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

Philip Camp, Alexander F. Pshenichnikov, Alexey O. Ivanov, Ekaterina Alexandrovna Elfimova

Research output: Contribution to journalArticlepeer-review

Abstract

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.
Original languageEnglish
JournalSoft Matter
DOIs
Publication statusPublished - 29 Mar 2016

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