Physical mechanics investigations of moving contact lines

Moving contact line (MCL) is the triple-phase region (TPR) formed by two impermeable fluids moving on a solid surface. TPR covers multiple scales, where the interactions among phases influence the dynamic behaviors of the entire fluid field. Owing to its significant applications and rapid development in the fields of energy, aerospace, biology, etc., new challenges emerge in MCL problems. Scaling analysis is an important tool to characterize self-similar expansion of the MCL. Focusing on the scaling relations of MCLs, we review the progresses of physical mechanics investigations under "mechano-electro-thermalchemical" multifield coupled conditions for MCL on rigid/flexible solid surfaces with complex geometries, including hydraulic interior corner, micro-pillar-arrayed surface, dissolvable surface, lag zone in hydraulic fracturing, etc. Through a combined study of multiscale experiments, large-scale molecular dynamics simulations, molecular kinetic theory and hydrodynamics, new phenomena were discovered, such as solid-like precursor film, single-file water-molecular precursor chain, and zigzag MCL. From the interface structure at atomic level to the flow characteristics at continuum level, we discuss the scaling laws of self-similar expansion, and the physical mechanisms and dynamic rules, such as driving source, energy dissipation, boundary conditions, etc. We explore the answers to the "Huh-Scriven paradox" under multifield circumstance, and outlook the prospects and applications of MCL.