Publications

2025

1. J. Phys. Chem. Lett.

   

   Supercritical ethanol–CO2 mixtures exhibit microscopic immiscibility: A combined study using X-ray scattering and molecular dynamics simulations

   J. Fan^*, T. Yoon, G. Vignat, H. Li, K. Younes, A. Majumdar, P. Muhunthan, D. Sokaras, T. Weiss, I. Rajkovic, and M. Ihme^*

   The Journal of Physical Chemistry Letters, 2025, accepted

   Abs PDF

   Supercritical mixtures of ethanol (EtOH) and carbon dioxide (CO2) are classified as type-I mixtures, with complete macroscopic miscibility. However, differences in molecular polarity and interactions suggest a distinct phase behavior at the microscopic level. Here, we combine Small Angle X-ray Scattering experiments and molecular dynamics (MD) simulations to investigate the microscopic structure of EtOH–CO2 mixtures under supercritical conditions. The structure factor exhibits non-linear composition-dependent behavior, revealing pronounced local density fluctuations. The complementary MD simulations, using optimized force field parameters, provide atomistic insight, showing that EtOH forms self-associated, hydrogen-bonded aggregates, while CO2 remains more uniformly distributed. Cluster analysis identifies a preferential EtOH-rich composition exceeding the bulk average, governed by a balance between energetic and entropic competition. These results demonstrate that, contrary to macroscopic expectations, the mixture exhibits significant microscopic heterogeneity and immiscibility, which may influence solubility, reactivity, transport properties, and thermodynamic response functions. These findings challenge the conventional views of type-I fluids and emphasize the necessity of revising mixture states and considering molecular polarity.

2024

1. Phys. Rev. Lett.

   

   Heterogeneous cluster energetics and nonlinear thermodynamic response in supercritical fluids

   J. Fan, N. Ly, and M. Ihme^*

   Physical Review Letters, 2024, 133(24): 248001.

   Abs DOI

   Microstructural heterogeneities arising from molecular clusters directly affect the nonlinear thermodynamic properties of supercritical fluids. We present a physical model to elucidate the relation between energy exchange and heterogeneous cluster dynamics during the transition from liquidlike to gaslike conditions. By analyzing molecular-dynamics data and employing physical principles, the model considers contributions from three key processes, namely, changing cluster density, cluster separation, and transfer of molecules between clusters. We show that the proposed model is consistent with the energetics at subcritical conditions and can be used to explain the nonlinear behavior of thermodynamic response functions, including the peak in the isobaric heat capacity.

2. Droplet

   

   A strategy to drive nanoflow using Laplace pressure and the end effect

   K.L. Zhang, H.Y. Xu, J. Fan, C.C. Ouyang, H.A. Wu, and F.C. Wang^*

   Droplet, 2024, 3: e136.

   (Cover paper)

   Abs DOI

   Nanofluidics holds significant potential across diverse fields, including energy, environment, and biotechnology. Nevertheless, the fundamental driving mechanisms on the nanoscale remain elusive, underscoring the crucial importance of exploring nanoscale driving techniques. This study introduces a Laplace pressure-driven flow method that is accurately controlled and does not interfere with interfacial dynamics. Here, we first confirmed the applicability of the Young–Laplace equation for droplet radii ranging from 1 to 10 nm. Following that, a steady-state liquid flow within the carbon nanotube was attained in molecular dynamics simulations. This flow was driven by the Laplace pressure difference across the nanochannel, which originated from two liquid droplets of unequal sizes positioned at the channel ends, respectively. Furthermore, we employ the Sampson formula to rectify the end effect, ultimately deriving a theoretical model to quantify the flow rate, which satisfactorily describes the molecular dynamics simulation results. This research enhances our understanding on the driving mechanisms of nanoflows, providing valuable insights for further exploration in fluid dynamics on the nanoscale.

3. Capillarity

   

   Impeding effect on droplet spreading by a groove on the substrate

   X. Huang, Y.Q. Li^*, J. Fan, H.A. Wu, and F.C. Wang^*

   Capillarity, 2024, 13: 1-9.

   Abs DOI

   Understanding the wetting behaviors of droplets on grooved surfaces is indispensable in surface science and offers promising avenues for advancing industrial processes. The droplet spreading on grooved surfaces can be discretized into a series of individual events that the droplet across each groove with variations in capillary forces and a subsequent re-equilibrium. In this work, a simplified model of droplet spreading on surface with an individual groove on both the left and right sides was utilized in order to elucidate the fundamental mechanisms underlying contact line pinning due to the groove. We examined the effects of the groove position and the wettability of solid surfaces. The contact line is observed to be pinned when the grooves are strategically positioned. However, by reducing the distance between the grooves, the contact lines can cross them. In such instances, the spreading process can be classified into four modes: Free spreading, impeding spreading, pinning, and depinning. The pinning and depinning phenomenon are explained by the balance between the driving force and pinning force on the contact line. Based on simulation results, the maximum pinning force exerted on the contact line by a certain solid surface can be theoretically predicted. Besides, the wettability of the solid surface also contributes to the impeding effect. This work provides theoretical guidance for the study of wetting on grooved surfaces at the nanoscale, which is essential for developing a comprehensive understanding of the interactions between droplets and structured surfaces, with potential applications in optimizing industrial processes and advancing surface science.

