Hydrodynamic interactions in colloidal ferrofluids: A lattice Boltzmann study

TL;DR

This study uses lattice Boltzmann simulations to explore how hydrodynamic interactions influence the dynamics and structure of colloidal ferrofluids, revealing their significant impact on decay rates and cluster formation.

Contribution

It provides a detailed quantitative analysis of hydrodynamic effects in dipolar colloidal suspensions using advanced simulation techniques, comparing with Brownian dynamics and Monte Carlo methods.

Findings

Hydrodynamic interactions slow decay of scattering functions by about twofold.

Hydrodynamics reduce cluster formation rate by roughly twofold.

Short-time hydrodynamic effects are less affected by dipolar interactions.

Abstract

We use lattice Boltzmann simulations, in conjunction with Ewald summation methods, to investigate the role of hydrodynamic interactions in colloidal suspensions of dipolar particles, such as ferrofluids. Our work addresses volume fractions $\phi$ of up to 0.20 and dimensionless dipolar interaction parameters $\lambda$ of up to 8. We compare quantitatively with Brownian dynamics simulations, in which many-body hydrodynamic interactions are absent. Monte Carlo data are also used to check the accuracy of static properties measured with the lattice Boltzmann technique. At equilibrium, hydrodynamic interactions slow down both the long-time and the short-time decays of the intermediate scattering function $S(q,t)$, for wavevectors close to the peak of the static structure factor $S(q)$, by a factor of roughly two. The long-time slowing is diminished at high interaction strengths whereas the short-time slowing (quantified via the hydrodynamic factor $H(q)$) is less affected by the dipolar interactions, despite their strong effect on the pair distribution function arising from cluster formation. Cluster formation is also studied in transient data following a quench from $\lambda = 0$; hydrodynamic interactions slow the formation rate, again by a factor of roughly two.