Repellent materials

A new discovery by scientists at Queen’s University Belfast has changed the face of research into water-repellent “ultrahydrophobic” materials creating a wealth of potential practical applications.

Drs Graham Saunders and Steven Bell of Queen’s University School of Chemistry and Chemical Engineering, together with PhD student, Iain Larmour, have developed a very simple method for treating metals that results in extremely high hydrophobicity using readily available starting materials and standard laboratory equipment in a process that only takes a few minutes.

The significance of the discovery lies in the ease of fabrication and the flexibility of the method. Dr Saunders said, “There have been numerous attempts to emulate the extraordinary water repellency of lotus leaves, but very few synthetic surfaces can match these natural systems. Those that do are unsuitable for practical applications because they are difficult and costly to fabricate or can be applied only to a very limited number of materials. Our method produces robust surfaces displaying hydrophobicity that surpasses that of lotus leaves – ultrahydrophobicity. Furthermore the method is cheap and quick, and can be extended to a wide range of metals.”

It is the structure of lotus leaves – nanohairs on microbumps which are coated with a waxy substance – that causes the hydrophobicity and the Queen’s team’s discovery has successfully mimicked that surface structure.

The process is simple. The objects to be treated are immersed in a metal-salt solution which coats them with a textured metal layer, thinner than a human hair, which resembles the structure of lotus leaves. The object is then dipped into a solution of a chemical surface-modifier, which covers the textured coating with a second, even thinner layer of water-repelling molecules. The resulting surface is so hydrophobic that water droplets deposited on the surface form almost perfect spheres and coated objects can be immersed for days but are found to be completely dry when they are pulled from the water.

The flexibility and simplicity of the approach means that the method can be applied to metal objects of any reasonable shape and size. Dr Bell said, “The team experimented with samples of various shapes and sizes and more complex metal objects, including a model of a pond skater made from copper. Pond skaters use superhydrophobic legs to walk on water, and our model, despite being 10x the mass of a pond skater of the same size, when treated, floated comfortably on water. Although this is a light-hearted example it does illustrate how readily our method can be applied.”

Future practical applications of this discovery are likely to include biomedical devices, liquid separation, and reducing turbulent flow in water-bearing pipes, among others.