The present work investigates free damped oscillations of an oil drop in water after its release from a capillary tube. Both pure heptane drops and diluted crude oil drops are considered (in the second case the interface is covered by amphiphilic spe
cies, natural components of crude oil). Shadowgraph images of the drops are taken by means of a high speed camera and the drop contour is detected by image processing. The axisymmetric drop shape is then decomposed into spherical harmonics, which constitute the eigenmodes of oscillations predicted by the Rayleigh-Lamb theory. Time evolution of each mode is then obtained. The frequency and the damping rate of the principal mode (n=2) are accurately determined and compared with theoretical values for an immobile clean drop oscillating around spherical shape. For pure heptane drops, theoretical value of the frequency agrees well with experiments whereas the damping rate is significantly underestimated by theory. The experimental results clearly show that the different modes are coupled. Energy is thus transfered from mode n=2 to n=3, which probably explains the observed enhancement of the damping rate. The effect of the interface viscoelastic behaviour, induced by adsorbed amphiphilic species on the free oscillations was examined. No significant effect was observed in the experiments conditions (small amplitude oscillations and moderate aging).
We study the flow response to an externally imposed homogeneous magnetic field in a turbulent swirling flow of liquid sodium -- the VKS2 experiment in which magnetic Reynolds numbers Rm up to 50 are reached. Induction effects are larger than in the f
ormer VKS1 experiment. At Rm larger than about 25, the local amplitude of induced field components supersedes that of the applied field, and exhibits non-Gaussian fluctuations. Slow dynamical instationarities and low-frequency bimodal dynamics are observed in the induction, presumably tracing back to large scale fluctuations in the hydrodynamic flow.
