Nicolas Francois discusses the dynamics of waves at the surface of a liquid and the motion of the fluid particles of which these waves are comprised. What is perceived as fluid motion on a surface perturbed by wave is a motion of the surface shape, which is always different from the fluid flow along the surface. Studying the interaction between waves and the fluid particles is related to solving the Navier-Stokes equations in the Lagrangian viewpoint; this is an outstanding question of fundamental physics with a broad range of environmental and industrial applications such as harvesting wave energy or mitigating the impact of pollution at the ocean surface. Experimental models are essential to make progress on this difficult topic.
In this perspective, several experimental studies carried out by our group use Faraday waves as such experimental model. Faraday waves are parametric oscillations that appear at the surface of a liquid which is vertically vibrated. These non-linear waves can be viewed as an ensemble of quasi-particles, termed oscillons that move erratically within a lattice. In parallel with this description of the waves, it has been found that the horizontal motion of fluid particles is not bounded to the wave lattice. Surprisingly, fluid particles move as they would in two-dimensional (2D) hydrodynamic turbulence.
Our studies of the motion of both oscillons and fluid particles in Faraday waves have revealed unexpected aspects of surface hydrodynamics.
- the presence of 2D turbulence in a 3D system;
- the creation of vorticity at a liquid surface perturbed by a field of oscillons;
- the coupling between the disordered motion of oscillon and the turbulent agitation of fluid particles;
- the fate of these mechanisms when the liquid surface is «polluted» by substances having complex rheological properties.
Nicolas Francois was awarded his PhD in physics of fluids and polymers from the Université de Bordeaux. Since 2012, Nicolas has worked as an experimentalist in the Physics of Fluids Laboratory. Nicolas studied Lagrangian aspects of Turbulence and surface hydrodynamics and is also interested in the physics of complex fluids and granular matter.