A ferrofluid is a colloidal suspension of nanometer-sized magnetic particles in a nonmagnetic carrier fluid. As you might expect, it can be easily manipulated with external magnetic fields and often exhibits different patterns and instabilities. For example, when a sufficiently strong magnetic field is applied perpendicular to the flat surface of a ferrofluid, the Rosensweig instability produces a stationary array of peaks protruding above the surface. When a similar field is applied to a ferrofluid droplet immersed in a confined immiscible liquid, the labyrinthine instability produces horizontal fingering as the two fluids interpenetrate. A new experiment reveals a hybrid situation in which those two normally distinct instabilities occur simultaneously. Scientists from Taiwan and Brazil immersed a ferrofluid droplet in a thin layer of a miscible nonmagnetic fluid. The images of the experiment, with a side view in the upper panels and a top view in the lower ones, show what the researchers found after switching on the field. The Rosensweig instability grows rapidly to its greatest amplitude in 0.43 s (left panels), at which time diffusion is already affecting the base of the droplet, decreasing the magnetic body force that sustains the peak against gravity and surface tension. At 1.2 s (middle panels), the peak is clearly decaying as the fingering progresses and after 5 s (right panels) the surface is again flat and radial diffusion dominates. (C.-Y. Chen, W.-K. Tsai, J. A. Miranda, Phys. Rev. E 77, 056306, 2008 [SPIN].)    — Stephen G. Benka