A coupling between the two processes. When the forcing amplitude is increased,
the bouncing of the drop becomes subharmonically modulated. This is reminiscent
of an elastic or inelastic ball on a vibrating plate where periodic, period doubling and chaotic behaviours have been
described. In our experiment, there is a range of droplet sizes for which the period
doubling becomes complete so that the droplet bounces on the surface only once in
two periods. The waves generated by this bouncing have the Faraday frequency and
a larger amplitude. Correspondingly, a symmetry breaking occurs: the droplet starts
propagating spontaneously on the liquid bath. The walker forms a coherent self-propelled
structure.