Fluid dynamics

Water flows out of touch

Superhydrophobic surfaces reduce the frictional drag between water and solid materials, but this effect is often temporary. The realization of sustained drag reduction has applications for water vehicles and pipeline flows.

The molecules of a fluid tend to adhere to solid surfaces even if the fluid is flowing at a high speed. This no-slip condition is the cause of substantial frictional drag, which dictates the fuel consumption of vehicles and the pumping costs for pipeline flows. A potential way of reducing drag in water flows is to create superhydrophobic surfaces, which consist of special coatings that contain air pockets at the solid–fluid interface, allowing the water to slip more readily. However, this slip effect is usually small, and when such surfaces are immersed in water, the trapped air is typically quickly flushed away. Writing in Science Advances, Saranadhi et al.1 make use of the discovery2,3 that heating a superhydrophobic surface can cause it to become engulfed in a stable vapour layer. The authors demonstrate that, under such conditions, surprisingly large slip amplitudes are maintained for extended times.

The non-wetting properties of hydrophobic surfaces can be greatly improved by introducing micro- or nanoscale surface roughness. This allows air to be trapped in the surface recesses and, like water on a lotus leaf, the droplets in contact with such materials remain highly mobile. However, when the material is completely immersed in water, the trapped air will eventually diminish, and water molecules will adhere to the surface. To extend the lifetime of the superhydrophobic nature and desired slip properties of a material, air (or another gas) needs to be replenished at the surface.

One way of coating a submerged surface with a gas film is to rapidly heat it. Just above the fluid's boiling point, vapour forms as individual bubbles at the hot surface — a process called nucleate boiling. When the surface temperature is further increased above what is called the Leidenfrost temperature, boiling produces a continuous vapour film. For normal materials, the Leidenfrost temperature is much higher than the fluid's boiling point, and, when the material is cooled, the film suddenly collapses, causing the system to return to nucleate boiling. By contrast, when a superhydrophobic surface is heated to a high temperature and subsequently cooled, the vapour film persists — even down to the liquid's boiling point — and the system can avoid nucleate boiling2. By using this method3, vapour layers can be sustained on preheated superhydrophobic spheres falling through water, and the drag can be reduced by as much as 75% compared with that on ordinary (non-superhydrophobic and unheated) spheres.

Saranadhi and colleagues have pushed these developments a decisive step further, establishing that vapour films can be sustained indefinitely under water as long as the superhydrophobic surface is continuously heated. Surprisingly, the authors found that the films remain intact and functional, even in a turbulent flow environment in which the solid–fluid interface is subjected to severe fluctuating stresses — conditions under which ordinary superhydrophobic surfaces would quickly degrade.

In the authors' experiment, preheated water was contained in the gap between two concentric cylinders and driven by rotation of the inner cylinder (Fig. 1). The superhydrophobic surface of the inner cylinder was heated to 160 °C and a stable vapour layer formed, reducing drag by up to 90% compared with that on an ordinary surface. Saranadhi et al. calculated that the amount of slip at the heated surface was approximately 100 times larger than the maximum slip obtained using standard superhydrophobic surfaces. They attributed this result to the unusually large thickness of the vapour layer (about 50 micrometres) and the fact that the layer fully encloses the surface — including its microstructure, which in unheated superhydrophobic surfaces would be partially exposed to the surrounding liquid.

Figure 1: Sustaining a superhydrophobic surface.

Saranadhi et al.1 carry out an experiment that uses water contained between two concentric cylinders. The inner cylinder rotates with respect to the outer cylinder, driving water flow through the system. The velocity of the water flow is illustrated here by a fluid-velocity profile, in which the length of each arrow is proportional to the velocity at that point (in the reference frame of the inner cylinder). The surface of the inner cylinder is superhydrophobic — it is irregular and rough, which causes air to be trapped in the recesses. By continuously heating the inner cylinder, the authors observe the appearance of a vapour film that coats the superhydrophobic surface. This film reduces frictional drag at the interface between the inner cylinder and the water, and is maintained even when the system is subjected to large shear stresses and turbulent flow conditions. (Adapted from ref. 1.)

Materials that can sustain large slip amplitudes in macroscopic flows have long been sought. At present, these materials work only under special conditions (in this case, high temperature) and are unlikely to find widespread application. Additionally, replenishing the vapour film currently consumes more energy than is saved by the reduction in drag. Nevertheless, the emergence of materials that, in principle, can provide partial slip at solid–fluid interfaces is a big step forward and will spur further research.

The authors' results might also be relevant to research on turbulence. In wall-bounded flows (such as pipes), turbulence is essentially sustained by the friction between the fluid and the wall. Introducing partial slip and observing how turbulence adapts to these new conditions could provide fundamental insights into the turbulence-sustaining mechanism.

The quest for materials that promote low-friction fluid transport bears some resemblance to the search for room-temperature superconductors4 (materials with zero electrical resistance). Whereas present-day superconductors5 work only at temperatures below 0 °C, for their classical-physics counterpart — fluid-drag reduction — the largest sustained slip obtained by the authors occurs at temperatures above room temperature. If either effect can hit the room-temperature target, it is likely to have a major impact on global energy consumption.Footnote 1


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Correspondence to Björn Hof.

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Hof, B. Water flows out of touch. Nature 541, 161–162 (2017). https://doi.org/10.1038/541161a

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