Flow and heat transfer characteristics of nanoconfined fluids

Nanoconfined fluids (NCF) are fluids confined within nanoscale spaces—such as nanochannels, nanotubes, nanopores, or other complex nanostructures—with typical characteristic sizes below 100 nm. At these scales, the large surface-to-volume ratio suppresses body forces and amplifies surface forces, thus surface forces largely govern transport phenomena; meanwhile, van der Waals and electrostatic interactions are also playing a dominating role. As a result, the transport process in NCF is strongly dependent on solid-fluid interfacial properties. Additionally, because the spatial characteristic size of NCF is comparable to the mean free path of fluid molecules, classical fluid mechanics theories rooted in the continuum hypothesis are no longer applicable, creating an urgent need for novel theoretical descriptions that account for nanoscale effects. The flow and heat transfer characteristics of NCF hold significant guiding value for technologies such as membrane separation, oil and gas extraction, energy conversion, and chip cooling. To date, research on NCF flow and heat transfer characteristics has remained limited, and both their theoretical models and microscopic mechanisms are still incomplete. Building on the unique properties of NCF, this paper reviews the latest research progress in basic theories, simulations, and experiments of the flow and heat transfer characteristics of NCF. For flow characteristics, this review only elaborates on key advancements recently occurred. Single-layer water transport is ultra-fast, dominated by fluid-solid interfacial friction, with theoretical models linking flow rate and driving pressure through friction coefficients rather than bulk viscosity. Miscible and immiscible multiphase flows differ sharply, shaped by wall wettability and slip effects, yet systematic flow pattern summaries are lacking. Phase-transition-driven flow—arising from capillary condensation and evaporation—offers low-energy benefits, but interfacial pressure departs from classical predictions: the Kelvin equation deviates at sub-nanoscales, and the Young-Laplace equation misses confined flow’s impact on interfacial dynamics. Complex surfaces can significantly regulate nanoscale flow, for example, rough surfaces alter molecular arrangement and viscosity, charged walls adjust alignment and slip length, and functional groups modulate transport via hydrogen bond networks.Heat transfer in nanospaces follows unique rules: non-Fourier conduction dominates when heat carrier mean free paths match confined dimensions, demanding models like the phonon Boltzmann equation for precise characterization. Anomalous thermal conductivity exhibits size dependence, layered distribution, and anisotropy. The temperature jump is related to the interfacial thermal resistance, as well as the strength of the solid-liquid interaction and the pressure. Axial heat conduction becomes non-negligible, reducing the Nusselt number and invalidating classical correlations. Viscous dissipation, amplified by high shear rates and Brinkman number, can elevate fluid temperature above walls, requiring modified equations integrating slip and temperature jump effects. By leveraging these distinct nanoscale heat transfer mechanisms, effective heat transfer enhancement strategies for NCF include wettability regulation to reduce interfacial thermal resistance, hydrogen bond network formation to strengthen fluid internal thermal diffusion, and external field application to modulate thermal transport.Finally, this paper summarizes the current challenges. Existing theoretical models for nanoscale flow and heat transfer characteristics, developed by adjusting classical parameters, lack sufficient physical rigor, thus requiring the establishment of a unified framework based on fundamental equations that incorporate nanoscale effects. The understanding of underlying mechanisms remains at the phenomenological level, and quantum effects, as well as other emergent phenomena, have not been fully explored, which calls for advanced multiscale simulation techniques and experimental tools. Most of the existing findings are based on molecular dynamics simulations, lacking adequate experimental validation. Specifically, high-precision measurements of single-layer water flow in sub-nanoscale channels, interfacial thermal resistance, and evaporation flux in nanochannels are still insufficient. Future research should adopt innovative characterization techniques and advanced experimental platforms to accurately quantify nanoscale flow and heat transfer behaviors, thereby establishing a closed-loop research cycle integrating simulation, experiment, and theory.