Anomalous Dynamics of Magnetic Anisotropic Colloids Studied by XPCS

Abstract

The influence of an applied magnetic field on the collective dynamics of novel anisotropic colloidal particles whose shape resembles peanuts is reported. Being made up of hematite cores and silica shells, these micrometer-sized particles align in a direction perpendicular to the applied external magnetic field, and assemble into chains along the field direction. The anisotropic dynamics of these particles is investigated using multispeckle ultrasmall-angle X-ray photon correlation spectroscopy (USA-XPCS). The results indicate that along the direction of the magnetic field, the particle dynamics strongly depends on the length scale probed. Here, the relaxation of the intermediate scattering function follows a compressed exponential behavior at large distances, while it appears diffusive at distances comparable or smaller than the particle size. Perpendicular to the applied field (and along the direction of gravity), the experimental data can be quantitatively reproduced by a combination of an advective term originating from sedimentation and a purely diffusive one that describes the thermal diffusion of the assembled chains and individual particles.

Observation of liquid glass in suspensions of ellipsoidal colloids

“…These predictions also have implications for molecular liquid crystalline materials ( 25 , 26 ). However, the few investigations of glass-forming suspensions of ellipsoids have all been focused on static structures obtained by driving particles in external gravitational or electric fields ( 27 – 30 ). The only investigations on steric effects of ellipsoid shape have been performed in two-dimensional (2D) films ( 26 , 31 – 33 ).…”

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…This enabled the measurement of velocity fluctuations at the early stage of sedimentation in brownian colloidal suspensions [110]. Similar approach is required for probing the anisotropic dynamics in an applied magnetic field [106,109] and in confined geometry of microfluidic channels [111]. In these cases, the measured microstructure and dynamics display strong anisotropy.…”

“…A common feature of these systems is hyperdiffusive dynamics characterized by a compressed exponential decay of g 2 (q, t) [9,105]. Although such a non-diffusive behavior could arise due to various reasons, a generally assumed mechanism is the presence of dynamic heterogeneities and associated internal stress relaxations [105,109].…”

Potential and limits of a colloid approach to protein solutions

“…It is only through the use of recently introduced techniques such as differential dynamic microscopy or X-ray photon correlation spectroscopy that these properties have become accessible at high concentrations close to the glass transition for these model ellipsoids also. 63,79 During the last 2 decades, enormous experimental and theoretical effort has also been devoted to the design and synthesis and to the understanding of the phase behaviour of so-called patchy colloidal particles, i.e. particles with site-specifically engineered surfaces (see Fig.…”