The Polymer/Colloid Duality of Microgel Suspensions

Colloidal dispersions have been studied for decades as a result of their utility in numerous applications and as models for molecular and atomic condensed phases. More recently, a number of groups have exploited in such studies submicrometer-sized hydrogel particles (microgels) that have environmentally tunable sizes. The experimental convenience of tuning the dispersion's colloidal volume fraction while maintaining a constant number density of particles provides a clear advantage over more tedious studies that employ traditional hard-sphere particles. However, as studies delved deeper into the fundamental physics of colloidal dispersions comprising microgel particles, it became abundantly clear that a microgel's utility as a tunable hard sphere was limited and that the impact of softness was more profound than previously appreciated. Herein we review the brief history of microgel-based colloidal dispersions and discuss their transition from tunable hard spheres to a class of soft matter that has revealed a landscape of physics and chemistry notable for its extraordinary richness and diversity.