Reversible Switching between Superhydrophilicity and Superhydrophobicity
Taolei Sun, Guojie Wang, Lin Feng, Biqian Liu, Yongmei Ma, Lei Jiang*, Daoben Zhu
The extent to which water can wet a surface is dependent on a number of factors, including the chemistry and the roughness of the surface.Various methods have been used to change the extent of wetting dynamically, but they cover only a limited range of water contact angles. The surface can be reversibly switched from being superhydrophilic to being superhydrophobic with a very small change in temperature. On its own, poly(N-isopropylacrylamide) will switch from being hydrophilic to being mildly hydrophobic when the temperature is raised from 25 oC to 40oC.At the lower temperatures, the C=O and N-H groups are partnered by water molecules, and intermolecular hydrogen bonding dominates; when the temperature is raised, intramolecular hydrogen bonding takes over, ejecting the water molecules, and the chains adopt a more compact form. This transition is enhanced by depositing the polymer onto patterned silicon substrates. As the pattern size was decreased (finer grooves), they observed an increase in the range of contact angles achieved on switching. Detailed investigation of the substrate showed a large fraction of irregular nanoparticles produced by sputtering from neighboring regions and thus a very large surface area.
Angew. Chem. Int. Ed., 2004, 43, 357-360