Entropic bonds, fluid-fluid transitions, identity crises and topological order in hard particle systems
Sharon C. Glotzer · StatPhys 27 Main Conference · 2019
Entropy, information, and order are important concepts relevant for materials to machines, for biology to economics. Entropy is typically associated with disorder; yet, the counterintuitive notion that particles with no interactions other than excluded volume might self-assemble from a fluid phase into an ordered crystal has been known since the mid-20th century. Studies in this current century show that entropy alone can produce order and complexity beyond anything previously imagined. In this talk, we show how the emergent, directional entropic interactions in hard particle systems that produce complex colloidal crystals can be fruitfully considered entropic bonds. We show that, as for molecular liquids and protein solutions, multi-step crystallization pathways are possible for hard particle fluids, involving a purely entropic fluid-fluid phase transition preceding crystallization. We show that when crystallization in hard particle fluids fails and instead glasses form, the underlying mechanism can be understood as an identity crisis in alchemical space. Finally, we expand the library of known topological phases by demonstrating the existence of topological phases in dense colloidal crystals. We show that transitions in dense packings lead to the existence of stable, topologically ordered thermodynamic phases at densities considerably below the putative densest packing limit, potentially providing a way of constructing materials that are robust in the presence of fluctuations.