Guillaume Lagubeau

On the elasticity of an inertial liquid shock

A drop of low viscosity hitting a solid may bounce, provided that the material is highly hydrophobic. As a model of such a situation, we consider here the case of a very hot solid. Then, as discovered by Leidenfrost, a thin layer of vapour sustains the drop, preventing any contact with the substrate. On hitting such a solid, a drop rebounds, and we discuss here the elasticity of the shock. Two very different cases are described: at a large velocity, the weaker the impact velocity, the weaker the elasticity; at a small velocity, a quasi-elastic regime is found. The boundary between the two domains is set by a Weber number, which compares the kinetic and surface energies of the drop, of order unity.

Viscous mechanism for Leidenfrost propulsion on a ratchet

An evaporating drop placed on a ratchet self-propels, as discovered by Linke et al. in 2006. Sublimating platelets do the same, and we discuss here a possible viscous mechanism for these motions. We report that the flow of vapor below the levitating material is rectified by the asymmetric teeth of the ratchet, in the direction of descending slopes along each tooth. As a consequence, the resulting viscous stress can entrain the material in the same direction, and we discuss the resulting self-propelling force.

Supershear Rayleigh waves at a soft interface.

We report on the experimental observation of waves at a liquid foam surface propagating faster than the bulk shear waves. The existence of such waves has long been debated, but the recent observation of supershear events in a geophysical context has inspired us to search for their existence in a model viscoelastic system. An optimized fast profilometry technique allows us to observe on a liquid foam surface the waves triggered by the impact of a projectile. At high impact velocity, we show that the expected subshear Rayleigh waves are accompanied by faster surface waves that can be identified as supershear Rayleigh waves.

Statics and dynamics of magnetocapillary bonds

When ferromagnetic particles are suspended at an interface under magnetic fields, dipole-dipole interactions compete with capillary attraction. This combination of forces has recently given promising results towards controllable self-assemblies as well as low-Reynolds-number swimming systems. The elementary unit of these assemblies is a pair of particles. Although equilibrium properties of this interaction are well described, the dynamics remain unclear. In this paper, the properties of magnetocapillary bonds are determined by probing them with magnetic perturbations. Two deformation modes are evidenced and discussed. These modes exhibit resonances whose frequencies can be detuned to generate nonreciprocal motion. A model is proposed that can become the basis for elaborate collective behaviors.

Empirical mode decomposition profilometry: small-scale capabilities and comparison to Fourier transform profilometry.

We present the empirical mode decomposition profilometry (EMDP) for the analysis of fringe projection profilometry (FPP) images. It is based on an iterative filter, using empirical mode decomposition, which is free of spatial filtering and adapted for surfaces characterized by a broadband spectrum of deformation. Its performances are compared to Fourier transform profilometry, the benchmark of FPP. We show both numerically and experimentally that using EMDP improves strongly the profilometry small-scale capabilities. Moreover, the height reconstruction distortion is much lower: the reconstructed height field is now both spectrally and statistically accurate. EMDP is thus particularly suited to quantitative experiments.