Confinement-Controlled Self Assembly of Colloids with Simultaneous Isotropic and Anisotropic Cross-Section

The phase behavior of building blocks with mushroom cap-shaped particle morphology is explored under 2D and quasi-2D confinement conditions. Fast confocal microscopy imaging of the particles sedimented in a wedge cell reveals a range of mono- and bilayer structures partially directed by the isotropic and anisotropic profiles of the particle geometry. The sequence of phases tracked with increasing confinement height includes those reported in spheres, in addition to the more complex rotator and orientation-dependent phases observed for a class of short rod-like colloids. In the later case, the major particle axis reorients with respect to the substrate. Closest packing considerations provide rationale for the observed 1 Delta (hexagonal)-1Buckled-1Sides (rotator)-2 square(square)-2 Delta (hexagonal)-2Sides (rotator) structural transitions with height.