A checkerboard of water

Water droplets cling in flat squares and dance in round globes on a smart surface created by South Korean researchers. In this picture, the surface has been designed to attract or push away water in a chequerboard pattern. Exposure to light wipes away that pattern, and an alternative can be ’written in’ with no etching required. 

Kilwon Cho and colleagues from Pohang University of Science and Technology made the surface that switches reversibly from extreme water repulsion (superhydrophobicity) to extreme water attraction (superhydrophilicity) via exposure to UV or visible light. To create the pattern shown in the picture, a hydrophobic surface was covered by a waffle-shaped aluminium mask. UV irradiation made the exposed surface hydrophilic. In these regions, water spreads out over as much surface as possible. In the remaining hydrophobic sections, water gathers itself into a sphere to avoid touching the surface. On exposure to visible light, the surface becomes entirely hydrophobic again, ready to be re-patterned with UV light. 

In close-up, the surface consists of a forest of string-like organic molecules - fluorinated azobenzenes - on top of layers of nanoparticles covering a silicon wafer. The azobenzene molecules usually line up in a series of straight chains, like a group of trees with water-repelling fluorine atoms sticking out at the top. This makes the surface hydrophobic. When illuminated by UV light, azobenzenes switch from straight-chain (trans) form to bent (cis) form, though the change is reversible. The curled-up azobenzene molecules now hide away the fluorine atoms, and the surface becomes slightly more hydrophilic. 

But on a perfectly smooth silicon wafer, bent or straight azobenzenes don’t make much difference to the water-attracting properties of a surface. The hydrophobic-hydrophilic switching effect was instead accentuated by underlying nanoparticle layers, which made the surface rough. The rougher the surface, the greater the subsequent hydrophilic switch when covering string-like molecules were illuminated with UV light. 

The work combines well-understood physical and chemical concepts to make a new system, said Richard Jones of Sheffield University, UK. The rough surface amplifies the effects of the chemical switching mechanism. 

Apart from its striking water-covered appearance, the new switchable surface could be useful in many applications, predicted Cho. ’The results of our research can be used for the construction of future generation smart devices such as microfluidic devices, protein separation systems, bioanalysis and optical storage devices,’ he told Chemistry World. In microfluidics, for example, tiny droplets of water are moved around in channels to combine small amounts of chemicals. Using the switchable surface, hydrophilic channels could be patterned into the surface reversibly, with no etching needed. 

Richard Van Noorden