Flame-impingement-induced superhydrophilicity of soda-lime-silica glass surface

The importance of superhydrophilicity of glass surfaces lies in their self-cleaning abilities. The need for antifogging characteristics of soda-lime-silica (SLS-) based window glasses requires feasible solutions. Superhydrophilicity is generally achieved by textured surfaces with suitable features or any chemical modification including thin films. Fabrication of textured surfaces usually involves sophisticated facilities that are often expensive. This paper reveals a novel approach to achieving superhydrophilic SLS surfaces by flame-impingement. The chemical energy of methane gas was converted into thermal energy by a flame torch to reach temperatures just above the softening point of SLS glass. The glass surface was exposed to the flame at a distance of around 100 mm for 10 s. The surface was transformed into a superhydrophilic state with a static contact angle of nearly zero after the treatment. This property was remarkably retained on exposure of the surface to the ambient atmosphere for 3 years of aging. The subsurface structural modifications accountable for the alteration in wetting behavior by the influence of flame-impingement were investigated. High-resolution X-ray photoelectron spectroscopy of the O1s spectral line evidenced the repolymerization of vicinal silanols into bridging oxygens (BOs), accompanied by the loss of hydrous species (SiOH/H2O) in the near-surface region. The repolymerized BOs acted as adsorption sites of water molecules to promote superhydrophilicity. Atomic force microscopy exhibited the conversion of an open silica tetrahedral network with nonbridging oxygens into closed rings. The high surface energy of the residual surface nanostructure at the solid/vapor interface was accountable for the superhydrophilicity.