(299c) Multi-Scale Kinetics of a Field-Directed Colloidal Phase Transition

Suspensions of polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. The accomanying fluid-solid transition makes magneto-rheological (MR) fluids useful as shock absorbers, while electro-rheological (ER) fluids can be used as haptic controllers and tactile displays in micro-electronics devices. However, kinetic arrest ultimately prevents the formation of equilibrium phases encoded by the colloidal interactions or particle shape. In this work, we demonstrate that, by toggling the field strength at varied frequencies, such gels formed from MR fluids can be annealed. This allows the arrested structure to relax periodically to equilibrium. There is a stark boundary as a function of magnetic field strength and toggle frequency distinguishing percolation from phase separation. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. Such directed assembly may be harnessed to create novel materials from dispersions of colloids and nanoparticles.