For many years the chemical industry has been able to make a material waterproof. Material designers have copied nature, such as the composition of a lotus leaf that prevents moisture getting in. Similarly, chemists have developed materials and coatings that produce high adhesion.
But until now, mankind has yet to create a practical material that can switch from ‘sticky’ to ‘slippery’ and back again. This is because previous research in this field has only led to the production of very fragile materials.
However, a new study has found a much more robust product that uses a layer of boron nitride and an electric current so that, as the online journal Phys.org reports, “… it can be switched back and forth between states with high and low wetting and adhesion.”
A material’s stiction (static friction) and wetting are usually dictated by the nanostructure of the surface and how it interacts with another surface, including the geometry of the atoms, but the relationship between these two states has often been little understood.
But now a breakthrough has been made, and the results published in the online journal Nature, which states that, “Stiction and adhesion can be reversibly switched by applying different electrochemical potentials to the sample [a hexagonal boron nitride monolayer on rhodium], causing atomic hydrogen to be intercalated or not.”
As one of the lead researchers, Thomas Greber from the Physics Institute at the University of Zürich, explained, “Just as the material graphene consists of only one layer of carbon atoms, our boron nitride has a thickness of only one atomic layer.”
The online journal Phys.org explained in more detail, stating that, “This ultrathin layer can be grown on a rhodium single crystal. The atoms on the rhodium surface and in the boron nitride form a hexagonal pattern, but the distances between the atoms in the two materials are different. Thirteen atoms in boron nitride take the same space as twelve rhodium atoms, so that the two crystals do not fit together perfectly. Because of this mismatch, the boron nitride hexagons must bend, they appear as a frozen wave with a wavelength of 3.2 nanometres and a height of about 0.1 nanometres.”
The ‘two-dimensional nanowave’ allows the surface to have low stiction when in contact with water. However, by putting the material in acid and applying an electrical current the surface changes. This is because, “… hydrogen atoms creep under the boron nitride layer and change the bond between nitrogen and rhodium. This makes the boron nitride flat. Suddenly the adhesion of a water drop on the surface changes 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.”
As a result, the materials ‘stickiness’ and ‘slipperiness’ can be freely switched back and forth.
A video clip of the experiment in progress can be seen here.
But the true discovery is in how robust this new material is, as fellow researcher Stijn Mertens of the Institute of Applied Physics at the Vienna University of Technology explained, “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. 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.”
With this level of sturdiness and stability the range of uses will be wide. It is only to be seen how long it takes the chemical industry to develop the discovery further so that it can be brought to market. Whilst chemists analyse the results to confirm their validity, those of a more business way of thinking are considering what applications this discovery may have. And ultimately asking themselves; “Where would you apply a material that is reversible between the states of adhesion and non-adhesion?”