Spontaneous rotation by symmetry breaking of a capillary wave source*

When a millimetric object is deposited onto the interface of a vibrating liquid bath, the floating body generates an extended capillary wavefield with associated surface streaming flows. It has recently been shown that chiral objects placed at the interface are able to steadily rotate in a determined direction via an imbalance of wave stresses as a consequence of their imposed geometric asymmetry. Here, we consider symmetric (achiral) objects which spontaneously begin to rotate in either direction. This symmetry breaking phenomenon occurs above a critical driving acceleration, with this threshold depending on the driving frequency, fluid parameters, and wave source geometry. We characterize the dependence of the steady rotation speed on the experimental parameters, and rationalize our observations with a simple mathematical model drawing inspiration from other physical systems that exhibit spontaneous symmetry breaking.