Super-lattice, rhombus, square, and hexagonal standing waves in   magnetically driven ferrofluid surface
  Standing wave patterns that arise on the surface of ferrofluids by (single frequency) parametric forcing with an ac magnetic field are investigated experimentally. Depending on the frequency and amplitude of the forcing, the system exhibits various patterns including a superlattice and subharmonic rhombuses as well as conventional harmonic hexagons and subharmonic squares. The superlattice arises in a bicritical situation where harmonic and subharmonic modes collide. The rhombic pattern arises due to the non-monotonic dispersion relation of a ferrofluid. 
Hyun-Jae Pi, So-yeon Park, Jysoo Lee, and Kyoung J. Lee (Department of   Physics, Korea University)