Designing Rapid Phase Transitions and Shape-Shifting Materials Using Patterned Colloids

The recent development of methods of conferring well-defined shape and interaction anisotropies to Brownian colloidal particles renders their phase behaviour and dynamics closer to that of molecules, but on greatly expanded spatial and temporal scales.  We focus here especially on interaction anisotropy, which is present in the simplest form in Janus spheres with an attractive and a repulsive.   We determine the phase diagram of such spheres by Brownian dynamics simulations using various ensembles, revealing a fluid phase and two solid phases, in one of which the particle orientation is disordered, forming a “rotator” phase, while in the other, the spheres orient spontaneously into a lamellar phase. We determine that in addition to reduced temperature, aniostropic pressure influences the phase diagram and we use special boundary conditions to control both pressure and its anisotropy.  We thereby find that the rotator-to-lamellar solid-solid transition to be controllable by small changes in pressure anisotropy, with negligible changes in system volume. The solid-to-solid phase transition is very rapid and nearly hysteresis free, and modelled well by the Maier-Saupe theory devised initially for isotropic-nematic transitions in liquid crystal molecules.  From trajectory analysis, we also determine the nucleation rate, critical cluster size and shape for the rotator-lamellar transition. We also compare results to those for “lock-and-key” particles with shape anisotropy, and summarize the implications for future work.