4. Adv. Geo-Energy Res

   

   Enhancement of oil transport through nanopores via cation exchange in thin brine films at rock-oil interface

   X.Y. Hong, X. Jin^*, M. Shen, J. Fan, H.A. Wu, and F.C. Wang^*

   Advances in Geo-Energy Research, 2024, 12: 22-34.

   Abs DOI

   Interactions at the oil/brine/rock interfaces play a pivotal role in the mobility of crude oil within reservoir matrices. Unraveling the microscopic mechanisms of these interactions is crucial for ion-engineered water flooding in secondary and tertiary oil recovery. In this study, the occurrence and transport behavior of crude oil in kaolinite nanopores covered with thin brine films was investigated by molecular dynamics simulation. There is an apparent interface layered phenomenon for the liquid molecules in slit pores and the polar oil components primarily concentrate at the oil/brine interfacial region and form various binding connections with ions. The interfacial interactions between the polar oil components and brine ions exhibit an inhibitory effect on the transport of crude oil through nanopores. The interaction mechanism between acetic acid molecules and hydrated ions was elucidated by interaction modes and interaction intensity, which was proved to illustrate the flow difference in different brine film systems. Moreover, a strategy of exchanging the binding sites of divalent cations with acetic acid molecules by monovalent cations with a higher concentration was proposed. The cation exchange scheme was further validated, demonstrating an enhancement in the oil mobility within nanopores. These findings deepen our understanding of oil/brine/rock interfacial interactions and provide a significant molecular perspective on ion-engineered water flooding for enhanced oil recovery.

5. Geoenergy Sci. Eng.

   

   Exploring wettability variations on minerals surfaces: Insights from spreading coefficient and interaction energy analysis

   J.N. Fan, J. Fan, X.Y. Hong, H.Y. Xu, H.A. Wu^*, and F.C. Wang^*

   Geoenergy Science and Engineering, 2024, 234: 212672.

   Abs DOI

   The wettability of minerals plays a pivotal role in determining the distribution of residual oil and devising effective flooding strategies for enhanced oil recovery. Utilizing molecular dynamics simulations, we investigated the surface wettability of various typical minerals. The simulation results illustrate that the modeled crude oil spreads completely on most mineral surfaces. We proposed an efficient approach to calculate the spreading coefficient of the modeled crude oil on mineral surfaces, yielding accurate predictions of the spreading state — whether it is completely spread or not. Compared with the conventional spreading simulation method, our approach offers significant time savings in the calculations. Additionally, our findings underscore the influence of various crude oil components on mineral surface wettability, emphasizing the crucial role of microscopic interactions between these components and the mineral. These insights contribute to a refined understanding of mineral surface wettability and its correlation with crude oil composition, offering valuable guidance for optimizing reservoir management and production strategies.

2023

1. Microfluid. Nanofluidics

   

   Boundary slip moderated by interfacial hydrogen bond dynamics

   J.C. Li, K.L. Zhang, J. Fan, H.A. Wu^*, and F.C. Wang^*

   Microfluidics and Nanofluidics, 2023, 27: 86.

   Abs DOI

   Understanding the slip behaviors on the graphene surfaces is crucial in the field of nanofluidics and nanofluids. The reported values of the slip length in the literature from both experimental measurements and simulations are quite scattered. The presence of low concentrations of functional groups may have a greater impact on the flow behavior than expected. Using non-equilibrium molecular dynamics simulations, we specifically investigated the influence of hydroxyl-functionalized graphene surfaces on the boundary slip, particularly the effects related to hydrogen bond dynamics. We observed that hydroxyl groups significantly hindered the sliding motion of neighboring water molecules. Hydrogen bonds can be found between hydroxyl groups and water molecules. During the flow process, these hydrogen bonds continuously form and break, resulting in the energy dissipation. We analyzed the energy balance under different driving forces and proposed a theoretical model to describe the slip length which also considers the influence of hydrogen bond dynamics. The effects of the driving force and the surface functional group concentration were also studied.

2. Colloids Surf. A

   

   Local molecular asymmetry mediated self-adaptive pinning force on the contact line

   X. Huang, J. Fan, H.A. Wu^*, and F.C. Wang^*

   Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2023, 674: 131987.

   Abs DOI

   When a droplet is placed on a solid surface, the pinning force on the contact line is the key factor determining the dynamics of liquid droplet. As the apparent contact line position is not moving, the pinning force can still be adjusted to balance the changing driving force. However, the underlying mechanism of the self-adaptive variations in the pinning force remains elusive. In this work, molecular dynamics simulations for evaporation of a liquid droplet on a chemically heterogenous substrate were carried out. Our investigation disclosed that the liquid atom on the solid surface experiences a tangential force from the substrate, unless it was located in an absolutely symmetrical position. There exists a non-uniform distribution of liquid atoms in the layer near the solid surface. The dependence of the capillary force on these liquid atoms on their locations relative to the solid atoms is the molecular origin of the self-adaptive pinning force on the contact line. The contact line position may be unchanged on the macroscopic scale, but the local distribution of liquid atoms had a subtle adjustment on the microscopic scale, giving rise to the pinning force on the contact line. A theoretical model was proposed to quantitatively describe the link between the pinning force on the contact line and the molecular asymmetry in the local distribution of liquid atoms.

