Field induced anisotropic cooperativity in a magnetic colloidal glass

The translational dynamics of a repulsive colloidal glass-former is probed by time-resolved X-ray Photon Correlation Spectroscopy. In this dense dispersion of charge-stabilized and magnetic nanoparticles, the interaction potential can be tuned, from quasi-isotropic to anisotropic by applying an external magnetic field. This powerful control parameter finely tunes the anisotropy of the intricate energy landscape in the colloidal glass-former, which is seen here as a new tunable model-system to probe the dynamical heterogeneities at the approach of the glass transition. Both structural and dynamical anisotropies are reported on interparticle lengthscales associated with highly anisotropic cooperativity, almost two orders of magnitude larger in the field direction than in the perpendicular direction and in zero field. The translational dynamics of magnetic nanoparticles in a repulsive colloidal glass is probed by XPCS. Under an external magnetic field, the slight perturbation of the interaction potential induces a highly anisotropic cooperativity of the dynamics, almost hundred-fold larger in the field direction.