Wave propelled ratchets and drifting rafts

¹  Matières et Systèmes Complexes, Université Paris Diderot, CNRS - UMR 7057 Bâtiment Condorcet, 10 rue Alice Domon et Léonie Duquet 75205 Paris cedex 13, France, EU
²  GRASP-Photopôle, Institut de Physique B5, Université de Liège - B-4000 Liège, Belgium, EU
³  Matériaux et Phénomènes Quantiques, Université Paris Diderot, CNRS - UMR 7162 Bâtiment Condorcet, 10 rue Alice Domon et Léonie Duquet 75205 Paris cedex 13, France, EU

Corresponding author: antonin.eddi@univ-paris-diderot.fr

Received: 24 January 2008
Accepted: 26 March 2008

Abstract

Several droplets, bouncing on a vertically vibrated liquid bath, can form various types of bound states, their interaction being due to the waves emitted by their bouncing. Though they associate droplets which are individually motionless, we show that these bound states are self-propelled when the droplets are of uneven size. The driving force is linked to the assymetry of the emitted surface waves. The direction of this ratchet-like displacement can be reversed, by varying the amplitude of forcing. This direction reversal occurs when the bouncing of one of the drops becomes sub-harmonic. As a generalization, a larger number of bouncing droplets form crystalline rafts which are also shown to drift or rotate when assymetrical.