2022

1. SCPMA

   

   Molecular transport under extreme confinement

   F.C. Wang, J.H. Qian, J. Fan, J. Li, H.Y. Xu, and H.A. Wu^*

   Science China Physics, Mechanics & Astronomy, 2022, 65: 264601.

   (Invited Review, Cover paper)

   Abs DOI

   Mass transport through the nanoporous medium is ubiquitous in nature and industry. Unlike the macroscale transport phenomena which have been well understood by the theory of continuum mechanics, the relevant physics and mechanics on the nanoscale transport still remain mysterious. Recent developments in fabrication of slit-like nanocapillaries with precise dimensions and atomically smooth surfaces have promoted the fundamental research on the molecular transport under extreme confinement. In this review, we summarized the contemporary progress in the study of confined molecular transport of water, ions and gases, based on both experiments and molecular dynamics simulations. The liquid exhibits a pronounced layered structure that extends over several intermolecular distances from the solid surface, which has a substantial influence on static properties and transport behaviors under confinement. Latest studies have also shown that those molecular details could provide some new understanding on the century-old classical theory in this field.

2. Energy

   

   Formation mechanism and structural characteristic of pore-networks in shale kerogen during in-situ conversion process

   H.Y. Xu, H. Yu^*, J. Fan, J. Xia, H. Liu, and H.A. Wu^*

   Energy, 2022, 242: 122992.

   Abs DOI

   In-situ conversion process (ICP) appears to be a promising approach to enhance hydrocarbon recovery of shale reservoirs. During the ICP, the pore-networks gradually generate underground and serve as the conduits for the storage and transport of hydrocarbon, which finally decides the recovery ability of the reservoir. Reactive molecular dynamics simulations are used to study the pyrolysis behavior of kerogen under the reservoir conditions. The pyrolysis proceeds in four stages: energy accumulation, oil window, gas window, and steady stage. And during this process, the kerogen pyrolytic pore-networks form under the combined actions of chemical bond breaking and physical deformation. Further analysis demonstrates that the structural characteristic is dependent on the maturities of kerogen and pyrolysis temperature. Low-maturity kerogen creates high-quality pore-networks (∼15% porosity), while the pores in medium- and high-maturity kerogen are scarce and isolated (∼1% porosity). In the simulation, the optimal pyrolysis temperature of pyrolysis is about 2300 K to develop high-quality pore-networks. In addition, a conversion relationship between the simulation temperatures and ICP engineering temperatures is established using Arrhenius equations, and the optimal temperature for ICP engineering is suggested to be ∼730 K.

2021

1. Nat. Mater.

   

   Hydrophilicity gradient in covalent organic frameworks for membrane distillation

   S. Zhao#, C.H. Jiang#, J. Fan# , S.S. Hong, P. Mei, R.X. Yao, Y.L. Liu, S.L. Zhang, H. Li, H.Q. Zhang, C. Sun, Z.B. Guo, P.P. Shao, Y.H. Zhu, J.W. Zhang, L.S. Guo, Y.H. Ma, J.Q. Zhang, X. Feng^*, F.C. Wang^*, H.A. Wu, and B. Wang^*

   Nature Materials, 2021, 20: 1551–1558.

   Abs DOI

   Desalination can help to alleviate the fresh-water crisis facing the world. Thermally driven membrane distillation is a promising way to purify water from a variety of saline and polluted sources by utilizing low-grade heat. However, membrane distillation membranes suffer from limited permeance and wetting owing to the lack of precise structural control. Here, we report a strategy to fabricate membrane distillation membranes composed of vertically aligned channels with a hydrophilicity gradient by engineering defects in covalent organic framework films by the removal of imine bonds. Such functional variation in individual channels enables a selective water transport pathway and a precise liquid–vapour phase change interface. In addition to having anti-fouling and anti-wetting capability, the covalent organic framework membrane on a supporting layer shows a flux of 600 l m–2 h–1 with 85 °C feed at 16 kPa absolute pressure, which is nearly triple that of the state-of-the-art membrane distillation membrane for desalination. Our results may promote the development of gradient membranes for molecular sieving.

2. J. Chem. Phys.

   

   Anomalously low friction of confined monolayer water with a quadrilateral structure

   J. Li, Y.B. Zhu, J. Xia, J. Fan, H.A. Wu^*, and F.C. Wang^*

   The Journal of Chemical Physics, 2021, 154: 224508.

