Hierarchical organization of chiral rafts in colloidal membranes

Abstract

Liquid–liquid phase separation is ubiquitous in suspensions of nano- particles, proteins and colloids. It has an important role in gel for- mation, protein crystallization and perhaps even as an organizing principle in cellular biology1,2. With a few notable exceptions3,4, liquid– liquid phase separation in bulk proceeds through the continuous coalescence of droplets until the system undergoes complete phase separation. But when colloids, nanoparticles or proteins are confined to interfaces, surfaces or membranes, their interactions differ fun- damentally from those mediated by isotropic solvents5,6, and this results in significantly more complex phase behaviour7–13. Here we show that liquid–liquid phase separation in monolayer membranes composed of two dissimilar chiral colloidal rods gives rise to thermo- dynamically stable rafts that constantly exchange monomeric rods with the background reservoir to maintain a self-limited size. We visu- alize and manipulate rafts to quantify their assembly kinetics and to show that membrane distortions arising from the rods’ chirality lead to long-range repulsive raft–raft interactions. Rafts assemble into cluster crystals at high densities, but they can also form bonds to yield higher-order structures. Taken together, our observations demon- strate a robust membrane-based pathway for the assembly of mono- disperse membrane clusters that is complementary to existing methods for colloid assembly in bulk suspensions14–16. They also reveal that chiral inclusions in membranes can acquire long-range repulsive interactions, which might more generally have a role in stabilizing assemblages of finite size13,17. Nature 513, 77 (2014). doi:10.1038/nature13694