Colloidal crystallization and banding in a cylindrical geometry.

Colloidal crystallization takes advantage of the strong interfacial forces and tunable interactions that organize particles into regular structures at small scales. Thus, colloidal crystallization and patterning provide a powerful and simple method to functionalize planar surfaces with applications to optical, catalytic, sensing, and cleansing materials. Nevertheless, the ability to pattern topologically more complex surfaces such as curved, confined, or soft substrates can open new avenues for novel, "intelligent", and responsive materials. We present one step in this direction by characterizing colloidal crystallization inside circular capillaries: a nearly periodic banding is observed, and the colloidal packing is dictated by confinement produced by the wedge-like region formed by a capillary confined meniscus. The packing consists of a succession of hexagonally close-packed regions, which are separated by narrow regions of "buckled phase crystals".