Magnetic-field-induced structural transitions in a ferrofluid emulsion - art. no. 011403

A ferrofluid emulsion, subjected to a slowly increasing magnetic field, exh ibits a complicated structural behavior: a gas of Brownian particles change s to columnar solid structures due to induced dipole interaction. Two trans ition (intermediate) structural regimes are observed: (i) randomly distribu ted chains and particles and (ii) distinct thin columns and randomly distri buted chains and particles. Three structural transition magnetic fields are found, one marking each structural transition, from the initial to the fin al structural regime. A structural diagram of the structural transition mag netic fields, H-c, versus particle volume fractions, phi, is constructed ex perimentally. Theoretical models of scaling calculations, based upon the do minant magnetic interaction in each structural regime, give the three struc tural transition magnetic-field relations as H-C1 proportional to phi (-1/2 ), H-C2 proportional to phi (-1/4), and H-C3 proportional to (phi (gamma)/G (2))exp(piG/phi ((gamma /2))), where gamma =0.39 and G=0.29 for our sample. The final end shape of columns and the relative position between columns s how that the end-end repulsion between chains is important in the structura l formation.