The leaves of lotus flowers exhibit unique properties enabling them to clean themselves and remain unsoiled even when exposed to muddy water. These self-cleansing properties are due to the ability of the leaf-surfaces to strongly repel water—a characteristic of hydrophobic surfaces. The surfaces of the leaves are covered with micro and nanometer scale structures causing water droplets to roll off instead of spreading and absorbing dirt particles.

A wide range of superhydrophobic materials ranging from paints, roof tiles and fabrics have been developed using rough surfaces treated with special fluorochemical or silicone coatings, or with micron-sized particulates.

Fig. 1: Nanotextured surface with tunable wettability.

Now researchers at the Pohang University of Science and Technology, Korea1 have produced novel surfaces where their ability to repel or attract water is controlled externally. The team led by Kilwon Cho has created a new superhydrophobic surface, which can reversibly be turned into a water-attracting surface by exploiting ion-pairing interactions.

To engineer their tunable surface, the researchers grafted a positively charged brush-like polymer onto a semiconductor supported, nanostructured gold surface, allowing the the degree of wetting—wettability—to be reversibly programmed between superhydrophobicity and superhydrophilicity by careful selection of the counteranion (Fig.1).

“The wettability of the surface was easily modulated by changing the counteranions, because different anions possess different surface free energy values,” says Cho. By exchanging chloride ions with counteranions of different water affinity, the researchers switched the wettability of their surface from superhydrophobic to superhydrophillic.

To maximize surface roughness and wettability, they deposited highly branched micrometer-sized gold clusters, bearing nanoscopic protuberances, onto a silicon surface using an electrochemical method. These micro- and nanoscopic protuberances provide a high-porosity, densely packed support which provided a myriad of sites partially filled with the polymer brushes, further enhancing ion-pairing. “We have demonstrated that the small change in wettability that results from ion exchange on a flat surface can be amplified by introducing surface nanostructure,”says Cho.

The researchers believe that, because of their easy binding and removal, the use of these tunable ion-paired counteranions might be expanded to research fields like microfluidics and biological and biomedical domains, which require a precise control over surface wettability.