PERIODIC STRUCTURES AND SUBSTRUCTURES IN MAGNETIC SUSPENSIONS

We investigate the phase transitions induced by external fields in mag netic colloidal suspensions. We report video microscopy experiments sh owing the onset of unidimensional phase-segregated patterns composed o f self-similar periodic structures induced by a pulsed magnetic field. Similar periodic patterns are shown to occur in the presence of combi ned shear flow and dc magnetic fields. The wavevector of these pattern s can be tuned by the shear viscosity or the frequency of the pulsed f ield.