Surface Waves and Wave Resistance in Magnetic Fluids

We present here the experimental determination of the dispersion relation of surface waves of a ferro uid submitted to a steady vertical magnetic eld. The results are compared to a linear theory where all the magnetic characteristics of the uid have been taken into account. The knowledge of the dispersion equation is then used to analyse the capillary-wave resistance, that is the drag force associated to the emission of waves by a moving disturbance at a free uid surface. It undergoes a transition from zero to a nite value as the speed of the disturbance reaches a certain critical value. The e ect of viscosity is explored, and a magnetic uid is shown to allow controlling the critical speeds. Contrary to the theoretical model, however, the measured wave resistance reveals a non monotonic speed dependence after the threshold.