The team from the University of Aix-Marseille and Liverpool University, explains that the mathematics behind the invisibility cloak involves a geometric transform – which takes a point, inflates it and renders anything that lies inside the resulting bubble unreachable by the waves.

The team's cloak is a shallow metal cylinder, measuring 10 cm across. The cylinder does not have solid walls but instead consists of a series of rods arranged in 100 identical sectors and seven concentric rings. The object functions just like a whirlpool. The liquid enters through the gaps between the rods, swirls around the concentric rings and then enters the far side of the cylinder so as to leave the central region entirely free of liquid. The trick is to transform the waves so that they have a greater velocity along the circumference of the rings than along the radii, therefore slowing down the liquid as it approaches the centre and forcing it out the far side.