Since they were first synthesized, carbon nanotubes have attracted considerable attention from the research community for, among other things, their ability to form arrays with nanoscale roughness. This property encourages the entrapment of air within the nanopores, which can give surfaces coated with the arrays an exceptionally water-repellant nature. Although researchers have tested this property using gently placed drops of water, few have analyzed how the arrays respond to rapidly falling droplets. Snapshots of a water droplet before, during and after initial impact with a nanotube-covered substrate demonstrate the surface’s superhydrophobic properties. Reprinted with permission of Applied Physics Letters.Nikhil Koratkar and colleagues at Rensselaer Polytechnic Institute in Troy, N.Y., have examined the water-repelling properties of nanotube-covered surfaces under dynamic conditions. Their findings may aid in the development of materials with applications in spray painting, ink-jet printing and other processes. To synthesize the surfaces, the researchers deposited arrays of multiwalled carbon nanotubes onto silicon substrates. By varying the temperature and gas flow rate in the reactor, they changed the packing density and thickness of the rods to yield arrays with contact angles of 140° and 163°. These values represent the angles formed between the water drops and surfaces under static conditions. The scientists next placed each coated substrate in a chamber to prevent interference from air currents. During observation with a high-speed digital camera from Redlake Inc. of Tucson, Ariz., they released droplets of water from various heights above the surfaces with a microsyringe. Operating at a speed of 3000 fps, the camera and associated analysis software allowed the scientists to characterize the velocity of any given droplet just before impact. They also quantified the diameter of the drop as it spread across the surface, as well as the contact angle at various time points.Close examination of the images reveals distinctly different behaviors for each surface. In a trial involving an array with a 160° contact angle, the incoming droplet made contact, flattened and then rebounded completely (see image). Once a droplet made contact with the 140° array, however, it pulsated violently up and down but did not rebound. Pushing out the air pocket at the water-array interface, the drop eventually soaked in as a result of strong van der Waals interactions between the water and the nanotubes. In both cases, droplet speed was 0.56 m/s. Experiments at higher and lower velocities confirmed the trends.The experiments showed the investigators the fine distinction between arrays with good and poor dynamic hydrophobicity. In future experiments, they will seek to determine the minimum contact angle necessary for the arrays to repel impending droplets.Koratkar noted that the superhydrophobic arrays could be adapted for application to aircraft wings and fuselages, where they would retard ice formation by shedding water and, potentially, by lowering its freezing point. Other research groups have found ways to anneal carbon nanotubes to metal substrates, which could render such coatings more resistant to wear and tear. Further applications for the technology exist in microelectronic precision solder-drop dispensing, in spray cooling and in other settings where water repellency must be maintained under dynamic conditions.Applied Physics Letters, July 9, 2007, Vol. 91, 023105.