Curved colloidal crystals of discoids at near-critical liquid-liquid interface.

The assembly of disc-shaped particles at curved liquid-liquid interfaces was studied by using confocal microscopy. The interface is formed by a phase-separating critical liquid mixture of 2,6-lutidine and heavy water, where the colloids spontaneously assembled forming a dome. The novelty of this system is three-fold. First, the domes can be constructed and annihilated remotely and reversibly, which allows dynamic control of the colloidal assembly. Second, the effect of curvature can be investigated by analyzing domes of different radii ranging from 5 µm to 125 µm. Third, the slow dynamics due to hydrodynamic interaction among the particles can be utilized to investigate the time-evolution of defect morphology. Unlike the widely studied repulsive colloids, the interparticle potential near the critical point has an attractive component. I contrasted the packing and defects morphology of a solid-like and liquid-like dome differing in particle number density. In the solid-like dome, a chain of 5- and 7-fold coordinated particles was observed. The analysis of trajectories showed that particles were bound in a potential well of a depth of about ten times the thermal energy, which matched well with the calculation of the pair-potential by considering the attractive critical Casimir force among the particles. In the liquid-like dome, 6-fold particles separated by clusters of 5- and 7-coordinated particles were observed, which is suggestive of liquid-solid coexistence. The uniqueness of this system will open up a new research avenue to investigate the effect of varying curvature on the crystallization, defects, and phase diagram of colloidal assemblies.