Ferrofluidic torsional pendulum driven by oscillating magnetic field.

A thin disk-shaped container filled with a ferrofluid and suspended in a horizontal linearly polarized ac magnetic field can perform torsional vibrations around its vertical diameter. In contrast to a recently studied spherical pendulum, the cell is sensitive to the field direction: It exposes its edge to the stationary or slowly oscillating magnetic field, and its broad side to the field of a high frequency. When the amplitude of the latter field is increased, the state of rest gets destabilized, yielding to oscillations near the equilibrium.

Onset of convection in colloids stratified by gravity.

Settling of the equilibrium (barometric) distribution of colloidal grains in a liquid requires a very long time in comparison with that necessary for temperature equilibrium establishment. This mismatch enables different scenarios for the onset of thermal convection. If the stratified state has had no time to be settled, the colloid behaves as a pure fluid where only stationary convection arises.

Nonlinear dynamics of a ferrofluid pendulum.

A ferrofluid torsion pendulum in an oscillating magnetic field exhibits a rich variety of nonlinear self-oscillatory regimes. The dynamics is governed by the system of coupled differential equations for the in- and off-axis components of the fluid magnetization and the pendulum angular deflection. In the limiting case of high driving frequency, the system reduces to the sole Rayleigh-type equation.

Comment on "Structure of ferrofluid dynamics".

We compare known equations for ferrofluid magnetization with a recently proposed rectified Debye theory [H. W. Müller and M.