Super-hydrophobic nano-surfaces

Figure 2. Schematic of air trapped between hydrophobic microfeaturesof a super-hydrophobic surface. Reproduced from [8].

Water repellency is important in many industrial and biological processes, including the prevention of the adhesion of snow to antennas and windows, self-cleaning traffic indicators, metal refinement, stain-resistant textiles, bioanalysisand cell motility. An application with major economic and environmental impact is drag reduction on marine vessels. According to a recent study for the International Maritime Organization [1], international shipping was responsible for annual emissions of around 843 million tonnes of CO2 in 2007, or around 3% of total man-made carbon emissions.

The main contributor to ship drag at normal operational speeds is, as on an aircraft at cruise, turbulent skin friction. A number of methods are being considered to reduce this (e.g., riblets, micro-bubbleinjection, polymer injection), but all have one thing in common: they aim to reduce the shear stress occurring at a non-slip water/surface interface, so limiting the maximum possible reduction to the laminar boundary-layer value. With super-hydrophobic surfaces, on the other hand, the aim is to reduce skin-friction by inducing a finite slip velocity in the sea water at the ship’s surface. Recent work in this area has shown much promise: micro-machined surfaces made of chemically hydrophobic materials can be constructed that capture and sustain micro-scale air pockets within the surface (see figure) [2]. The relatively shear-free interface between the air pockets and the water produces a substantial reduction in total macroscopic skin friction; in the low-speed experiments of Danielloet al. up to 50% drag reduction was recorded [2].

For large-scale high-speed marine applications, much smaller micro (possibly nano) cavities/features will be required in order to maintain a coherent air-water interface at the large static pressures experienced. Nanoscale superhydrophobic surfaces are beginning to be developed which demonstrate this is agenuine possibility. For example, nanopin surfaces have been fabricated with superhydrophobicity, achieving water droplet contact angles of greater than 178 degrees (see figure) [3]. However, at such small scales, the air pockets/cavities cannot be assumed to reduce wetted area in proportion to their surface coverage. This is because in microscopic air pockets the strain rate must become extremely high to produce any significant slip in the water phase, so the air cannot be treated as inviscid despite its relatively low dynamic viscosity. Non-equilibrium effects, such as slip at the gas/solid and gas/liquid interfaces will offset this scale-dependent drag-reduction limit, making the search for optimum scale and topography a delicate and subtle design problem. An accurate simulation tool that can capture the interplay between micro/nano non-equilibrium gaseous/liquid flows and interface interactions is essential.

 

[1] 58th session, Marine Environment Protection Committee, 6-10 October 2008
[2] Daniello RJ, Waterhouse NE, Rothstein JP. 2009. Physics of Fluids 21:085103
[3] Hosono E, Fujihara S, Honma I, Zhou H. 2005. Journal of the American Chemical Society 127:13458-13459