Interfacial polymer phase segregation and self-assembly of square colloidal crystals

Polystyrene colloids exhibit unique bimodal dynamics at the air–water interface in the presence of PEG polymers. We attribute this to agglomeration of the polymer into polymer-rich and polymer-sparse domains at the interface. Colloids adsorbed to polymer-depleted regions retain mobility and self-assemble into two-dimensional crystals with square symmetry, which is ascribed to an undulated contact line on the particles and the droplet's negative Gaussian curvature. To our knowledge, this is the first example of thermodynamically favoured two-dimensional square crystals formed by means of interfacial colloid assembly.