Superhydrophobic surfaces efficiently repel water in a phenomenon that is also known as the lotus effect. Indeed, the lotus leaf is a symbol of purity in many cultures because of its ability to remain clean: when rain falls onto the leaf, the drops of water that form on the surface roll off, taking any dirt with them. Such surfaces can be made by nano and microstructuring surfaces so that protrusions trap air underneath the liquid. In contrast to smooth, unstructured surfaces in which the contact line between a water droplet and the surface advances continuously, the contact line has to overcome an air gap between protrusions on a super liquid repellent surface.

To better understand this effect, a team led by Hans-Jürgen Butt has now imaged water droplets (around 5 µl in volume) advancing and receding on superhydrophobic arrays of micropillars for the first time using confocal microscopy. The surface was gradually tilted until the drop distorts in shape and at some inclination angle, the contact line between the droplet and the surface becomes more elliptical.

Contrary to what is currently believed, the images reveal that the liquid surface gradually “bends down” until it touches the top face of the foremost micropillar with an advancing contact angle of 180°. The advancing contact angle can even exceed 180°. On the receding side, pinning the top faces of the micropillars determines the apparent receding contact angle, say the researchers. The receding contact angle is a direct measure of the lateral adhesion force of the drop to the surface and determines the angle at which the drop rolls off, explains Butt.

Helping design better waterproof materials

“A drop on a superhydrophobic surface basically rolls forward rather than jumping from protrusion to protrusion,” he says. “We also found that that there is no well-defined advancing contact angle on a superhydrophobic surface so the receding contact angle (rather than the apparent advancing contact angle) therefore characterizes the liquid repellency of the surface.”

The findings could help researchers design better waterproof materials with applications from self-cleaning surfaces and antifouling coatings to fog collection systems to supply fresh water in arid environments.

Guihua Yu of the Texas Materials Institute, who was not involved in this study, says that Butt and colleagues are reporting on an interesting fundamental work using scanning confocal microscopy to image the wetting behaviour in detail of water drops on superhydrophobic micropillar arrays. "This paper truly represents an important step forward towards a better understanding of the wetting phenomena of liquid drops on textured surfaces – it’s a beautiful piece of work."

The Max Planck researchers say that they now want to try sliding drops at a defined constant velocity and see how the contact line at the front and rear on a superhydrophobic surface moves. “Perhaps events at the front and rear are connected,” Butt tells nanotechweb.org. “For example, depinning at the rear might be causing a capillary wave to travel over the drop’s surface and touch down at the front to the next protrusion.”

The results are detailed in Physical Review Letters DOI:http://dx.doi.org/10.1103/PhysRevLett.116.096101.