   Abs DOI

   In this work, we explored how the structure of monolayer water confined between two graphene sheets is coupled to its dynamic behavior. Our molecular dynamics simulations show that there is a remarkable interrelation between the friction of confined water with two walls and its structure under extreme confinement. When the water molecules formed a regular quadrilateral structure, the friction coefficient is dramatically reduced. Such a low-friction coefficient can be attributed to the formation of long-range ordered hydrogen bond network, which not only decreases the structure corrugation in the direction perpendicular to the walls but also promotes the collective motion of the confined water. The regular quadrilateral structure can be formed only if the number density of confined water falls within a certain range. Higher number density results in larger structure corrugations, which increases the friction, while smaller number density leads to an irregular hydrogen bond network in which the collective motion cannot play the role. We demonstrated that there are four distinct stages in the diagram of the friction coefficient vs the number density of confined water. This research clearly established the connection between the dynamic characteristics of confined monolayer water and its structure, which is beneficial to further understand the mechanism of the high-speed water flow through graphene nanocapillaries observed in recent experiments.

3. Energy Fuels

   

   Enhanced gas recovery in kerogen pyrolytic pore-network: Molecular simulations and theoretical analysis

   H.Y. Xu, H. Yu^*, J. Fan, J. Xia, F.C. Wang, and H.A. Wu

   Energy & Fuels, 2021, 35(3): 2253-2267.

   Abs DOI

   Enhanced gas recovery (EGR) is believed to be a promising technology to improve the production of shale gas reservoirs and simultaneously reduce the emissions of greenhouse gas via the injection (sequestration) of carbon dioxide, to which great effort has been devoted by scholars. However, traditional investigations are generally limited to the ideal model of nanochannels and statistic characterization of competitive adsorption, neglecting the nanoporous structure of the kerogen matrix and the complex dynamic behavior during the EGR process. In this work, we present a comprehensive study of the EGR process in a realistic kerogen pore network (matrix) which is obtained from the artificial pyrolysis of bulk kerogen through reactive force field molecular dynamics (ReaxFF MD) simulations. The influence of pore properties (e.g., porosity) of the kerogen matrix under different maturities, and the proportions (i.e., methane and carbon dioxide) of injection gas with various injection pressures are revealed and meticulously discussed. In addition, the underlying mechanisms including diffusion and displacement effects behind the EGR process are analyzed by combining them with with particle trajectory capture technology. In particular, based on the MD simulation results, an analytical model to depict the dynamic recovery process in the kerogen matrix is proposed by coupling consideration of recovery time and capacity, which are examined against the simulation and experimental data. The hybrid recovery strategy is developed by utilizing the advantages of depressurization and gas-injection recoveries to achieve the optimization of both recovery time and capacity. The insights acquired from this work would be helpful for efficient exploitation of shale gas reservoirs and pave the way to capture the realistic EGR processes within the kerogen matrix from molecular and theoretical perspectives.

4. J. Colloid Interface Sci.

   

   A generalized examination of capillary force balance at contact line: On rough surfaces or in two-liquid systems

   J. Fan, J. De Coninck, H.A. Wu^*, and F.C. Wang^*

   Journal of Colloid and Interface Science, 2021, 585: 320-327.

   Abs DOI

   We investigate the capillary force balance at the contact line on rough solid surfaces and in two-liquid systems. Our results confirm that solid-liquid interactions perpendicular to the interface have a significant influence on the lateral component of the capillary force exerted on the contact line. Surface roughness of the solid substrate reduces the mobility of liquid and alters how the perpendicular solid-liquid interactions transfer into a force acting parallel to the interface. A quantitative relation between surface roughness and the transfer strategy is proposed. Moreover, when a liquid is in coexistence with another immiscible liquid on a solid, the capillary forces exerted on liquids of both sides are involved in our theoretical model. The contact angle can be predicted by calculating three interfacial tensions. These arguments are then verified by molecular dynamics simulations. Our findings set up the generalized theoretical framework for the capillary force balance at the contact line and broaden its application in more realistic scenarios.

5. Energy Fuels

   

   Transport of shale gas in micro/nano-porous media: Molecular to pore-scale simulations

   H. Yu, H.Y. Xu, J. Fan, Y.B. Zhu, F.C. Wang, and H.A. Wu^*

   Energy & Fuels, 2021, 35(2): 911-943.

   (Invited Review, Front cover paper)

