Grain boundary (GB) premelting, the formation of liquid layers along GBs below the melting point, can significantly alter material properties. However, the phenomenon remains poorly understood due to observational challenges. We study GB premelting through both experiments using temperature-sensitive microgel colloids and simulations of hard spheres. We observe that the premelted liquid layer thickens logarithmically before being interrupted by melting at each GB. Additionally, the onset and ending volume fractions of premelting depend on GB misorientation, consistent with a simple theoretical model, and coincide with the equilibrium melting and freezing volume fractions of a single crystal. Simulations further reveal that premelting of GBs with different misorientation angles exhibits mutual influence for finite grain sizes. These findings advance our understanding of premelting and can be applied to determine liquid distribution in polycrystals, which is critical for mechanical stability and materials processing.

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• Crystal melting
• Phase transitions
• Wetting
• Colloidal crystal
• Optical nanoscopy