Superhydrophobic nanowire surfaces for drop movement using magnetic fields

We present a novel method to control water drop movement on superhydrophobic surfaces through magnetic fields. Water drops with paramagnetic particles can be moved on fractal Si-nanowire superhydrophobic surfaces at high speeds by displacing a magnet. A drop pinned to a surface defect can be combined with another drop containing paramagnetic particles thus allowing movement of the newly formed drop. A drop can also be split using two magnetic fields. Under a magnetic field, paramagnetic particles form chain-like clusters. The movement generating force appears as a consequence of chains pushing against the drop skin at the drop bottom, with chains acting as a lever and drop surface tension as a fulcrum. Our findings demonstrate the feasibility of using magnetic fields to move, combine and split aqueous drops on non-patterned superhydrophobic surfaces with the only driving force of magnetic fields, which appears as a promising way to manipulate small discrete amounts of water.