Field-induced clearing in sphere-sphere and rod-sphere binary mixtures of charged colloidal particles

Binary mixtures of larger and smaller colloids having charges of equal sign display unusual response to low frequency electric fields E. We previously reported the anomalous orientation - perpendicular to the electric field - of highly diluted rod-like particles when immersed in a sea of smaller spherical particles charged with same sign [1]. A correlated but more general phenomenon is observed in mixtures of large (dilute) and small (semi-dilute) spherical particles, whose total scattering cross section for light decreases as a low frequency electric field is applied to the dispersion [2]. Such field-induced clearing is general for mixtures and appears to be due to the build up of an anisotropic E2-dependent distribution of small spheres around the large ones. Having ruled out other mechanisms, we interpret this behavior as resulting from O(E2) electro-osmotic flows whose magnitude is greatly enhanced by the presence of the small particles. Various evidences suggest that this structuring, unaccountable on the only basis of dipole-dipole electric interaction, is at the origin of the anomalous torque in mixtures of rods and spheres. Overall, this work strengthens the notion that non-linear electrokinetic response is an essential part in the description of the electro-optic behavior of complex colloidal dispersions. 1 - F. Mantegazza, M. Caggioni, M.L. Jimenez and T. Bellini, Anomalous field-induced particle orientation in dilute mixtures of charged rod-like and spherical colloids, Nature Physics, 1, 103 (2005) 2 - L. Fornasari, F. Mantegazza, M. L. Jimenez, M. Buscaglia, and T. Bellini, Field-induced clearing in sphere-sphere and rod-sphere binary mixtures of charged colloidal particles, Physical Review E 79, 060401R (2009)