428586 Predicting out-of-Equilibrium Phase Behavior in the Dynamic Self-Assembly of Colloidal Crystals

Monday, November 9, 2015: 3:50 PM
255C (Salt Palace Convention Center)
Zachary Sherman, Massachusetts Institute of Technology, Cambridge, MA and James Swan, Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA

Crystals self-assembled from colloidal particles are useful materials with applications as photonic crystals, microwires, templates for inverse opals and macroporous membrances, lithographic masks, and thermal and chemical sensors. During fabrication however, gelation and glassification often leave these materials arrested in defective or disordered metastable states. We show how time-dependent, pulsed interparticle interactions can avoid kinetic barriers and yield well-ordered crystalline domains for a suspension of hard, spherical colloidal particles interacting through short-range attractions. This dynamic self-assembly process is analogous to the flashing Brownian rachet. Although this is an inherently unsteady, out-of-equilibrium process, we can predict its outcome using appropriate time averages of equilibrium equations of state. While this conclusion holds for arbitrary interparticle potentials, we compute a phase diagram for actuations which switch between two specific interparticle potentials with short-ranged attraction and hard sphere repulsion. The predicted phase behavior is tested and validated by examining the fluid/crystal coexistence of such dynamically self-assembling dispersions in Brownian dynamics simulations of sedimentation equilibrium and homogeneous nucleation. We also show that our dynamic self-assembly scheme offers control and tunability over the crystal growth kinetics and can even stabilize nonequilibrium structures.

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