Thermodynamics of the Stockmayer fluid in an applied field

Compressibility of ferrofluids: Towards a better understanding of structural properties

This paper addresses a computational method aimed at obtaining the isothermal compressibility of ferrofluids by means of molecular dynamics (MD) simulations. We model ferrofluids as a system of dipolar soft spheres and carry out MD simulations in the NPT ensemble. The obtained isothermal compressibility computed via volume fluctuations provides us with a strong evidence that dipolar interactions lead to a higher compressibility of dipolar soft sphere systems: the stronger the dipolar interactions, the bigger is the deviation of the compressibility from the one of a system with no dipoles. Furthermore, we use the isothermal compressibility to calculate the structure factor of ferrofluids at low values of wave vectors, i.e. in the range where it is difficult to predict its behaviour because of a problem with accounting for long-range particle correlations that give the main contribution to the structure factor in this range. Our approach based on the interpolation of the structure factor and the computed isothermal compressibility allows us to obtain the smooth structure factor in the range of low wave vectors and the reliable fractal dimension of the clusters formed in the system.

Pressure and compressibility factor of bidisperse magnetic fluids

In this work, we investigate the pressure and compressibility factors of bidisperse magnetic fluids with relatively weak dipolar interactions and different granulometric compositions. In order to study these properties, we employ the method of diagram expansion, taking into account two possible scenarios: (1) dipolar particles repel each other as hard spheres; (2) the polymer shell on the surface of the particles is modelled through a soft-sphere approximation. The theoretical predictions of the pressure and compressibility factors of bidisperse ferrofluids at different granulometric compositions are supported by data obtained by means of molecular dynamics computer simulations, which we also carried out for these systems. Both theory and simulations reveal that the pressure and compressibility factors decrease with growing dipolar correlations in the system, namely with an increasing fraction of large particles. We also demonstrate that even if dipolar interactions are too weak for any self-assembly to take place, the interparticle correlations lead to a qualitative change in the behaviour of the compressibility factors when compared to that of non-dipolar spheres, making the dependence monotonic.

Sedimentation equilibria in polydisperse ferrofluids: critical comparisons between experiment, theory, and computer simulation

The sedimentation equilibrium of dipolar particles in a ferrofluid is studied using experiment, theory, and computer simulation. A theory of the particle-concentration profile in a dipolar hard-sphere fluid is developed, based on the local-density approximation and accurate expressions from a recently introduced logarithmic free energy approach. The theory is tested critically against Monte Carlo simulation results for monodisperse and bidisperse dipolar hard-sphere fluids in homogeneous gravitational fields. In the monodisperse case, the theory is very accurate over broad ranges of gravitational field strength, volume fraction, and dipolar coupling constant. In the bidisperse case, with realistic dipolar coupling constants and compositions, the theory is excellent at low volume fraction, but is slightly inaccurate at high volume fraction in that it does not capture a maximum in the small-particle concentration profile seen in simulations. Possible reasons for this are put forward. Experimental measurements of the magnetic-susceptibility profile in a real ferrofluid are then analysed using the theory. The concentration profile is linked to the susceptibility profile using the second-order modified mean-field theory. It is shown that the experimental results are not consistent with the sample being monodisperse. By introducing polydispersity in the simplest possible way, namely by assuming the system is a binary mixture, almost perfect agreement between theory and experiment is achieved.