Dynamic interference patterns from ferrofluids

Time-dependent interference was observed above a critical magnetic field H-c1 in ferrofluids by a novel technique - magnetic thermal lens effect. The onset of the dynamics is associated with the formation of linear structures in the system. When a second critical field, H-c2, was reached, a lattice of quasi-periodic columns was observed by optical microscopy and random rotating patterns were replaced by stable rings, indicating ordering in the system. This technique provides a useful way to detect microstructures that are much smaller than the optical wave length thus unobservable by conventional optical technique. We found that the result crucially depends on boundary conditions.