Observation of a water droplet motion by using an oxide nanowire transistor covered by a nanofiber mesh

The motion, in particular the flow speed and dropping height, of a water droplet was observed using a tin oxide (SnO₂) nanowire transistor with a polyurethane (PU) nanofiber mesh as a selective filter. The changes in the SnO₂ nanowire transistor characteristics, particularly the threshold voltage and on-current, were due to the adsorbed water molecules that acted as electron donors on the surface of the oxide nanowire semiconducting channel. The role of the PU nanofiber mesh, allowing the passage of water vapor while blocking liquid water, was to restrict the direct contact between the water droplet and the oxide nanowire semiconducting channel and electrodes, which could cause abnormal transistor characteristics. The selective filtering properties of the PU nanofiber mesh could be controlled by changing the number of PU layers.