Spontaneous Pattern Formation Induced by Benard-Marangoni Convection for Sol-Gel-Derived Titania Dip-Coating Films: Effect of Co-solvents with a High Surface Tension and Low Volatility

Evaporation-driven surface tension gradient in the liquid layer often causes the convective flow, i.e., Benard-Marangoni convection, resulting in the formation of cell-like patterns on the surface. Here, we prepared sol-gel-derived titania films from Ti(OC3H7i)(4) solutions by dip coating and discussed the effect of the addition of co-solvents with a high surface tension and low volatility on the spontaneous pattern formation induced by Benard-Marangoni convection. Propylene glycol (PG, with a surface tension of 38.6 mN m(-1)) and dipropylene glycol (DPG, with a surface tension of 33.9 mN m(-1)) were added to the coating solutions containing 2-propanol (2-Pr, with a surface tension of 22.9 mN m(-1)) for controlling the evaporation-driven surface tension gradient in the coating layer on a substrate. During dip coating at a substrate withdrawal speed of 50 cm min(-1) in a thermostatic oven at 60 degrees C, linearly arranged cell-like patterns on a micrometer scale were spontaneously formed on the titania gel films, irrespective of the composition of coating solutions. Such surface patterns remained even after the heat treatment at 200 and 600 degrees C, where the densification and crystallization of the titania films progressed. The width and height of the cell-like patterns increased with increasing PG and DPG contents in the coating solutions, where the addition of PG resulted in the formation of cells with a larger height than DPG.