Thickness estimation of the silica-like thin layers via swelling-driven wrinkling instability

Abstract

In the present study, we report a simple, yet facile route to thickness characterization of the UV/ozone (UVO) treated polydimethylsiloxane (PDMS) thin layer via osmotically-driven wrinkling instability. Through the UVO oxidation process of PDMS, a bi-layered film with local moduli-mismatch regions (i.e., the top UVO exposed region of a thin stiff (less-elastomeric) SiOx layer and the bottom region of an unmodified, elastomeric PDMS foundation) was generated. When ethanol directly dropped on top of a bi-layered film and swelled preferentially a lower elastomeric PDMS foundation, the wrinkles were produced in the confined area within the three-phase contact trace of the ethanol droplet, and then vanished completely as the ethanol evaporated irreversibly. The wrinkle wavelength was observed to be magnified as the mixing ratio of base monomer and curing agent increased from 7:1 to 8:1 to 9:1 to 10:1. The increasing wavelength as a function of increasing UVO treatment time (tUVO) reflected increasing thickness of the silica-like layer. For a given ratio, the SiOx thickness was found to increase similarly to the wrinkling wavelength as the tUVO increased. The thickness varied from 22.3 ~ 43.4 nm to 51.1 ~ 99.3 nm as the tUVO varied from 12 min to 60 min. A hydrophobic recovery of a hydrophilic thin SiOx film created by UVO treatment was observed by examining the wrinkle wavelength as a function of elapsed time. As time passed by, a decrease in the wrinkle wavelength confirmed the hydrophobic-to-hydrophilic transition.