ABSTRACTPrecursor liquid is a nanoscale liquid creeping ahead of the macroscopic edge of spreading liquids, whose behaviors tightly correlate with the three-phase reaction efficiency and patterning accuracy. However, the important spatial–temporal characteristic of the precursor liquid still remains obscure because its real-time spreading process has not been directly observed. Here, we report that the spreading ionic liquid precursors in a silicon corner can be directly captured on video usingin situscanning electron microscopy.In situspreading videos show that the precursor liquid spreads linearly over time (\documentclass[12pt]{minimal}
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}{}${\rm{\Delta }}L\ \sim\ {\rm{\Delta }}T$\end{document}) rather than obeying the classic Lucas–Washburn law (\documentclass[12pt]{minimal}
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}{}$l\ \sim\ {t}^{1/2}$\end{document}) and possesses a characteristic width of ∼250–310 nm. Theoretical analyses and molecular dynamics simulations demonstrate that the unique behaviors of precursor liquids originate from the competing effect of van der Waals force and surface energy. These findings provide avenues for directly observing liquid/solid interfacial phenomena on a microscopic level.