Morphology-Controlled Transition between Interfacial Slip and Drag over a Pinned Surface Nanobubble under...

Surface nanobubbles are known to influence interfacial slip and near-wall hydrodynamics, yet the role of actively controlled nanobubble morphology in regulating momentum transfer at solid-liquid interfaces remains insufficiently understood. Here, nonequilibrium molecular dynamics simulations are used to investigate a pinned surface nanobubble located at a wettability step in a confined water nanochannel. An electrowetting-inspired interfacial forcing (Fext) is applied near the solid surface to tune the effective solid-liquid wettability, thereby inducing reversible deformation of the pinned nanobubble from a flattened configuration to a more protruding morphology. The resulting velocity fields, effective slip lengths, and wall shear stresses reveal a pronounced nonmonotonic dependence of hydrodynamic drag on nanobubble morphology. For small protrusion angles, the gas-liquid interface behaves as a slip boundary that reduces near-wall viscous dissipation. As the protrusion angle increases, the nanobubble increasingly acts as a geometric obstacle, inducing flow recirculation and increasing flow resistance. Bubble deformation is also accompanied by internal gas-phase circulation that couples with the external shear flow and modifies the near-wall hydrodynamics. These results clarify how nanobubble morphology mediates the transition between slip enhancement and drag increase and provide insight into flow regulation at gas-structured interfaces.