= 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 \b[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.
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