Dewetting on the Surface of Rutile
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Dewetting on the Surface of Rutile Shelley R. Gilliss, N. Ravishankar§ and C. Barry Carter Dept. of Chemical Engineering and Materials Science, University of Minnesota 421 Washington Ave. S.E., Minneapolis MN 55455 USA § Indian Institute of Science, Bangalore, India ABSTRACT After annealing a continuous SiO2 film on the (001) surface of TiO2, the film dewets and then spreads to form a complex pattern. The final droplet morphology displays a densely branching morphology similar to those seen in computer-simulated models. It is proposed that Bénard–Marangoni convection cells form within the film before dewetting occurs. The formation of Bénard–Marangoni convection cells prior to dewetting results in the uniform size and spacing of the droplets on the surface. These convection cells form at temperature when the TiO2 substrate dissolves into the SiO2 thin film. The change in composition results in regions of differing surface tensions and therefore leads to the formation of the convection cells. INTRODUCTION The rutile (TiO2) surface plays a critical role in processes such as gas sensing and catalysis [1-3]. The behavior of Ti in silicate melts has been widely investigated due to its importance in the glass industry and igneous petrology. TiO2–SiO2 glasses are particularly interesting because very low amounts of titanium in silicates can significantly affect physical and chemical properties of glasses and melts including thermal expansion, tensile strength, chemical durability, softening temperature, color, compressibility, elastic constants, sound velocity, viscosity, nucleation rate, density, partial molar volume, glass transition temperature, liquidus phase relations, and liquid immiscibility [4-9]. The SiO2– TiO2 glassy system is also used as anti-reflection coatings and refractive index controlled planar waveguides [10-12]. In the present study, thin-films of amorphous SiO2 have been deposited onto single-crystal (001) rutile (tetragonal polymorph of TiO2, space group P42/mnm) and heat treated at high temperature to investigate the wetting behavior of silica glass on rutile. At high temperature, amounts of TiO2 dissolve into the glass film and precipitate out during cooling. It is proposed that this change in composition of the SiO2 film produces Marangoni–Bénard instabilities in the film and result in a complex pattern forming on the surface. It is proposed that the glass film undergoes three distinct stages of wetting during the heat treatment of the specimens. First, the Bénard convection cells form within the film. Second, the film dewets. Due to the periodic fluctuations in the film due to the presence of Bénard cells, the film dewets into droplets of uniform size and spacing. Finally, the in the third stage, the film spreads upon cooling. A Marangoni-type instability at the periphery of the spreading front produces patterns within the droplets. To the author’s knowledge, Bénard cells caused by solutal convection have not been previously reported observed in ceramic systems. Yamaguchi and Kanai [13] re
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