   Abs DOI

   As the typical unconventional reservoir, shale gas is believed to be the most promising alternative for the conventional resources in future energy patterns, attracting more and more attention throughout the world. Generally, the majority of shale gas is trapped within the tight shale rock with ultralow porosity (<10%) and ultrasmall pore size (as less as several nanometers). Thus, the accurate understanding of gas transport characteristic and its underlying mechanism through these microporous/nanoporous media is critical for the effective exploitation of shale reservoir. In this context, we present a comprehensive review on the current advances of multiscale transport simulations of shale gas in microporous/nanoporous media from molecular to pore-scale. For the gas transport in shale nanopores using molecular dynamics (MD) simulations, the structure and force parameters of various nanopore models, including organic models (graphene, carbon nanotubes, and kerogen) and inorganic models (clays, carbonate, and quartz), and flow simulation strategies (such as nonequilibrium molecular dynamics (NEMD) and Grand Canonical Monte Carlo simulations) are systematically introduced and clarified. The significant MD simulation results about gas transport characteristic in shale nanopores then are elaborated respectively for different factors, including pore size, ambient pressure, nanopore type, atomistic roughness, and pore structure, as well as multicomponent. Besides, the two-phase transport characteristic of gas and water is also discussed, considering the ubiquity of water in shale formation. For the lattice Boltzmann method (LBM) and pore network model (PNM) approaches to conduct pore-scale simulations, we briefly review its origins, modifications, and applications for gas transport simulations in a microporous/nanoporous shale matrix. Particularly, the upscaling methods to incorporate MD simulation into LBM and PNM frameworks are emphatically expounded in the light of recent attempts of MD-based pore-scale simulations. It is hoped that this Review would be helpful for the readers to build a systematical overview on the transport characteristic of shale gas in microporous/nanoporous media and subsequently accelerate the development of the shale industry.

2020

1. Phys. Rev. Lett.

   

   Microscopic origin of capillary force balance at contact line.

   J. Fan, J. De Coninck, H.A. Wu^*, and F.C. Wang^*

   Physical Review Letters, 2020, 124(12): 125502.

   Abs DOI

   We investigate the underlying mechanism of capillary force balance at the contact line. In particular, we offer a novel approach to describe and quantify the capillary force on the liquid in coexistence with its vapor phase, which is crucial in wetting and spreading dynamics. Its relation with the interface tension is elucidated. The proposed model is verified by our molecular dynamics simulations over a wide contact angle range. Differences in capillary forces are observed in evaporating droplets on homogeneous and decorated surfaces. Our findings not only provide a theoretical insight into capillary forces at the contact line, but also validate Young’s equation based on a mechanical interpretation.

2. ACS Appl. Mater. Interfaces

   

   Preparation of twisted bilayer graphene via wetting transfer method

   Y. Hou, X.B. Ren, J. Fan, G.R. Wang, Z.H. Dai, C.H. Jin, W.X. Wang, Y.B. Zhu, S. Zhang, L.Q. Liu^*, and Z. Zhang^*

   ACS Applied Materials & Interfaces, 2020, 12(36): 40958-40967.

   Abs DOI

   Assembling monolayers into a bilayer system unlocks the rotational free degree of van der Waals (vdW) homo/heterostructure, enabling the building of twisted bilayer graphene (tBLG) which possesses novel electronic, optical, and mechanical properties. Previous methods for preparation of homo/heterstructures inevitably leave the polymer residue or hexagonal boron nitride (h-BN) mask, which usually obstructs the measurement of intrinsic mechanical and surface properties of tBLG. Undoubtedly, to fabricate the designable tBLG with clean interface and surface is necessary but challenging. Here, we propose a simple and handy method to prepare atomically clean twisted bilayer graphene with controllable twist angles based on wetting-induced delamination. This method can transfer tBLG onto a patterned substrate, which offers an excellent platform for the observation of physical phenomena such as relaxation of moiré pattern in marginally tBLG. These findings and insight should ultimately guide the designable packaging and atomic characterization of the two-dimensional (2D) materials.

3. Energy Fuels

   

   Nano-confined transport characteristic of methane in organic shale nanopores: The applicability of continuous model

   H. Yu, H.Y. Xu, J. Xia, J. Fan, F.C. Wang, and H.A. Wu^*

   Energy & Fuels, 2020, 34(8): 9552-9562.

   Abs DOI

   In recent years, since the fast mass transport emerges from some low-dimensional nanostructures (e.g., graphene and CNTs), the applicability of the continuous model (e.g., hydrodynamics) for describing the nanoscale flow has been intensely challenged, even for shale gas. As the typical tight rock with numerous nanopores, most scholars considered that gas transport capacity (permeability) within a shale organic matrix (i.e., kerogen) would be significantly enhanced as well, due to the nanoscale slip effect. Herein, we perform comprehensive molecular dynamics (MD) simulations to reveal the realistic gas transport behavior through shale kerogen nanopores. The results show that, interestingly, all the velocity profiles for different kerogen (type I, II, and III) nanopores display no-slip parabolic shape, which are quantitatively satisfied with the continuous model under various conditions, including pressure drop (0.25–1 MPa), pore size (2–8 nm), and ambient pressure (5–50 MPa) and temperature (300–390 K). In particular, using potential energy surface (PES) and particle trajectory capture (PTC) technologies, we find that, confined by the rough kerogen walls and ultra-high reservoir pressure, the methane molecules collide with the walls frequently but just go round and round without moving along the walls (confined reflection), and thus, the tangential momentum (slip velocity) is negligible at the walls. Importantly, this work demonstrates that traditional consideration of the slip effect is redundant for methane transport in organic shale (kerogen) nanopores and will overestimate the gas permeability immensely (as much as 100 times). These new insights would be helpful for the precise understanding and accurate modeling of methane transport within nanoporous shale rocks.

