  Suspensions of colloidal microplates in contoured 2D elastic fluids sheets
are dominated by the bending mechanics and shear rigidity of the plates and the
contrasting in-plane shear flow of the 2D fluid. Using the phase separated
phospholipid membranes of individual giant unilamellar vesicles as models of
contoured 2D suspensions, where solid domains act as colloids in a fluid
membrane, we explore bending elasticity-driven assembly. The plate-shaped
domains are varied between 1-10 {\mu}m in diameter, with 4-100 plates per
vesicle depending on size, contributing a solid area of 17 plus minus 3%. Three
classes of reversible plate arrangements evidence inter-plate attractions and
repulsions: persistent hexagonal vesicle-encompassing quasi-lattices,
persistent closely associated configurations (chains or concentrated lattices),
and a dynamic disordered state. The vesicle-encompassing quasi-lattice is
stable to vesicle dehydration by 30% relative to an inflated sphere. Excess
area or membrane slack, for a fixed composition, dominates the preferred
configuration while domain size and number contribute pattern intricacy.
Different from the gradual variations in domain interactions and tunable
positions in two-colloid systems, multibody interactions vary sharply within a
particular range of excess area, producing cooperative assembly reminiscent of
a phase transition.
