Hygromorphic actuator from a metal oxide film driven by a nano-capillary forest structure

Abstract

We have developed a hygromorphic metal oxide actuator using an electrochemical method to produce a superhydrophilic free-standing nano-capillary forest of titanium oxide with a high aspect ratio (~80). This metal oxide film has an inhomogeneous initial gap at the top and bottom surfaces between the tubes due to flexure during fabrication. The actuation mechanism is as follows. First, when a drop of water is applied on the surface of a titanium oxide nano-capillary forest (TNF), the water penetrates through the film instantaneously, and the titanium oxide nano-capillaries are pulled together by interplay of the capillary force and van der Waals force. When water has fully filled in the gaps between the capillaries, the free-standing TNF film remains unbent for ~2 min. Then, as the water evaporates, the film bends further in the forward direction. When the water has completely evaporated, the van der Waals force alone acts on the capillaries, and the TNF film returns to its initial state. This TNF possesses great stability and repeatability for long-term usage having a high bending energy density of ~1250 kJ m–3 and unique capabilities. It may lead to novel stimuli-responsive systems, including energy collection and storage, as well as robotics applications. Arrays of titanium oxide nanocapillaries that bend in response to ordinary water drops may help gather free energy from rainfall events. The approach by Jinkee Lee and co-workers from South Korea's Sungkyunkwan University uses electrochemical methods to grow thin metal oxide tubes, micrometers long but only 100 nanometers wide, into a freestanding forest-like thin film. Because of slight flexing during fabrication, each capillary is surrounded by a variable, nanometer-scale gap. When a water drop contacts the film, it immediately penetrates the gaps and pulls the capillaries together. This bends the film forward until the drop evaporates and the initial upright state is restored. Initial calculations suggest that energy-harvesting cantilevers made from the nanoforest film would generate a larger mechanical force per gram than most existing actuators. A new hygromorphic actuator made by hydrophilic metal oxide film was developed, which is moved by the fluid spread on film and the imbibition within a nano-capillary forest. This system possesses a great stability and repeatability for long time and has a very high energy density of ~1250 kJ m–3. The research results suggest that the actuating nano-capillary forest film could be applicable to humidity-responsive actuators, high-efficiency energy converters and others.