  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.
