## Abstract / Description of output

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 hardsphere

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.

and computer simulation. A theory of the particle-concentration profile in a dipolar hardsphere

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.

Original language | English |
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Journal | Soft Matter |

DOIs | |

Publication status | Published - 29 Mar 2016 |