Non-equilibrium thermodynamics of magnetic colloids with tunable dipolar interactions

Abstract

We describe the stochastic rotational dynamics and magnetization curves of a suspension of interacting magnetic colloidal particles of a ferrofluid under an external magnetic field. The orientation of the magnetic dipole moments of these particles is considered as a fluctuating variable, so within the framework of mesoscopic irreversible thermodynamics, we derive an equivalent Fokker–Planck equation (FPe), which gives us the orientational distribution function (Odf) of the magnetic dipole moments of these particles. The numerical solution of the FPe allows us to observe the temporal evolution of the orientation of the magnetic dipoles to the equilibrium limit, once an external magnetic field is applied to the ferrofluid. We observe that the dipoles orient better in the direction of the field when the particles are bigger or the temperature of the solvent is decreased. Decreasing the average distance between the magnetic particles does not give us an important improvement in their orientation with respect to the external magnetic field. Finally, with the solution of the FPe and the partition function, we determine an average magnetic dipole moment of the particles and, changing the magnetic field applied to the ferrofluid, we obtain a set of magnetization curves which fits to experimental results by tuning the interactions among the particles. These results are useful for different applications of magnetizable composite particles.