4. J. Phys. Chem. C

   

   Roughness factor-dependent transport characteristic of shale gas through amorphous kerogen nanopores

   H. Yu, H.Y. Xu, J. Fan, F.C. Wang, and H.A. Wu^*

   Journal of Physical Chemistry C, 2020, 124(3): 12752-12765.

   (Cover paper)

   Abs DOI

   In the past decades, shale gas has been recognized as the promising unconventional resource for global energy storage, and a clear understanding of the gas-transport characteristic within nonporous shale organic matter (i.e., kerogen) is fundamental for the effective development of shale reservoirs. In this regard, previous studies were generally conducted based on the ideally smooth nanochannels (e.g., graphite slit or tube) without considering the atomistic-scale roughness of the walls. Herein, using molecular dynamics (MD) simulations, we perform a systematical investigation on the gas-transport characteristic through amorphous organic nanopores constructed by realistic kerogen molecules. The results show that the gas-transport velocity in amorphous organic nanopores drops dramatically (40, 70, and 90%) only with tiny roughness factors (0.3, 0.6, and 1.2%) when compared with ideally smooth nanochannels. Further analysis of the potential energy surface and the particle trajectory justifies the entirely different gas-transport mechanisms in ideally smooth (surface diffusion) and relative rough (viscosity diffusion) organic nanopores. Besides, based on the insights of numerous MD simulations (pore sizes: 3–9 nm and system pressures: 5–50 MPa), a new analytical model that is able to consider the key effect of roughness factor on gas transport in organic-rich shale is developed, which is well verified with the experimental results. It is particularly found that the gas-transport capacity in organic-rich shale (∼1 nm of slippage length) would be enormously overrated as much as 2 orders of magnitude by the traditional cognition based on ideally smooth nanopores (∼100 nm of slippage length).

5. Energy Fuels

   

   Two-phase transport characteristic of shale gas and water through hydrophilic and hydrophobic nanopores

   H.Y. Xu, H. Yu^*, J. Fan, Y.B. Zhu, F.C. Wang, and H.A. Wu^*

   Energy & Fuels, 2020, 34(4): 4407-4420.

   Abs DOI

   Previous attempts to characterize shale gas transport in nanopores are not fully successful due to the fact that the presence of water within shale reservoirs is generally overlooked. In addition, shale is known as a wettability-varying (hydrophilic and hydrophobic) rock depending on various components and maturity grades. Herein, toward this end, we performed a comprehensive study about two-phase transport characteristic of shale gas and water through hydrophilic and hydrophobic nanopores by integrating the molecular dynamics (MD) simulations and analytical models. Using MD simulations, we showed that water molecules prefer to accumulate at the walls (water film) in hydrophilic nanopores while form the water cluster at the center region of hydrophobic nanopores, which significantly alters the shale gas transport behavior. For hydrophilic nanopores, the existence of water film weakens the gas–walls collisions (slip effect), resulting in a viscosity dominant transport mechanism. In contrary, shale gas transport in hydrophobic nanopores is mainly contributed by slip effect where the gas–gas collisions (viscosity) is abated by the water cluster. On this basis, we proposed an analytical model to quantitatively depict the shale gas transport behavior in moist nanopores, which is well verified by MD simulations results. Particularly, according to our flow model, the gas transport capacity decreases to only 15% when mixing with 50% water molecules for both hydrophilic and hydrophobic nanopores, which would be greatly overestimated by traditional models neglecting the presence of water molecules. The deep insights gained in this work will further the exploitation and development of shale reservoirs.

6. Phys. Fluids

   

   Evaporation-driven liquid flow through nanochannels

   J. Fan, H.A. Wu^*, and F.C. Wang^*

   Physics of Fluids, 2020, 32(1): 012001.

   Abs DOI

   Evaporation-driven liquid flow through nanochannels has attracted extensive attention over recent years due to its applications in mass and heat transfer as well as energy harvesting. A more comprehensive understanding is still expected to reveal the underlying mechanisms and quantitatively elucidate the transport characteristics of this phenomenon. In this study, we investigated evaporation-driven liquid flow through nanochannels using molecular dynamics simulations. The evaporation flux from the solid-liquid interface was higher than that from the middle region of the channel or the liquid-vapor interface. This finding may explain why experimental observations of evaporation flux through nanochannels exceed the limits predicted by the classical Hertz–Knudsen equation. Upon increasing the interaction strength between liquid atoms, the liquid exhibited enhanced solid-liquid interfacial evaporation and higher surface tension, albeit with reduced total flux. We also found that lyophilic channels exhibited higher evaporation fluxes than lyophobic channels, which can be interpreted by a Gibbs free energy analysis. The energy conversion analysis indicated that the effective pressure gradient exerted on a liquid flow by evaporation depends on the channel length. This was consistent with our simulations. Evaporation-driven liquid flow through nanochannels could be modeled quantitatively using this knowledge.

7. J. Nat. Gas Sci. Eng.

   

   Multiscale gas transport behavior in heterogeneous shale matrix consisting of organic and inorganic nanopores

   H. Yu, J. Fan, J. Xia, H. Liu, and H.A. Wu^*

   Journal of Natural Gas Science and Engineering, 2020, 75: 103139.

