Underwater Superoleophobic Sapphire (0001) Surfaces

Abstract

Owing to its excellent mechanical, thermal, and optical properties, sapphire (crystalline Al2O3, alpha alumina) is extensively used to make optical windows for harsh conditions, for example, underwater surveillance in the oil industry. However, under these conditions the sapphire surfaces are continuously exposed to oil and other fouling mixtures which can lead to contamination of the surface. Hence, making the surface underwater oleophobic would be highly desirable. Here we show that the properties of the bare sapphire (0001) surface can vary enormously from underwater oleophilic to underwater superoleophobic just depending on the crystal miscut, polishing method, and initial cleanliness state. We further show that the key factor governing this behavior is the in-air hydrophilicity of the surface. Rendering the surface hydrophilic (in air) significantly improves the underwater oleophobic performance. This effect can be explained as the trapping of water on the surface forming a “protective layer” that hi...