Wettability evaluation of hydrophilic-hydrophobic nanohybrid silica thin films using picoliter water droplets

Water wettabilities of hydrophilic-hydrophobic nanohybrid silica thin films were investigated by contact angles of extremely small size water droplets (8 pL-1 nL) and using an antifog analyzer. The nanohybrid silica was prepared via our unique sol-gel process based on tetramethoxysilane (TMOS) and metyltrimethoxysilane (MTMS) with a hydroxyacetone catalyst, in which hydrophilic portion was generated from hydrolysis of microporous silica prepared from TMOS and hydrophobic portion was directly prepared from MTMS. The sizes of these domains were controlled by the growth time of the microporous precursor polymers in solution. The hydrophilic and hydrophobic domains in the nanohybrid surface were visualized and confirmed by a lateral-mode friction force microscopy using the cantilever modified with self-assembled monolayers of mercaptohexadecanol. Using a small size of water droplets (< 100 pL), the contact angles for the nanohybrid silica films were lower than those for the wholly hydrophilic silica. The small size of the water droplet has a characteristic effect on the lower contact angle on the nanohybrid silica. The contact angles using the small size 70 pL of the water droplet on the nanohybrid silica films at [MTMS]/[TMOS] ratios from 0.25 to 0.75 were lower than those using the conventional size 2 microL. The dependence of the [MTMS]/[TMOS] ratio in the preparation on the antifog parameter was similar to that on the contact angle using the small size 70 pL of the water droplet. The use of the small water droplet for the contact angle measurement was suitable for the evaluation of the antifog films.