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					                                                                                          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                             mzou@uark.edu
                                                                                                                                   • Film thickness increases with
                                                                                                                                                                                  surfaces.                                                                          Robert “Drew” Fleming                   dxf04@uark.edu
                                                                                                                                     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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posted:7/31/2012
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