This image shows a droplet being deposited on the switchable surface through a very thin glass tube. Image: Vienna University of Technology.
This image shows a droplet being deposited on the switchable surface through a very thin glass tube. Image: Vienna University of Technology.

When rain falls on a lotus leaf, the leaf doesn't get wet: thanks to its special structure, the water droplets roll off without wetting the surface. Artificial materials can be made similarly water-repellent, but producing a surface with switchable wetting properties has proved extremely challenging.

Now, researchers from the Vienna University of Technology in Austria, KU Leuven in Belgium and the University of Zürich in Switzerland have found a way to switch the surface of a single layer of boron nitride back and forth between states with high and low wetting and adhesion. They report this work in a paper in Nature.

"One of the most interesting physical properties of a surface is its stiction or static friction," explains Stijn Mertens from the Vienna University of Technology. "This force has to be overcome for an object on the surface to start sliding."

The nanostructure of a surface influences its stiction to a great extent. The details of the contact between the surface and another object (for example, a water droplet) depends on the geometry of the atoms on the surface and other properties, which determine adhesion, stiction and wetting. The relationship between stiction and wetting, however, is so far only poorly understood.

"Just as the material graphene consists of only one layer of carbon atoms, our boron nitride – which contains as many boron as nitrogen atoms – has a thickness of only one atomic layer," says Thomas Greber from the Physics Institute at the University of Zürich. This ultrathin layer can be grown on a single rhodium crystal. The atoms on the surface of the rhodium crystal and the atoms making up the layer of boron nitride form a hexagonal pattern, but the distances between the atoms in the two materials are different. Thirteen atoms in boron nitride take up the same space as 12 rhodium atoms, and so the two crystals do not fit together perfectly. Because of this mismatch, the boron nitride hexagons must bend, producing a frozen wave with a wavelength of 3.2nm and a height of about 0.1nm.

"Precisely this two-dimensional nanowave influences the wetting of the surface by water," says Mertens. But the boron nitride superstructure can be made flat again by using a simple trick. Putting the material in acid and applying an electrical voltage causes hydrogen atoms to creep under the boron nitride layer, altering the bonding between the nitrogen and rhodium atoms and flattening the boron nitride.

This causes the adhesion of a water drop on the surface to change dramatically – even though the drop is 100,000 times bigger than the tiny waves in the boron nitride. If the voltage is decreased, this effect is reversed: "We can switch the surface again and again between these two states," explains Mertens.

To investigate the wetting of the surface and apply the voltage at the same time, the researchers built an instrument specially for this purpose, in which a liquid droplet is deposited on the surface through a very thin glass tube. The droplet is made bigger and smaller while at the same time its shape is recorded. Whether the droplet shape is flat or more rounded depends on the properties of the surface.

Techniques for switching the wettability of a surface back and forth have been around for a while. For example, organic molecules that change their shape on exposure to light of a certain color can be attached to the surface. However, such molecules are much more complex and fragile than the materials studied here.

"Our surface consists of only a single layer of atoms, is completely inorganic and does not change even if we heat it in vacuum to 1000°C," agree Mertens and Greber. "This means that this material could also be used for applications where organic molecules would long be destroyed, ranging from daily life to space travel."

This story is adapted from material from the Vienna University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.