Instabilities of concentration stripe patterns in ferrocolloids

Equations describing the kinetics of the phase separation in ferrocolloids in a Hele-Shaw cell under the action of a rotating magnetic field are propo sed. Numerical simulation on the basis of a pseudospectral technique demons trates that upon the action of a rotating field on a magnetic colloid which undergoes the phase separation a periodical system of stripes parallel to the plane of a rotating magnetic field stripes is created. The period of a structure found numerically satisfactorily corresponds to the one calculate d on the basis of the energy minimum. Thus, the undulation instability lead ing to the formation of chevron structures takes place if the tangential co mponent of a rotating magnetic field is eliminated, whereas the normal comp onent is increased at the same time. If during the development of the undul ation deformations of a concentration pattern the magnetic Bond number is l arge enough the secondary instabilities may occur leading to the fingering of stripes to bring about merging and break-up of stripes. It is shown that an increase in the magnetic Bond number leads to the onset of the instabil ity at the boundaries between the regions with homogeneous orientation of s tripes as well as to formation of the characteristic hairpin patterns.