Magnetic colloid pattern formation at magnetic field induced phase separation

A. Cebers

Institute of Physics, University of Latvia, Salaspils-1, LV-2169, Latvia

Abstract
Magnetic field induced phase separation of the magnetic colloid in the Hele-Shaw cell is considered. Phase diagramm of the magnetic colloid in dependence on the thickness of the Hele-Shaw cell and deviation from the critical point is derived which shows that transition to the ordered phase - the periodic system of the stripes is continuous for the magnetic Bond number greater as critical one. Equations which describe the kinetics of the phase separation under the action of the magnetic field of the different orientation with the respect to the cell among which it is possible to mention normal, tangential, rotating in the plane normal to the boundaries of the cell, oblique and conical are derived. By numerical simulation different pattern formation phenomena are studied and the main points to be mentioned are the following: under the action of the normal field in dependence on the deviation from the critical point labyrinthine or hexagonal patterns are formed, under the action of the tangential field the system of the parallel stripes is formed, under the action of the magnetic field rotating in the plane normal to the boundaries of the Hele-Shaw cell the periodic system of the parallel stripes is formed. Different structure transformation processes are observed at the change of the magnetic field orientation and its value - under the action of the normal magnetic field with the magnetic Bond number greater as the magnetic Bond number of the rotating field on the system of the parallel stripes formed under the action of the rotating field the development of the chevrons is observed, the chevron pattern under the action of the magnetic field with greater value of magnetic Bond number undergoes secondary instability leading to the formation of the hairpins. Under the action of the oblique field on the hexagonal pattern the formation of the system of the parallel stripes if the inclination angle of the field is enough large is observed. During this process of the structure transformation the formation of the dislocations and their migration is observed. Figs 15, Refs 32.