Colloidal Suspensions of Rodlike Nanocrystals and Magnetic Spheres under an External Magnetic Stimulus: Experiment and Molecular Dynamics Simulation

Abstract

Using experiments and molecular dynamics simulations, we explore magnetic field-induced phase transformations in suspensions of nonmagnetic rodlike and magnetic sphere-shaped particles. We experimentally demonstrate that an external uniform magnetic field causes the formation of small, stable clusters of magnetic particles that, in turn, induce and control the orientational order of the nonmagnetic subphase. Optical birefringence was studied as a function of the magnetic field and the volume fractions of each particle type. Steric transfer of the orientational order was investigated by molecular dynamics (MD) simulations; the results are in qualitative agreement with the experimental observations. By reproducing the general experimental trends, the MD simulation offers a cohesive bottom-up interpretation of the physical behavior of such systems, and it can also be regarded as a guide for further experimental research.

Orientational order and translational dynamics of magnetic particle assemblies in liquid crystals

“…Still, our model provides a realistic description of other experimental systems. For example, we have recently used our model to interpret the response of a real system 20 -consisting of LC pigment nanorods (of typical width of 40 nm) and magnetic spheres (of diameter 10 nm)-under an external magnetic field.…”

“…We have chosen this value since (i) corresponds to a well studied mono-dispersed GB system, 53 and (ii) it is in the accessible limit, though in the lowest, of experimentally investigated real rodlike liquid crystalline colloidal suspensions. 19,20 The DSS particles interact via a soft repulsive potential and a dipole-dipole interaction. 22,23 The diameter of the DSS particles is set s s * = s s /s 0 = 0.25 (i.e.…”

“…22,23 The diameter of the DSS particles is set s s * = s s /s 0 = 0.25 (i.e. it is four times smaller than the width of the rods comparable with that in corresponding experimental systems 19,20 ). A systematic examination of mixtures with spheres diameter equal or greater than the width of the rods has been reported elsewhere.…”

“…in ref. 20) of magnetic particles in LC matrices. The fact that the orientational state of the matrix (isotropic versus nematic) does not significantly change the cluster types and their size distributions suggests, on the one hand, that the (free) energy landscape determining the cluster formation is similar to the case of pure DSS particles (without the matrix).…”

“…Indeed, the global order parameter S (s) of the DSS particles can not measure the orientational order of the clusters in the LC matrix. Below, we describe the method we have used 20 to monitor the orientational order of the clusters (see also ESI, † Section I): (i) initially, we calculate the nematic director (n r ) for the entire system, (ii) secondly, we construct the ordering matrix of each cluster and from the extracted eigenvalues S A plot of the S cl b order parameters at various temperatures in the N u and SmB phase for r* = 0.44 is given in Fig. 5.…”