Simulation of Water Flow in a Nanochannel with a Sudden Contraction or Expansion.

Water flow in a nanoscale channel is known to be affected by strong water-wall interactions; as a result, the flow considerably deviates from the conventional continuum flow. Nanochannel with a sudden contraction or expansion is the most fundamental morphological nanostructure in many nanoporous systems such as shale matrix, mudrock, membrane, etc. However, the nanoconfinement effects of water flow in nanoporous systems and its effect on macroscopic flow behavior are still evolving research topics. In this work, our recently developed pore-scale lattice Boltzmann method (LBM) considering the nanoscale effects is extended to directly simulate water flow in nanoporous systems. The results show that the flow rate is dramatically decreased in hydrophobic nanopores because of additional flow resistances at the contraction and expansion junctions. This indicates that the bundle of capillary models or the permeability averaging method overestimates the water flow rate in nanoporous media if the contraction/expansion effects between different nanopores are ignored. This work highlights the importance of wettability of the nanochannel in the determination of dynamic water flow behaviors in the contraction/expansion nanosystem. Other important aspects, including velocity distribution, flow patterns, and vortex characteristics as well as pressure variation along the flow direction, are for the first time revealed and quantified. Large differences can be found comparing gas or larger-scale water flows through the same system. A new type of pressure variation curve along the axis of flow direction is found in the hydrophobic nanochannel with a sudden contraction/expansion. This work provides the fundamental understanding of water transport through the nanoscale system with contraction and expansion effects, giving implications to a wide range of industry applications.

Reassessing water slippage in hydrophobic nanostructures.

Reported data of measured slip lengths in nanostructures span several orders of magnitude, from a few nanometers to tens of micrometers. Small roughness on surfaces caused by structural defects or thermal fluctuations dramatically reduces slippage. Tiny bubbles entrapped on rough surfaces can also affect slippage. We used an asymptotic solution and a high density-ratio pseudopotential lattice Boltzmann model to systematically study the drag resistance of a rough surface with attached bubbles. As bubbles nucleate and grow, drag resistance is slightly reduced until the tri-phase contact line reaches the edges of roughness, where bubbles with small angles substantially reduce drag resistance. As bubbles grow to become a continuous gas layer on the surface, the drag resistance greatly decreases. However, the interface deformation from flat to curved shape greatly hinders liquid flow, and the vortex structures cause a wave-like fluctuation in the effective slip length. This finding sheds light on the controversies of reported large variations in the slip length of super-hydrophobic surfaces in nanostructures, e.g., carbon nanotubes.

Mesoscopic method to study water flow in nanochannels with different wettability.

Molecular dynamics (MD) simulations is currently the most popular and credible tool to model water flow in nanoscale where the conventional continuum equations break down due to the dominance of fluid-surface interactions. However, current MD simulations are computationally challenging for the water flow in complex tube geometries or a network of nanopores, e.g., membrane, shale matrix, and aquaporins. We present a novel mesoscopic lattice Boltzmann method (LBM) for capturing fluctuated density distribution and a nonparabolic velocity profile of water flow through nanochannels. We incorporated molecular interactions between water and the solid inner wall into LBM formulations. Details of the molecular interactions were translated into true and apparent slippage, which were both correlated to the surface wettability, e.g., contact angle. Our proposed LBM was tested against 47 published cases of water flow through infinite-length nanochannels made of different materials and dimensions-flow rates as high as seven orders of magnitude when compared with predictions of the classical no-slip Hagen-Poiseuille (HP) flow. Using the developed LBM model, we also studied water flow through finite-length nanochannels with tube entrance and exit effects. Results were found to be in good agreement with 44 published finite-length cases in the literature. The proposed LBM model is nearly as accurate as MD simulations for a nanochannel, while being computationally efficient enough to allow implications for much larger and more complex geometrical nanostructures.

Confinement Correction to Mercury Intrusion Capillary Pressure of Shale Nanopores.

We optimized potential parameters in a molecular dynamics model to reproduce the experimental contact angle of a macroscopic mercury droplet on graphite. With the tuned potential, we studied the effects of pore size, geometry, and temperature on the wetting of mercury droplets confined in organic-rich shale nanopores. The contact angle of mercury in a circular pore increases exponentially as pore size decreases. In conjunction with the curvature-dependent surface tension of liquid droplets predicted from a theoretical model, we proposed a technique to correct the common interpretation procedure of mercury intrusion capillary pressure (MICP) measurement for nanoporous material such as shale. Considering the variation of contact angle and surface tension with pore size improves the agreement between MICP and adsorption-derived pore size distribution, especially for pores having a radius smaller than 5 nm. The relative error produced in ignoring these effects could be as high as 44%--samples that contain smaller pores deviate more. We also explored the impacts of pore size and temperature on the surface tension and contact angle of water/vapor and oil/gas systems, by which the capillary pressure of water/oil/gas in shale can be obtained from MICP. This information is fundamental to understanding multiphase flow behavior in shale systems.

Multiscale and multiresolution modeling of shales and their flow and morphological properties.

The need for more accessible energy resources makes shale formations increasingly important. Characterization of such low-permeability formations is complicated, due to the presence of multiscale features, and defies conventional methods. High-quality 3D imaging may be an ultimate solution for revealing the complexities of such porous media, but acquiring them is costly and time consuming. High-quality 2D images, on the other hand, are widely available. A novel three-step, multiscale, multiresolution reconstruction method is presented that directly uses 2D images in order to develop 3D models of shales. It uses a high-resolution 2D image representing the small-scale features to reproduce the nanopores and their network, a large scale, low-resolution 2D image to create the larger-scale characteristics, and generates stochastic realizations of the porous formation. The method is used to develop a model for a shale system for which the full 3D image is available and its properties can be computed. The predictions of the reconstructed models are in excellent agreement with the data. The method is, however, quite general and can be used for reconstructing models of other important heterogeneous materials and media. Two biological examples and from materials science are also reconstructed to demonstrate the generality of the method.