Phenomenon of superhydrophobicity refers to the existence of very high water contact angles on solid surfaces (contact angle > 150°). This effect, which was originally observed in nature (e.g., on lotus leaves), is important for a wide range of scientific and technological applications, including development of coatings that possess self-cleaning property, reduction of viscous drag of solid surfaces subject to fluid flows, or prevention of surface fouling [1–4]. Furthermore, the ability of superhydrophobic solid surfaces with high water contact angles to support and stabilize smooth, nearly spherical aqueous droplets has led to a number of optical applications in which the surface-supported droplets act as optical resonant cavities . In general, a smooth, homogeneous solid surface can be made hydrophobic by reducing its surface energy using a suitable chemical modification. However, superhydrophobic wetting regime can only be achieved by combining chemical modification of the surface with surface roughness. This idea was independently established by Wenzel  and Cassie and Baxter , and the wetting of rough surfaces has been since widely studied both theoretically and experimentally [4, 8].
Recently, solids with nanometer-scale pores have become popular templates for creating superhydrophobic surfaces because of their inherent surface roughness. There exist multiple techniques for producing nanoporous surfaces such as lithography, particle deposition, template imprinting, or etching [4, 8]. In this letter, we focus on nanoporous alumina-based surfaces with self-organized hexagonal pore structure prepared by electrochemical anodization of Al. With its high nanopore density, low fabrication cost, mechanical strength, and thermal stability , anodic alumina has been one of the most attractive nanoporous substrates used for the synthesis of superhydrophobic surfaces. In addition to its favorable material characteristics, the size and separation distance of the alumina pores can be readily adjusted by changing the electrochemical anodization conditions which allows optimizing the wetting properties of the resulting superhydrophobic surface.
Up to date various hydrophobic and superhydrophobic surfaces have been synthesized using the alumina material system. McCarley et al.  and Javaid et al.  fabricated octadecyltrichlorosilane-modified hydrophobic alumina membranes for gas-separation. Wang et al.  prepared a trichlorooctadecyl-silane-modified alumina with a water contact angle of 157°. Park et al.  fabricated heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane-modified alumina membrane. Castricum et al.  modified alumina by methylchlorosilanes in toluene. Moreover, Kyotani et al. , Atwater et al.  and Yang et al.  obtained hydrophobic alumina membranes by fluorination treatment resulting in water contact angle of about 130°. Zhao et al.  and Kim et al.  fabricated a polyurethane-coated porous alumina template. The water contact angles measured in those studies were 152° and 160°, respectively. Feng et al.  modified alumina by polyethyleneimine and observed an increase of the water contact angle with the increasing immersion time in the boiling water during the surface coating procedure.
As summarized above, wetting properties of porous alumina surfaces have been modified by different chemicals including silanes. Silane molecules react strongly with the free surface hydroxyl groups of alumina, and they are among the most popular surface-modifying agents. Hexamethyldisilazane (HMDS) is a silane whose chemical activity derives from the presence of a highly reactive nitrogen atom within the compound. High silanization power of HMDS on various hydroxyl-bearing surfaces, including alumina has been demonstrated in a number of studies [21–26].
The HMDS modification of a standard alumina surface at 200°C has been investigated by Lindblad and Root . They exposed the alumina samples repeatedly to the HMDS vapor and reported that new Si-OH sites are formed after each reaction treatment which acts as additional reaction sites for further silanization reactions. They also carried out experiments at different reaction temperatures and demonstrated that Si-O-Si and Al-O-Si bridges are formed via release of methyl groups with the increasing temperature . Furthermore, the reaction mechanism of alumina surface with chlorotrimethylsilane was studied by Slavov et al. in the temperature range 80°C to 500°C. They concluded that silanization of alumina is a sequential reaction which produces methane as the only gaseous product . In 1998, the same group investigated the reaction of alumina with HMDS over the temperature range 150°C to 450°C. They proposed that the initial reaction of HMDS with the alumina surface occurs by the dissociative chemisorption of HMDS via reaction of coordinatively unsaturated Al+ and O-sites. Subsequent reaction of pendant -O-SiMe3 and -NH-SiMe3 groups with the surface hydroxyl groups leads to the production of ammonia, methane, hexamethyldisiloxane, and nitrogen as gaseous products .
In this letter, we report on the preparation and characterization of water-repellent surfaces based on HMDS vapor-treated anodic alumina with self-organized hexagonal nanopore structure. We investigate the relationship between the measured water contact angle, surface roughness, and surface chemistry, and determine the optimal silanization conditions that lead to the highest observed water contact angles. Despite the previous reports summarized above that show surface modification using HMDS, there is no account in the literature on the use of HMDS for modification of the wetting properties of nanoporous alumina surfaces. Different silanes such as chlorosilanes and fluorosilanes have been used for this purpose [10–14]. In those cases, however, hydrophobic nanoporous alumina surfaces were prepared by liquid-phase deposition in contrast to the vapor-phase deposition used in our work. Vapor-based treatment has the following important advantages over the liquid-based treatment: (1) It is simpler and shorter as it consists of fewer sample preparation stages. Prior to the liquid phase silanization, unmodified surfaces are cleaned by heating in air, boiled sequentially in hydrogen peroxide and distilled water to hydroxylate the surface, and then dried [10–14]. In contrast, our sample preparation procedure includes only boiling the sample in distilled water and drying. (2) It is performed under more controllable ambient conditions that do not require volatile organic compounds (ethanol, hexane, chloroform, toluene, etc.) for silane solutions which can affect the environment and human health. (3) It is less expensive as it requires smaller amounts of chemicals for a comparable surface coverage.