Skin-Friction Drag Reduction from Superhydrophobic Coatings of Textured Hyperbranched Polymers

Previous studies have shown that superhydrophobic surfaces have skin-friction drag reducing properties in the laminar flow region, but not in the turbulent flow region. The drag reducing properties of superhydrophobic surfaces are recognized to result from air voids trapped in the surface, and the apparent lack of effectiveness in the turbulent flow region could result from: (1) lack of robust air voids in the textured hydrophobic surface, (2) failure of the nano- or micro-textures, and (3) inadequate air void areas. Correction of these properties is believed to be a key to retaining drag reducing properties in the turbulent flow region. We propose to optimize our current superhydrophobic hyperbranched polymer surfaces with hierarchic nano/micro textures to satisfy these requirements. The obtained superhydrophobic surfaces will be tested for drag reduction in a water tunnel. In addition, numerical simulations will also be conducted to find the fraction of air voids in a textured superhydrophobic surface to achieve greater than 20% drag reduction in a fully-developed turbulent flow region. Our superhydrophobic hyperbranched polymer coatings will be easy to apply on large surfaces. If this technology is successful, it will be a lightweight and low-cost technology for drag reduction applications.BENEFITS: If superhydrophobic technology for drag reduction is successful, the technology would greatly reduce the cost of the fuel for marine vessel operation, increase the range and the speed, and the lesser amount of fuel required would increase the payload capacity. In addition, the increased efficiency would reduce the dependence on foreign oil. It could also significantly improve the performance (speed) of military vessels and find application in underwater weapons such as torpedoes and other missiles. It would be a lightweight and low-cost means to achieve drag reduction in comparison with other technologies that have been investigated, e.g., polymer or bubble injection. The superhydrophobic materials could also have applications in other areas, and potential applications are envisioned in the following fields: 1. Protecting clothing from water and from dirt. This is an especially desirable property for army combat uniforms where clothing readily gets wet and dirty. 2. Reducing surface contamination by pathogenic organisms. A superhydrophobic coating also having biocidal properties would provide protection by reducing surface contamination. 3. Anti-fouling marine coatings. The super non-stick properties of the superhydrophobic surfaces will provide an even better ability to prevent the build-up of marine organisms on ships’ hulls compared to the silicone-based coatings that are effective as anti-fouling marine coatings. 4. Inhibiting metal surface corrosion. The corrosion of metals typically requires the presence of water on the metal surface. A superhydrophobic coating will minimize the presence of water at the metal surface and hence will minimize corrosion. This enhanced corrosion resistance will be useful for both military and civil aircraft, for ground vehicles and for the superstructure of ships. 5. Water-repellent windshields. A superhydrophobic clear coating on a windshield surface will allow rain droplets to be easily blown off the windshield surface and hence improve visibility during rain storms. 6. Microfluidic devices. Flow in the micro-channels is a problem and can be improved by drag-reducing coatings. The value of these unique materials as superhydrophobic coatings could be in the multi-millions of dollars in products and license fees.