Article pubs.acs.org/JPCC
Underwater Superoleophobic Sapphire (0001) Surfaces Naureen Akhtar,*,† Vårin R. A. Holm,† Peter J. Thomas,‡ Benny Svardal,‡ Simen H. Askeland,† and Bodil Holst† †
Department of Physics and Technology, University of Bergen, P.O. Box 7803, N-5020, Bergen, Norway Christian Michelsen Research AS, P.O. Box 6031, NO-5892, Bergen, Norway
‡
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 hinders the deposition of oil droplets.
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INTRODUCTION Optical surfaces with desirable wettability1,2 are technologically important in oil/water-related applications. When such applications involve harsh conditions such as sea environments, material robustness becomes an additional requirement. Sapphire (alpha alumina, Al2O3), due to its robustness and transparency, is widely utilized as an optical window in technologies applied in the offshore oil and gas industry, such as optical sensors and cameras. Optical technologies can facilitate increased production and cost efficiency, reduce safety risks, and avoid production interruptions by providing online environmental surveillance for remote installations. However, for subsea installations, where optical windows are constantly exposed to hydrocarbons, precipitated salts, and biological specimens, surface fouling could lead to operational difficulties. Maintenance interventions under water are costly and can involve risks related to both human safety and environment. Hence, there is a strong and increasing demand for antifouling surfaces.3,4 Such surfaces will also benefit a number of other technologies such as flow rate and turbidity measurements, forward-looking infrared imaging, and flare gas measurements. Sapphire is one of the hardest materials known in nature. It has a hexagonal closed-pack crystal structure with lattice constants a = 0.476 nm and c = 1.299 nm. The vicinal c-plane sapphire surfaces exhibit step-terrace structure with step height equal to c/6 (0.21 nm).5 Sapphire substrates are usually prepared by cutting the surface at a small angle to a particular plane followed by polishing. The final quality of the surface depends on the surface-processing method. Different techniques are used for polishing of sapphire surface such as ultrafine diamond powder polishing, chemical−mechanical polishing, electrical−chemical polishing, electrical−chemical−mechanical © 2015 American Chemical Society
polishing, abrasive-free polishing, magneto-fluid polishing, ultrasonic polishing, and annealing.6,7 The polishing procedure, however, leaves a damaged surface layer which can only be removed through surface treatment such as annealing. The final surface structures depends on both the surface treatments and crystal miscut angle.8 Since the wettability of a surface is governed by the surface-free energy as well as the surface structures,9 it is plausible that the wetting behavior of sapphire surfaces can vary depending on the surface preparation method. However, up until now this has not been investigated in the literature. Here, we present a study of the underwater−oil wettability of sapphire (0001) surfaces with different surface qualities and crystal miscut. Surface features are investigated by AFM, and the onset of surface wetting by oils (underwater) is studied through contact angle measurements. Furthermore, a real-time contamination monitoring setup is used to study the long-term (>12 h) contamination performance of sapphire surfaces.
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EXPERIMENTAL SECTION Materials. Specification of sapphire (0001) windows used in this study are given in Table 1. The windows are labeled as cp, cp0, and epi-polished. For the measurements, two different crude oils from the Norwegian sector labeled crude oil A and crude oil B were used in addition to hexadecane (Sigma-Aldrich). See Table 2 for a detailed description of the crude oil properties.
Received: April 19, 2015 Revised: May 31, 2015 Published: June 1, 2015 15333
DOI: 10.1021/acs.jpcc.5b03741 J. Phys. Chem. C 2015, 119, 15333−15338
Article
The Journal of Physical Chemistry C Table 1. Specification of the Sapphire (0001) Windows Used in the Experiments Presented Here cp
cp0
epi-polished
supplier
Freudiger
Freudiger
crystal growth method diameter (mm) thickness (mm) crystal miscut surface finish
Verneuil method
Verneuil method
12.67−12.73 with bevel edge of 45°, 0.2 mm 1.55−1.60
12.67−12.73 with bevel edge of 45°, 0.2 mm 1.55−1.60
MTI corporation Czochralski (CZ) method 12.7 ± 0.1 1.57 ± 0.05
30° random orientation
0 ± 0.5°
0 ± 0.3°
chemically polished (both sides), 40/20a
chemically polished (both sides), 40/20a
epi-polished (one side)b
a
Scratch/Dig number. bOne side is epi-polished, and the other side is rough with Ra ≈ 2 μm. Figure 1. Diagram of the underwater−oil contact angle measurement setup. Camera and illumination were placed on left and right side, respectively (not shown here).
Table 2. Properties of Crude Oils Used for Contact Angle Measurements platform/field API S.G sulfur pour point TAN nickel vanadium visc.(20 °C)
crude oil A
crude oil B
Troll B 35.9° 0.8452 0.14 mass % −15 °C 0.44 mg KOH/g 0.4 wppm 0.4 wppm 5.7 cSt
Oseberg 38.5° 0.8325 0.24 mass % −18 °C 0.26 mg KOH/g 1.3 wppm 1.7 wppm 4.9 cSt
Atomic Force Microscopy. AFM images of the sapphire surfaces were obtained using a Scientec 5100 equipped with Si n-type cantilever with a tip radius
