Bio-inspired humidity responsive switch for directional water droplet delivery

In nature, the fibrils of awns can make an open-close motion in response to humidity changes. Shorebirds can suck water droplets by the repeated open-close motion of the beaks. Through the fusion of these inspirations, we report a directional droplet delivery device in response to humidity. A hydrogel-actuated switch was fabricated through the integration of polyacrylic acid hydrogel, cellulose membrane and superhydrophilic copper wires. It can realize directional droplet delivery. In the open state, the droplet was propelled directionally towards the tip-site of the conical channel by virtue of the Laplace pressure difference. When the switch shifted to be closed and the channel turned into a parallel one, the droplet was pinned due to the disappearance of the Laplace pressure difference. The switch for directional liquid delivery can attract broad interest in the field of microfluidics, droplet manipulation, etc.