Non-Magnetic Solid Body in Ferrofluid Containers: Wall Effects

Experimental and numerical investigations of a magnetic ponderomotive force acting on non-magnetic body immersed in ferrofluid were carried out. The study was performed on a nonmagnetic sphere in a cylindrical container filled with the magnetic fluid and magnetized by a uniform magnetic field. The symmetry of the experimental setup allowed the simplification of the problem: 2D axisymmetric numerical simulation corresponded to the 1D experimental measurements. The magnetic ponderomotive force turned out to be non-monotonic: it has two extrema and one or three zero values depending on the geometrical parameters of the container and magnitude of the applied field intensity. It was shown that wall effects are crucial for this problem, because the ponderomotive force in the vicinity of the container's bottom (or top) may change its sign (direction). This behaviour can be described only in the framework of the inductive approach, which takes into account all demagnetizing fields generated by the ferrofluid container. On the contrary, the simplified non-inductive approach is unable to explain the magnetic force behaviour, especially the wall effects. (paper)