8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Wed 711:10-14:00P5.133Anistropic diffusion of spindle type hematite particlesaligned in a magnetic fieldMathias Reufer, 1 Peter Schurtenberger, 2 and Wilson Poon 11 University of Edinburgh, Institute for Condensed Matter and Complex Systems,Kings Buildings, JCMB, Mayfield Road EH9 2NG, Edinburgh, United Kingdom2 University of Lund, Lund, SwedenCharacterization of anistropic dynamics as observed in systems consisting of aligned particleswith anisotropic shape or interactions, active particles in a chemical potential, or bacteriaperforming chemotaxis is a notoriously difficult task. It has been reserved to laborious, complex,and expensive techniques such as particle tracking, ultra-small angle light scattering (USALS) orX-ray photon correlation spectroscopy (XPCS). We show that differential dynamic microscopy(DDM) is an excellent tool to access anisotropic dynamics. An ordinary microscope can be usedfor the study of the q-dependent dynamics of colloidal dispersions with particle sizes below theoptical resolution limit. Time series of digital video images are acquired with a camera, andimage differences are Fourier analysed as a function of the time delay between them. So far, thetechnique has been successfully applied to systems with isotropic dynamics such as sphericalcolloids [1] and bacteria swimming in random directions [2]. As a model system we use silicacoated spindle-type hematite particles dispersed in water at low concentration. Without appliedmagnetic field, the particles are randomly orientated and the measured value for the diffusioncoefficient is the mean over the diffusion coefficients (D a , D b , D c ) along the three axis of theparticle. With an applied field the particles align with the major particle axis perpendicular to thefield and the measured dynamics becomes anisotropic reflecting the anisotropy of the individualparticle. The data obtained with DDM is in excellent agreement with a model that allows theextraction of the orientational order parameter S 2 (B), which describes the mean orientation of theparticles with respect to the field.[1] R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008).[2] L. G. Wilson, V. A. Martinez, J. Schwarz-Linek, J. Tailleur, P. N. Pusey, and W. C. K. Poon,Phys. Rev. Lett. 106, 018101 (2011).133