Have you ever seen a gecko walking in a wall or in the ceiling? The way they move is quite impressive. They can hold on to the walls and ceiling in a very natural way and they can walk along the walls at ease. In order to do that the gecko uses two properties: stiction and adhesion. Stiction, or static friction, is the force that needs to be overcome so that two stationary objects in contact with each other can develop a relative motion. For example, when you start to push a book that is on a table, stiction is the force that holds the book in place, that keeps it ad before you apply enough force to make it move. Adhesion is the attachment of two surfaces and it is strongly related to stiction.
Fig1. A gecko is very good at controlling stiction and adhesion.
The gecko is really great at controlling both stiction and adhesion. It can quickly turn them on and off while he walks along the ceiling. Another impressive natural phenomena involves lotus leaves. Even when water falls on them they don’t get wet due to its molecular structure. The lotus leaves can’t turn on and off it’s wetting properties, but there may be some materials that can. A group of scientists from Belgium and Switzerland have obtained a surface in which it is possible to switch stiction and adhesion, similarly to what a gecko does, and this alters the wetting properties of the surface.
The scientists studied a monolayer (layer with the thickness of only one atom) of boron nitride on a rhodium crystal. The atoms in the boron nitride layer were arranged in a hexagonal pattern (similar to what is found in graphene) and were grown over a rhodium crystal. The rhodium is also arranged in a hexagonal pattern but the size of these hexagons are different from the boron nitride ones. The scientists realized that both stiction and adhesion can be reversibly switched on and off by applying different electrochemical potentials to the sample. Moreover, they discovered that these properties are related with an altered wetting of the material.
Fig2. Scientists have created a material that can switch stiction and adhesion, similarly to a gecko.
How does this happen? It involves changes at the atomic level. The electric field changes the geometry of the atoms in the material. In particular, it leads to an intercalation atomic hydrogen to the monolayer of boron nitrate. This changes the nanostructure of the surface and alters how it interacts with other objects. Even though the modifications occur at the atomic level, the resulting geometry leads to altered properties at the macroscopic level, such as stiction, adhesion and wetting of the surface.
From single-atom manipulation up to the everyday experience of moving ourselves stiction, adhesion and wetting seem to be related. This study has shown how the macroscopic static friction and adhesion can be linked to our understanding of water adsorption on this surface at the microscopic scale.
This work may not lead to a new super-hero (sadly gecko-man is not coming to save us) but it does greatly increases the understanding about stiction, adhesion and wetting. Since the structure of the monolayers of boron nitride is relatively simple it may me possible for scientists to describe this behavior theoretically down to the atomic level. Plus, the hybrid of hexagonal boron nitride and rhodium is very stable and could be used as a new class of switchable surfaces. These have great potential for application in the study of adhesion, friction and lubrication and could lead to several technological innovations.
By Kellen Manoela Siqueira
The full paper can be found in the June 2016 edition of Nature.