   Abs DOI

   The complicated nanoporous structure and significant heterogeneity of shale matrix make it challenging to fully understand the gas transport behavior in shale. Towards this end, herein a multiscale approach was presented via coupling molecular dynamics (MD) simulations, analytical model and pore network model (PNM). Using MD simulations, we showed that gas transport in organic nanopores manifests a typical slippage feature whereas gas slippage breaks down in inorganic nanopores, which should be attributed to “rough” potential energy surface (PES) of inorganic walls. Based on the results of MD simulations, a unified gas transport model was derived to describe gas flow behaviors in organic and inorganic pores by integrating continuum flow theory and gas-surface dynamics theory. In particular, gas transport process in heterogeneous shale matrix was performed with the help of pore network model and the influence of multiple factors on the gas permeability of shale matrix were analysed and discussed.

2019

1. Nanoscale

   

   Dehydration impeding ionic conductance through two-dimensional angstrom-scale slits

   Y.Z. Yu, J. Fan, J. Xia, Y.B. Zhu, H.A. Wu, and F.C. Wang^*

   Nanoscale, 2019, 11: 8449-8457.

   Abs DOI

   There has been long-standing academic interest in the study of ion transport in nanochannel systems, owing to its vast implications in understanding the nature of numerous environmental, biological and chemical processes. Here, we investigate ion transport through two-dimensional slits using molecular dynamics simulations. These slits with angstrom-scale height dimensions can be realistically replicated in the simulation, which leads to direct comparisons between simulations and experiments. In particular, this new confining geometry allows the size exclusion effect to be unambiguously decoupled from other mechanisms. As the slit size approaches the ultimate scale, dehydration at the entry impedes the ionic conductance significantly, and even induces a complete ion rejection. We demonstrate that energy barriers required to accomplish the ion permeation can be theoretically connected to the partial dehydration process. The proposed model is further validated by simulations. Our results offer insights into the atomistic details of ion permeation, which may also shed light on developing effective ways for water filtration and desalination.

2. J. Phys. Chem. C

   

   Charge asymmetry effect in ion transport through angstrom-scale channels

   Y.Z. Yu, J. Fan, A. Esfandiar, Y.B. Zhu, H.A. Wu, and F.C. Wang^*

   Journal of Physical Chemistry C, 2019, 123(2): 1462-1469.

   Abs DOI

   Structural and dynamic properties of ions confined in nanoslits are crucial to understand the fundamental mechanism underlying a wide range of chemical and biological phenomena. K+ and Cl– show similar ion mobilities in a bulk aqueous solution, whereas they exhibit a remarkable difference when transporting through an angstrom-scale channel. Our molecular dynamics simulations uncover that such discrepancy originates from the subtle differences in their hydration structures and preferable locations across the channel. Opposite charge causes different water dipolar orientations around ions, mediating the distance and tribological interactions between hydrated ions and channel’s walls. Hydrated Cl– ions experience a remarkable larger friction force inside the channel and consequently a smaller mobility compared with K+ ions. This charge asymmetry mechanism becomes dominant when the channel size approaches the molecular scale, which takes responsibility for the delicate distinction between dynamic transport behaviors of K+ and Cl– under strong confinement. We believe that these results would shed light on future design and optimization of new membranes for filtration and desalination applications.

2018

1. R. Soc. Open Sci.

   

   Molecular mechanism of viscoelastic polymer enhanced oil recovery in nanopores

   J. Fan, F.C. Wang^*, J. Chen, Y.B. Zhu, D.T. Lu, H. Liu, and H.A. Wu^*

   Royal Society Open Science, 2018, 5(6): 180076.

   Abs DOI

   Polymer flooding is a promising chemical enhanced oil recovery (EOR) method, which realizes more efficient extraction in porous formations characterized with nanoscale porosity and complicated interfaces. Understanding the molecular mechanism of viscoelastic polymer EOR in nanopores is of great significance for the advancement of oil exploitation. Using molecular dynamics simulations, we investigated the detailed process of a viscoelastic polymer displacing oil at the atomic scale. We found that the interactions between polymer chains and oil provide an additional pulling effect on extracting the residual oil trapped in dead-end nanopores, which plays a key role in increasing the oil displacement efficiency. Our results also demonstrate that the oil displacement ability of polymer can be reinforced with the increasing chain length and viscoelasticity. In particular, a polymer with longer chain length exhibits stronger elastic property, which enhances the foregoing pulling effect. These findings can help to enrich our understanding on the molecular mechanism of polymer enhanced oil recovery and provide guidance for oil extraction engineering.

2. Int. J. Heat Mass Transf.

   

   Pressure-dependent transport characteristic of methane gas in slit nanopores

   H. Yu, J. Fan, J. Chen, Y.B. Zhu, and H.A. Wu^*

   International Journal of Heat and Mass Transfer, 2018, 123: 657-667.

