Measuring the deformation of a ferrogel sphere in a homogeneous magnetic field.

A sphere of a ferrogel is exposed to a homogeneous magnetic field. In accordance to theoretical predictions, it gets elongated along the field lines. The time dependence of the elastic shear modulus causes the elongation to increase with time, similar to mechanic creep experiments, and the rapid excitation causes the sphere to vibrate. Both phenomena can be well described by a damped harmonic oscillator model. By comparing the elongation along the field to the contraction perpendicular to it, we can calculate Poisson's ratio of the gel. The magnitude of the elongation is compared to the theoretical predictions for elastic spheres in homogeneous fields.