Hierarchical organization of chiral rafts in colloidal membranes

Inclusions dissolved in a colloidal membrane assemble into highly uniform finite-sized liquid droplets or rafts consisting of thousands of molecules. The separation of suspensions into distinct liquid phases is important in processes such as gel formation, protein crystallization and in processes in cellular biology. Liquid–liquid phase separation in bulk proceeds through continuous coalescence of droplets until the system completely phase separates. But things get more complicated when suspended colloids, nanoparticles or proteins are confined to interfaces or membranes. Prerna Sharma et al. now show that liquid–liquid phase separation in monolayer membranes composed of two dissimilar chiral colloidal rods yields stable rafts that assemble into cluster crystals at high densities, but also form bonds to create higher-order structures. Detailed observations reveal that membrane distortions arising from the rods' chirality are crucial for stabilizing and assembling the monodisperse membrane clusters, suggesting that chiral inclusions in membranes might more generally have a role in stabilizing assemblages of finite size. Liquid–liquid phase separation is ubiquitous in suspensions of nanoparticles, proteins and colloids. It has an important role in gel formation, 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 fundamentally from those mediated by isotropic solvents5,6, and this results in significantly more complex phase behaviour7,8,9,10,11,12,13. Here we show that liquid–liquid phase separation in monolayer membranes composed of two dissimilar chiral colloidal rods gives rise to thermodynamically stable rafts that constantly exchange monomeric rods with the background reservoir to maintain a self-limited size. We visualize 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 demonstrate a robust membrane-based pathway for the assembly of monodisperse membrane clusters that is complementary to existing methods for colloid assembly in bulk suspensions14,15,16. 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.