When a system undergoes a transition from a liquid to a solid phase, itpasses through multiple intermediate structures before reaching the finalstate. However, our knowledge on the exact pathways of this process is limited,mainly due to the difficulty of realizing direct observations. Here, weexperimentally study the evolution of symmetry and density for variouscolloidal systems during liquid-to-solid phase transitions, and visualizekinetic pathways with single-particle resolution. We observe the formation ofrelatively-ordered precursor structures with different symmetries, which thenconvert into metastable solids. During this conversion, two majorcross-symmetry pathways always occur, regardless of the final state and theinteraction potential. In addition, we find a broad decoupling of densityvariation and symmetry development, and discover that nucleation rarely startsfrom the densest regions. These findings hold for all our samples, suggestingthe possibility of finding a unified picture for the complex crystallizationkinetics in colloidal systems.
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