Uphill Water Transport on a Wettability-Patterned Surface: Experimental and Theoretical Results

In nature, there exist many functional water-controlling surfaces, such as the water-repellent surface of lotus leaves, the superhydrophobic water-adhesive surface of rose petals, the water-harvesting surface of a beetle's back, and the water-transporting surface of the legs of Ligia exotica. These natural surfaces suggest that surface chemistry and hierarchical structures are essential for controlling the water behavior. We have reported the preparation of superhydrophobic and antireflection silicon nanospike-array structures using self-organized honeycomb-patterned films as three-dimensional dry-etching masks. Moreover, the surface wettability of the silicon nanospike-array structures can be easily transformed from superhydrophobic to superhydrophilic by changes in the surface chemistry. In this report, we show the preparation of water-controlling surfaces, such as water-harvesting and water-transporting surfaces, by the wettability patterning of silicon nanostructured surfaces. We prepared honeycomb-patterned films for dry-etching masks made from polystyrene and an amphiphilic polymer by casting a chloroform solution. After the fixation of the top layer of the honeycomb-patterned films on a single-crystal silicon substrate, reactive ion etching was performed. The as-prepared silicon nanospike-array structure showed superhydrophobicity, and the water contact angles were over 170°. After UV-O₃ treatment with photomasks, only the UV-irradiated surfaces showed superhydrophilicity, suggesting that we can obtain superhydrophobic- and superhydrophilic-patterned surfaces for which the patterns are the same as those of the photomasks. On the basis of these wettability-patterned surfaces, we demonstrated water harvesting by superhydrophilic dot-patterned surfaces and water transportation against gravity by superhydrophilic triangular-patterned surfaces. In particular, we investigated uphill water transport through the motion of droplets on tilting slopes based on the equation of motion. These results suggested that we can obtain superior microfluidic devices suitable for various applications through the use of optional wettability patterns.