   Abs DOI

   The transport mechanism of methane gas through the nanopores is a fundamental concern in the shale gas extraction. Shale gas is mainly stored in nanopores of shale formations over a wide range of pressure which is a fundamental thermodynamic variable dominating the properties of substances, rendering that it is important to understand the pressure-dependent transport characteristic of gas molecules in nanopores. Toward this end, we performed molecular dynamics (MD) simulations to investigate the transport characteristic and its mechanism of methane gas in slit nanopores under different pressures. A modified bin method considering local gas density was proposed to reveal the mass velocity profile of methane gas in nanopores. Combining the trajectories of representative molecules, we found that the underlying mechanism of methane gas transport characteristic behaves as the competition between gas intermolecular collisions and gas-wall interactions. The transport characteristic of methane gas in slit nanopores is dependent on the pressure. Meanwhile, the pore width can significantly influence the mass flux contributions of different transport mechanisms. Under the high pressure, viscous flow is the primary transport mechanism due to the frequent gas intermolecular collisions. Under lower pressure and in larger nanopores, Knudsen diffusion becomes the dominating transport mechanism due to the enhancement of gas-wall interactions (collisions). Narrower nanopores can significantly increase the mass flux contribution of surface diffusion due to the increased proportion of absorbed gas molecules. In particular, we proposed a theoretical model coupling multiple transport mechanisms, which can accurately predict the apparent permeability of methane gas in slit nanopores. Under lower pressure and in narrower nanopores, the enhancement of surface diffusion and Knudsen diffusion can increase the apparent permeability of methane gas by as much as 50 times or more, demonstrating the sensitivity of pressure and pore size for methane gas transport.

3. Int. J. Heat Mass Transf.

   

   Lattice Boltzmann method simulations about shale gas flow in contracting nano-channels

   X.Z. Li, J. Fan, H. Yu, and Y.B. Zhu

   International Journal of Heat and Mass Transfer, 2018, 122: 1210-1221.

   Abs DOI

   Shale matrix consists of numerous interconnected nanoscale slit pores with size ranging from 4 nm to 200 nm, naturally resulting in variable cross-section scenarios for shale gas transport. Understanding the behavior of shale gas flow in variable cross-section channels would have practical benefits to further reveal the gas flow mechanism. Herein, using multiple-relaxation time lattice Boltzmann method (LBM) simulations, we investigate the flow characteristic of shale gas in sudden and gradual contraction channels. The influences of Knudsen number and cross-section shrinking coefficient on the shale gas transport are discussed in details. We find that the relationship between centerline velocity and Knudsen number is dependent on the gas flow regime. The nonlinear pressure deviation shows a negative correlation with the Knudsen number. The slip mass flux and the second-order permeability correlation factor are found to have positive correlations with the cross-section shrinking coefficient, demonstrating that the shale gas apparent permeability would be underestimated without considering the influence of cross-section contraction. Moreover, we reveal that the shrinking of pore channel has a great influence on Knudsen minimum effect. A high cross-section shrinking coefficient and dramatically varied cross-section type will lead to a low critical Knudsen number of Knudsen minimum effect. The LBM results and insight obtained here would be conductive to understanding shale gas transport in contracting nano-channels.

2017

1. J. Univ. Sci. Technol. China

   Research progress of micro/nano mechanical problems in unconventional oil and gas exploitation

   J. Fan, H. YU, J. Chen, X.Z. Li, F.C. Wang, and H.A. Wu^*

   Journal of University of Science and Technology of China, 2017, 47(02): 142-154. (Invited Review)

   DOI
2. AIP Adv.

   

   Channel-width dependent pressure-driven flow characteristics of shale gas in nanopores

   J. Chen, H. Yu, J. Fan, F.C. Wang, D.T. Lu, H. Liu, and H.A. Wu^*

   AIP Advances, 2017, 7(4): 045217.

   Abs DOI

   Understanding the flow characteristics of shale gas especially in nanopores is extremely important for the exploitation. Here, we perform molecular dynamics (MD) simulations to investigate the hydrodynamics of methane in nanometre-sized slit pores. Using equilibrium molecular dynamics (EMD), the static properties including density distribution and self-diffusion coefficient of the confined methane are firstly analyzed. For a 6 nm slit pore, it is found that methane molecules in the adsorbed layer diffuse more slowly than those in the bulk. Using nonequilibrium molecular dynamics (NEMD), the pressure-driven flow behavior of methane in nanopores is investigated. The results show that velocity profiles manifest an obvious dependence on the pore width and they translate from parabolic flow to plug flow when the width is decreased. In relatively large pores (6 – 10 nm), the parabolic flow can be described by the Navier-Stokes (NS) equation with appropriate boundary conditions because of its slip flow characteristic. Based on this equation, corresponding parameters such as viscosity and slip length are determined. Whereas, in small pores (∼ 2 nm), the velocity profile in the center exhibits a uniform tendency (plug flow) and that near the wall displays a linear increase due to the enhanced mechanism of surface diffusion. Furthermore, the profile is analyzed and fitted by a piecewise function. Under this condition, surface diffusion is found to be the root of this anomalous flow characteristic, which can be negligible in large pores. The essential tendency of our simulation results may be significant for revealing flow mechanisms at nanoscale and estimating the production accurately.