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Hierarchical organization of chiral rafts in colloidal membranes

Liquid-liquid phase separation is ubiquitous in suspensions of nanoparticles, proteins and colloids. With a few notable exceptions, surface-tension-minimizing liquid droplets in bulk suspensions continuously coalesce, increasing in size without bound until achieving macroscale phase separation. In comparison, the phase behavior of colloids, nanoparticles or proteins confined to interfaces, surfaces or membranes is significantly more complex. Inclusions distort the local interface structure leading to interactions that are fundamentally different from the well-studied interactions mediated by isotropic solvents. Here, we investigate liquid-liquid phase separation in monolayer membranes composed of dissimilar chiral colloidal rods. We demonstrate that colloidal rafts are a ubiquitous feature of binary colloidal membranes. We measure the raft free energy landscape by visualizing its assembly kinetics. Subsequently, we quantify repulsive raft-raft interactions and relate them to directly imaged raft-induced membrane distortions, demonstrating that particle chirality plays a key role in this microphase separation. At high densities, rafts assemble into cluster crystals which constantly exchange monomeric rods with the background reservoir to maintain a self-limited size. Lastly, we demonstrate that rafts can form bonds to assemble into higher-order supra-structures. Our work demonstrates that membrane-mediated liquid-liquid phase separation can be fundamentally different from the well-characterized behavior of bulk liquids. It outlines a robust membrane-based pathway for assembly of monodisperse liquid clusters which is complementary to existing methods which take place in bulk suspensions. Finally, it reveals that chiral inclusions in membranes acquire long-ranged repulsive interactions, which might play a role in stabilizing assemblages of finite size.