Parametrically forced surface wave with a nonmonotonic dispersion relation.
Surface wave patterns that arise in a mechanically driven ferrofluid system under constant magnetic field are investigated (1) to find out what kind of spatial patterns emerge when the system acquires a nonmonotonic dispersion relation and (2) to compare its surface wave patterns with those produced in the magnetically driven system studied earlier. As the strength of the applied magnetic field increases, the initial subharmonic square lattice formed by the Faraday instability first transforms to rolls, then becomes a rhomboid lattice. The rolls and the rhomboid lattice are found to coexist for a finite range of parameter space forming patterns with mixed domains. Possible underlying mechanisms for the observed rhomboid lattice is discussed. None of the diverse superlattices observed in the magnetically driven ferrofluid system appears in the mechanically driven system studied here.