Surface acoustic wave momentum

It is commonly known that a merry-go-round can be rotated even by a child going along its edge. The angular momentum of the whole system should be equal to zero so that the movement of the child results in a merry-go-round movement in opposite direction. Here we show experimentally that a macroscopic cylindrical solid can be rotated due to the existence of the momentum of surface acoustic wave propagating along the solid's surface, what is an analogy with the merry-go-round case. An external mechanical momentum is not applied to the solid, and its movement looks from the outside like a motion under the action of an internal force. Our results can be considered as a complement to the effect known as radiation pressure, which is applied to some obstacle on the way of the wave. Pioneering results for this phenomenon, which is quadratic in the wave amplitude, have been obtained for acoustic waves in gases, fluids, solids, and as well as for light one century ago [1]–[5]. Now we consider the mechanical response of a piezoelectric cylindrical tube covered with a periodic unidirectional surface acoustic wave transducer [6] on wave excitation if the obstacle is absent.