Smoother Flow Via Superhydrophobic Rough Surfaces

Conventional wisdom has always held that smooth surfaces are best for moving quickly through water. This is the logic behind everything from the design of most sailing vessels to the shaved bodies of competitive swimmers.

However, there is evidence that a carefully designed rough surface may be an improvement on reducing drag. According to a study described in the recent Physics of Fluids, a research team from the aerospace and mechanical engineering departments at UCLA modeled rough surfaces that have great resistance to water and a unique design pattern to reduce drag in the water (otherwise known as skin-friction drag).

The modeling used a class of materials known as superhydrophobic - generally considered to be materials that have wetting angles (contact angles of the water droplet on the surface) of greater than 150°. A micro-roughening of a superhydrophobic surface can increase the wetting angle even further, enhancing the effect.

In theory, these materials can trap air bubbles and create a cushioning effect against the friction of the water. (The effect is somewhat analogous to ice skating, where an extremely thin surface layer of water creates less friction against the hydrophobic surface of the thin skate blades.) In practice, the turbulent flow patterns often encountered by real-world vessels tend to break up the air bubbles and dissipate the cushioning effect.

While modeling a previously studied superhydrophobic material, the team made a surprising discovery. Using a surface covered with small, superhydrophobic ridges oriented in the direction of flow, the team discovered drag reduction in both laminar and turbulent flow regimes. However, the drag reduction was greater in turbulent flow regimes than in laminar ones.
Typically, laminar flow provides smoother sailing (so to speak). It's possible that the cushion does not contribute much of a change in the lesser drag of laminar flow, but is essential for achieving the larger drop found in turbulent flow. Since turbulent flows are more common in the real world, developing and optimizing these coatings could have a wide variety of real-world applications.

One potential application would be for fuel reduction in cargo ships. A successful reduction in drag for these applications could literally save tons of fuel and consequently reduce pollution. It's also likely that these coatings would reduce biofouling-the collection of various organisms on ship hulls that can add 330 pounds of weight per meter of surface area in a 6-month period, and cost up to $70,000 to remove. The US Navy estimates that these nuisance materials increase their fuel costs by 40-50%.

The positive effects could extend to the air as well as the water. Airplane manufacturers have researched similar technologies as a means of cutting turbulence around the jets. Fuel savings could be even higher than those in maritime applications, and again fuel reduction equates to less pollution.

The concept will have to move past the modeling stage to start gathering commercial interest and significant financial backing, but expect research to continue in this promising field to optimize superhydrophobic materials and the drag-reducing effect of various patterns.