Electrically Adjustable, Super Adhesive Force of a Superhydrophobic Aligned MnO2 Nanotube Membrane

A superhydrophobic membrane of MnO2 nanotube arrays on which a water droplet can be immobilized by application of a small DC bias, despite a large contact angle, is reported. For a 3 mu L water droplet, the measured adhesive force increases monotonically with increasing negative voltage, reaching a maximum of 130 mu N at 22 V, 25 times the original value. It follows that the nearly spherical water droplet can be controllably pinned on the substrate, even if the substrate is turned upside down. Moreover, the electrically adjustable adhesion is strongly polarity-dependent: only a five-fold increase is found when a positive bias of 22 V is applied. This remarkable electrically-controlled adhesive property is ascribed to the change in contact geometry between the water droplet and MnO2 nanotube array, on which water droplets exhibit the different continuities of three-phase contact line. As the modulation in this manner is in situ, fast, efficient and environmentally-friendly, this kind of smart material with electrically adjustable adhesive properties has a wide variety of applications in biotechnology and in lab-on-chip devices.