(520e) Predicting out-of-Equilibrium Phase Behavior in Dynamic Self-Assembly of Colloidal Crystals

Crystals self-assembled from colloidal particles have useful properties such as optical activity and sensing capability. During fabrication however, gelation and glassification often leave these materials arrested in defective or disordered metastable states. We show how carefully tuned, periodically varying inter-particle interactions can be used to avoid kinetic barriers during self-assembly of well-ordered crystalline domains from a suspension of hard, spherical colloids interacting through a short-ranged attraction. The time-variation investigated is a simple periodic switching of the inter-particle interaction between an attractive state and a purely repulsive state and is akin to a flashing Brownian ratchet.  Although this is an inherently unsteady, out-of-equilibrium process, we observe a terminal, periodic-steady-state during Brownian dynamics simulations in which a condensed and ordered phase of particles coexists with a dilute one.  We hypothesize that appropriate time averages of equilibrium equations of state can be used to explain this coexistence. This hypothesis is tested and validated by Brownian dynamics simulations of sedimentation equilibrium and homogeneous nucleation in the dynamically self-assembling suspension.  We also show how this dynamic self-assembly process offers control and tunability over the crystal growth kinetics.  By accessing a self-assembly pathway leading to a dissipative terminal state, the rates of crystal growth and defect formation can be decoupled from typical thermodynamic constraints.