Interfacial and size effects in high-pressure rheology: A review

High-pressure rheology is the study of fluid behavior under extreme conditions, from macroscale high-pressure mantle fluids to microscale lubrication. Pressure and temperature directly affect intermolecular spacing, modulating both fluid-fluid and fluid-solid interactions, altering rheological properties. Interfacial and size effects at microscopic scales are yet underexplored though macroscopic studies have established the temperature and pressure effects on fluid viscosity. This review adopts an intermolecular and surface-force perspective to summarize contemporary high-pressure rheometer techniques and systematically evaluate their precision limits, measurement ranges, and applicability across macro- and microscales. By doing so, this review highlights key challenges in assessing viscosity under varying pressure regimes and size confinements. Building on these insights, this framework proposes a water-based multiscale viscosity framework comprising four characteristic regimes: an interfacial-dominated region (<3 nm), a confinement-controlled region (3-200 nm), a transitional region (200-1000 nm), and a bulk region (>1000 nm) regions. By examining interfacial and size-effect influence on viscosity measurements, this review reveals fundamental differences between macroscopic and microscopic viscosities and underscores their significance for diverse applications. As different solid interfaces and scales lead to different liquid rheological properties, accurate viscosity measurements require accurate mapping of specific conditions and instrument functions. This review emphasizes the necessity of accounting for the previously overlooked interfacial and size effects in high-pressure rheology and offers practical guidance for the design of lubricants for extreme environments, optimizing microelectromechanical systems, and developing cross-scale rheological models.