 Solution

Increase particle volume fraction slowly by controlled evaporation or gentle concentration until ordering becomes favourable, while preserving sufficient colloidal repulsion to prevent irreversible sticking. Hard-sphere-like monodisperse suspensions can crystallize by packing entropy; weak, reversible attractions can also favour ordered assembly if they allow defect correction. A controlled drying front or patterned surface may guide the orientation, and gentle sedimentation can create a concentration gradient that crosses the ordering range.

The practical requirement is slow structural change relative to Brownian motion, with uniform temperature, well-characterized ionic strength and a narrow size distribution. An abrupt salt addition or rapid drying can remove the repulsive barrier and trap aggregates instead of growing a crystal. Scattering or microscopy should distinguish crystalline peaks/order from merely increased turbidity.