Moving contact line of a gas film trapped underneath a droplet impacting a moving surface

The moving contact line of the gas film trapped beneath a droplet impacting a moving surface was investigated using high-speed photography, with particular attention to the upstream and downstream portions of the contact line. The gas film retracts to a bubble after it is trapped, leading to the downstream point moving in the direction opposite to the surface motion initially, and the upstream point moving in the same direction as the surface with a velocity higher than that of the surface. Both points ultimately move with the surface velocity, indicating that they slip on the moving surface before attaining it. Five liquids spanning a viscosity range of μ = 1.00–9.10 mPa s were tested at impact velocities V0 = 1.06–2.22 m/s and surface velocities Vs = 0–3 m/s. Increasing either the surface tension or the viscosity reduces the time and displacement required to reach the surface velocity, whereas the impact velocity is weakly related to the film retraction dynamics and the bubble diameter. A model based on the balance of surface tension, viscous shear, liquid inertia, and the internal–external pressure difference was proposed and validated through comparison with experiments. The slip length λ was estimated to be O(1 nm) through a scaling consistency check using the analytical framework of Luo and Gao [J. Fluid Mech. 1019, A36 (2025)] applied to the Cox–Voinov theory, consistent with the linear slip regime at wall shear rates up to γ̇ ≈ 3 × 109 s−1.