Anomalous field-induced particle orientation in dilute mixtures of charged rod-like and spherical colloids

Charged colloids in aqueous suspension often reconfigure counter-intuitively in the presence of an external electric field(1-7). Despite decades of substantial theoretical effort, a clear understanding of such electrokinetic phenomena is only available for the electric polarization and migration of diluted spheres in low fields(8). The interpretation of the electric response of charged rod-like particles is far less advanced. Here we report that diluted rod-like particles suspended in a sea of smaller spherical particles having similar charge align perpendicular to the external electric field. This 'anomalous' orientation, which contradicts the notion that charged rods in dilute suspensions always align parallel to external electric fields(9), seems to be universal in dilute mixtures of charged rod-like and spherical colloids. The dependence of the perpendicular orientation on frequency and particle charge, size and concentration indicates the presence of field-induced asymmetric crowding of spherical particles around each rod, which gives rise to asymmetric electro-osmotic flows that induce a negative torque.