSF EPS-1003970, CMS-0600642, CMS-0645040, DMR-0520550 nucleation sites that increase hydrophobic behavior) water does not readily adsorb, and does not • Arkansas Analytical Lab film adhesion and thus spread at all. Water that does adsorb exists in the form of discrete • Electron Optics Facility increase film thickness droplets with small areas of surface contact. • UA High Density Electronics Center (HiDEC) • Surface profilometer • Arkansas Biosciences Institute Investigations into how these behaviors affect the optical transmittances measurements of wetted surfaces are ongoing.
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