Temporal evolution of equilibrium and non-equilibrium magnetic field driven microstructures in a magnetic fluid

Abstract

Abstract We investigate the temporal evolution of magnetic field driven microstructures in a magnetic fluid under different experimental conditions such as volume fraction, surfactant concentration, magnetic field strength and ramp rate. The transmitted light intensity and nature of micro-nano structures were evaluated in an oil-in-water emulsion of average diameter ~200 nm, containing oleic acid capped Fe3O4 magnetic nanoparticles of average diameter ~10 nm. For low volume fractions and applied magnetic fields, the transmitted intensity showed a rapid decrease, followed by an increase with elapsed time while at high field strengths transmitted intensity was low and time independent. The critical magnetic field followed a power law decay with the volume fraction. The phase contrast microscopy results show very fast lateral movement of well separated chains and interconnected frozen disordered non-equilibrium structures at low and high magnetic fields, respectively. These results reveal that the competition between surface energy and dipolar energy is the key to the non-equilibrium structure formation. When the average distance among chains is greater than the escape distance, a fewer chains coalesce to form individual chains. At high field strengths, more chains and complex patterns are formed within the escape distance, while at low field strengths, the average inter chain spacing increased with time and followed a power law with time. Our findings are useful for better control on the magneto-optical response of magnetic fluid based devices.