Capillary flow in nanoslits: Transition from deviation to conformance with the Lucas–Washburn equation

The Lucas-Washburn (L-W) equation does not accurately describe capillary flow in graphene-based nanoslits, particularly in terms of channel width dependence. In this study, the dynamics of imbibition in nanoslits are explored using many-body dissipative particle dynamics across various channel widths. For smooth channel walls like those of graphene, the impact of channel width on the imbibition rate varies oppositely between narrower and wider channels. A local minimum in imbibition rate is observed, indicating decreased rates in narrower channels and increased rates in wider ones, across various wettabilities. Conversely, for rough channel walls, where wall slippage is absent, the L-W equation holds, and the imbibition rate increases linearly with channel width. This discrepancy is attributed to wall slippage on smooth surfaces, with the slip length found to increase with channel width before approaching an asymptotic value. For narrower nanoslits with smooth walls, the dynamic contact angle (CA) derived from the L-W equation with a slip condition can be less than the static CA, challenging the prevailing understanding. This “effective” dynamic CA does not accurately represent the meniscus at the liquid front but instead suggests enhanced surface wettability.