Despite their broadly recognized utility in optical and catalytic devices, globally ordered colloidal crystal lattices remain challenging to assemble. Complex, hierarchical free energy landscapes, replete with small differences between local minima, often characterize ensembles of these structures. This makes it difficult to produce a single desired morphology without defects. Currently this problem is surmounted via either the “top-down”, or “bottom-up” methodology whereby structures are engineered starting from the largest, or smallest relevant length scales, respectively. The ubiquitous design strategy in such approaches is energy minimization; many colloids have been computationally engineered with anisotropic pairwise interactions to achieve morphological control in theory. However, the complexity of these designs often makes experimental realization difficult. I will show how extensive computer simulations reveal that the introduction of polymeric co-solutes into crystallizing colloidal suspensions can be used to intelligently direct the resulting nano- and mesoscale colloidal structures by relying upon the polymer's entropic interactions alone. [1-4] These entropic interactions result entirely from the interplay between the polymer's internal degrees of freedom and the void structure of a material. This represents a novel third design paradigm that has the potential to significantly simplify control over colloidal polymorphism. I will elaborate on how to rationally design the co-solute structure to thermodynamically stabilize a single desired polymorph in a binary mixture, and the consequences that thermal perturbations have on this effect.  I will also offer insights into how to design temperature-dependent co-solute “switches” that allow the stability of a polymorph to be controlled via experimentally accessible parameters.  As a whole, this work represents a novel entropic route to polymorphic control that has not yet been explored.
 Mahynski, Panagiotopoulos, Meng, and Kumar, Nature Communications 5, 4472 (2014).
 Mahynski, Kumar, and Panagiotopoulos, Soft Matter 11, 280-289 (2015).
 Mahynski, Kumar, and Panagiotopoulos, Soft Matter 11, 5146-5153 (2015).
 Mahynski, submitted (2016).