Spontaneous Pattern Formation Induced by Bénard–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., Bénard–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 Bénard–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 °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 °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.