Hydrodynamic slip in nanoconfined flows: a review of experimental, computational, and theoretical progress.

Nanofluidics has made significant impacts and advancements in various fields, including ultrafiltration, water desalination, biomedical applications, and energy conversion. These advancements are driven by the distinct behavior of fluids at the nanoscale, where the solid-fluid interaction characteristic lengthscale is in the same order of magnitude as the flow conduits. A key challenge in nanofluidics is understanding hydrodynamic slip, a phenomenon in which fluids flow past solid boundaries with a non-zero surface velocity, deviating from the classical no-slip boundary condition. This review consolidates experimental, computational, and theoretical efforts to elucidate the mechanisms behind hydrodynamic slip in nanoconfined flows. Key experimental methods, such as the surface force apparatus, atomic force microscopy, and micro-particle image velocimetry are evaluated alongside emerging techniques like suspended microchannel resonators, dynamic quartz crystal microbalance, and hybrid graphene/silica nanochannels, which have advanced hydrodynamic slip characterization at the nanoscale. In addition to direct slip measurement techniques, methods like sum frequency generation spectroscopy, X-ray reflectometry, and ellipsometry are discussed for their roles in probing solid-liquid interfacial interactions, shedding light on the origins of hydrodynamic slip. The review also highlights the contributions of molecular dynamics simulations, including both non-equilibrium (NEMD) and equilibrium (EMD) approaches, in modeling interfacial phenomena and slip behavior. Additionally, it explores the influence of factors such as surface wettability, shear rate, and confinement on slip, emphasizing the interaction between liquid structuring and solid-liquid interactions. Advancements made so far have uncovered more complexities in nanoconfined flows which have not been considered in the past, inviting more investigation to fully understand and control fluid behavior at the molecular level.