Capillary dynamic under nanoconfinement: Coupling the energy dissipation of contact line and confined water

Understanding the dynamic imbibition behaviors through nanopores is a subject of great interest in many fields. Recent molecular dynamics (MD) simulations and pressure-driven experiments demonstrated the increased flow resistance of nanoconfined water, which proposed a challenge to the classical molecular kinetic theory (MKT) that the friction dissipation mainly occurs at the three-phase contact line (TPCL) during the dynamic imbibition process. To address this issue, a unified model that combines the friction of moving contact line and confined water behind the meniscus is proposed to capture the dynamic imbibition behaviors at the nanoscale. The model is successfully validated against the published literatures. The results demonstrate that (1) the friction of confined water in hydrophilic silica nanopores (<10 nm) is several times larger than that of bulk water, and the magnitude will increase as the pore dimension and contact angle decrease. (2) The increased resistance of confined water is on account of solid-liquid two-phase interaction, the friction at the TPCL is the result of three-phase interaction (liquid/air/solid). The liquid-gas interface will give rise to the higher friction at the TPCL. (3) Compared with the energy dissipation at the TPCL, the role of friction induced by nanoconfined water strongly depends on the pore size and wettability. In our work, when the silica nanopore is smaller than 12 nm, the friction of confined water behind the meniscus always plays an important role on the spontaneous imbibition; when the pore size is larger than 200 nm, the energy dissipation mainly occurs at the vicinity of TPCL, which is consistent with the classical theory.