First, the team creates a hydrophilic–superhydrophobic boundary in a 6 mm diameter tube. The top half of the inner surface is unmodified (that is, hydrophilic) and the bottom half is covered with a superhydrophobic nanotextured film.

Water dropped into the top of a vertically held glass tube collects spontaneously and is stopped at the boundary, forming a water column as if there was an invisible stop valve built inside the device. When more water is added, the stopped water column falls down suddenly due to the effect of gravity.

On the other hand, when the tube is inclined at an angle of 27°, water dropped from the hydrophilic side flows at the start, but then collects at the boundary to form a water column. When the amount of water is increased, the front of the water column forms an elongated ellipse-like droplet. When further water is added, only a portion of the elongated water droplet continues along the tube, while the rest of the water column remains at the boundary.

Full details are available in the journal Nanotechnology including videos of the device in action.