Dynamics of the Field-Induced Formation of Hexagonal Zipped-Chain Superstructures in Magnetic Colloids

Combining nuclear magnetic resonance and molecular dynamics simulations, we unravel the long-time dynamics of a paradigmatic colloid with strong dipole-dipole interactions. In a homogeneous magnetic field, ionic ferrofluids exhibit a stepwise association process from ensembles of monomers over stringlike chains to bundles of hexagonal zipped-chain patches. We demonstrate that attractive van der Waals interactions due to charge-density fluctuations in the magnetic particles play the key role for the dynamical stabilization of the hexagonal superstructures against thermal dissociation. Our results give insight into the dynamics of self-organization in systems dominated by dipolar interactions.

Figure 1

(a) Representative NMR spectra of the ionic ferrofluid sample with a concentration of 1 vol % measured at the beginning of a sequence (solid circles). Solid (blue) and dashed (black) curves represent the total NMR line shape function and individual peak terms, respectively. The (red) curve is the NMR spectrum of pure water. (b) Dependence on time of the amplitude and position obtained by fitting the line shape function $S_R(\nu )$ to the main peak of each NMR spectrum of the IFF measured every 30 s.

Figure 2

MD simulation snapshots (insets) at various times on the way towards equilibration. Full (red) and open (blue) symbols represent the quasi-instantaneous correlation functions parallel and perpendicular to the field, respectively.

Figure 3

Time dependence of the first maximum of the parallel (red) and perpendicular (blue) correlation functions (normalized to their equilibrium values). Vertical lines indicate the times corresponding to Figs. 2a, 2b. Results for the system without vdW interactions (black and gray symbols) are also included.