Experimental evidence for ordered precursor foreshadowing crystal nucleation in colloidal system

Despite its fundamental and technological importance, the microscopic pathway of crystallization process is still elusive. Here, we experimentally study the crystallization of a colloidal system in real space with confocal microscopy. We found a strong decoupling between density variation and local order development. The nucleation events were discovered to rarely start from denser regions. Instead, the relatively ordered structures were found to behave as the precursors of crystal nucleation. Our results provide strong evidence for high-ordered structures foreshadowing crystal nucleation.