Nanoconfined water flow. This schematic shows the nanoconfined water flows between two parallel plates (left) and in a nanotube (right). The flow behavior and the arrangement of water molecules in proximity to nanopore walls strongly depend on their intermolecular interactions with the walls. Such interactions can be significantly influenced by the surface properties of the walls, such as surface hydrophobicity, molecular roughness, surface patterning, adsorption of a surfactant layer or gas film, which can lead to distinct slip phenomena and flow profiles at various water/solid interfaces and in different nanoscale channels [1,15–21]. The slip (left) and non-slip (right) boundary conditions are shown illustratively for two representative cases.
Nanoconfined water flow. This schematic shows the nanoconfined water flows between two parallel plates (left) and in a nanotube (right). The flow behavior and the arrangement of water molecules in proximity to nanopore walls strongly depend on their intermolecular interactions with the walls. Such interactions can be significantly influenced by the surface properties of the walls, such as surface hydrophobicity, molecular roughness, surface patterning, adsorption of a surfactant layer or gas film, which can lead to distinct slip phenomena and flow profiles at various water/solid interfaces and in different nanoscale channels [1,15–21]. The slip (left) and non-slip (right) boundary conditions are shown illustratively for two representative cases.

Understanding and modeling nanoconfined waterflow is of both fundamental and practical importance in numerous engineering and industrial applications such as in nanomedicine, nanofluidics, tribology, water purification, and energy storage and conversion. Despite much effort and considerable advances over the last few decades, many challenging issues on nanoconfined water flow still remain unresolved, even including the most basic ones, such as “How does the water slip actually occur at different interfaces?” and “How does water nanoconfinement influence the flow capacity?”.

Many researchers in fluid mechanics have long attempted to address these issues. A novel practical model for confine water flow was recently developed by Wu et al. [1]. This work reports a numerical correlation between the surface wettability, nanopore dimensions and confined water flow based on a theoretical analysis of the data from dozens of molecular dynamics (MD) simulations and experiments in the literature. This model could find application in a wide range of engineering processes, especially flows in nanofludics, nanoporous petroleum reservoirs, and geophysical processes.

Both experimental measurements and MD simulations have been employed for investigating various aspects of nanoconfined water flow such as viscosity, boundary conditions, intermolecular and surface forces, and water structures at interfaces. Even with today’s rapid advances in experimental techniques, it is still difficult to precisely quantify a nanoconfined water flux (e.g. in carbon nanotubes) [2]. As a key complement to experiments, MD simulations are able to predict nanoconfined water flux and even reveal some undiscovered physical phenomena [3,4]. Nevertheless, both experiments and MD simulations on naoconfined water flow can be very expensive and time-consuming. In contrast, a practical and facile model with reasonable assumptions based on involved physical phenomena can provide useful predictions about nanoconfined water flow in different systems (e.g. nanoporous petroleum reservoirs). Previous attempts to establish practical macroscopic models for nanoconfined water flow were not fully successful [5,6]. One issue is that some important emerging new physical phenomena at the nanoscale were not considered, including different boundary conditions (slip, no-slip, and multilayer sticking) [5,6] and a varying apparent viscosity.

Based on the collected evidence from reported experiments and MD simulations on the underlying physical mechanisms of nanoconfined water flow, Wu et al. have developed a macroscopic model by employing a so-called effective slip concept [1]. The effective slip is considered to be a linear sum of true slip (depending on surface wettability) and apparent slip (caused by a spatial variation of the confined water viscosity depending on surface wettability and a nanopore dimension), and their numerical relationship is established [1]. The confined water fluxes predicted by this new macroscopic model agree well with the majority of those obtained from experiments and MD simulations (total of 53 cases). Wu et al. show that the flow capacity of nanoconfined water can be 10−1–107 times that calculated by the no-slip Hagen-Poiseuille equation, significantly influenced by the surface wettability and nanopore dimension, which quantitatively resolves this controversial issue. This work demonstrates the effectiveness of continuum fluid mechanics in modeling fluid flow at the nanoscale.

The behavior and flow of nanoconfined water and boundary conditions are fundamentally determined by the intermolecular interactions between water and nanopore walls [7–10], as illustrated. Nevertheless, the essential physics underlying the nanoconfined water flow remains to be fully elucidated. The advances in nanomechanical techniques such as surface forces apparatus (SFA), atomic force microscopy (AFM), and bubble/drop probe AFM provide useful experimental approaches to quantify the intermolecular forces associated with water confined between different surfaces [10–12]. Super high-resolution optical microscopy techniques can be applied for visualization of flow profiles at solid/liquid boundary to help quantify the slip phenomenon [13,14]. Surface-sensitive spectroscopic techniques such as sum-frequency vibrational spectroscopy and accurate molecular simulations can be coupled to characterize the orientation and structure of water molecules adjacent to different interfaces [8]. These experimental measurements and simulations are expected to provide insights into the interactions of water-water and waternanopore walls at the molecular level and nanoscale for diverse nanoconfined water flow behaviors. The macroscopic model developed by Wu et al. further advances the development of new nanofluidic devices, models and their applications [1].

Acknowledgements

This research was undertaken, in part, thanks to funding from the Canada Research Chairs program (H. Zeng). We gratefully acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta Innovates – Energy and Environment Solutions (AIEES), and NSERC/AIEES/Foundation CMG and AITF Chairs.

Author affiliations: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada; Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, T2N 1N4, Canada; College of Petroleum Engineering, China University of Petroleum, Beijing, China.

This article was originally published in Nano Today 16 (2017) 7-8.

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