The interfacial tension (IFT) is an important factor for the hydrocarbon flow in a porous reservoir. Methane, one main accompanied gas of hydrocarbon, has a crucial influence on the IFT. However, the methane effect is still unclear. In this work, the effects of the methane content, temperature, and pressure on the water–oil interface were investigated, employing molecular dynamics simulations. The interfacial density profiles were given and indicated that the methane molecules accumulate at the interface, leading to a decreasing IFT. As the methane mole fraction increases, the interfacial roughness and interfacial thickness increase and the induced deeper molecular penetrations and stronger miscibility initiate a decrease of the IFT. Further, an enhancing fluid diffusivity at the interface is observed, which accounts for the strengthening interfacial mobility. On the other hand, our calculations indicate that the IFT decreases with the rising temperature while increases with the strengthening pressure. Our study provides an in-depth understanding of the interfacial behavior in the ternary phase system and has some promise for the exploitation of shale oil.

Fluidic flow in carbon nanochannels or nanopores has shown great application prospects in nanofiltration. Therefore, enhancing water flux while maintaining their filtration property is crucial to further extend their applications. In this work, inspired by the hourglass-shaped aquaporin water channel, we proposed a method to optimize water flux in carbon nanochannels using conical carbon nanochannels. Adopting nonequilibrium molecular dynamics simulations, water flow in a series of conical carbon nanochannels was simulated. The results showed that the conical channel with apex angle of 19.2° (38.9°) had the optimum water flux when water flowed from the base (tip) side to the tip (base) side of the conical channel, and the water flux was nearly twice as the recently developed MoS2 desalination membrane. Then, the physical mechanism for the conical channel optimizing water permeation was revealed through detailed analyses of potential of mean forces, average number of hydrogen bonds and pressure distributions of the simulation systems. Finally, the microscopic water structures in these channels were also analyzed to further rationalize the optimizing mechanism. This work indicates that other than decreasing the membrane thickness, regulating the channel configuration is a more effective method to enhance water permeation rate in channels with limited channel sizes.Download high-res image (122KB)Download full-size image

•Molecular dynamics simulation was firstly used to address CO2 WAG in nanochannel.•The different roles of scCO2 slug and water slug in CO2 WAG injection were revealed.•The synergistic effects of scCO2 slug and water slug were proposed.CO2 water-alternating-gas (WAG) injection has been proven as a promising and effective technology to significantly enhance the recovery of the residual oil. However, the detailed displacing process and exquisite interaction mechanism still remain ambiguous. By using Nonequilibrium Molecular Dynamic Simulation (NEMD), three comparative displacement processes of adsorbed oil layer in nanoslit using water, supercritical carbon dioxide (scCO2), and CO2 WAG injection were conducted to study the enhanced oil recovery (EOR) mechanism of CO2 WAG injection. Simulation results indicate that the scCO2 and water play different roles in CO2 WAG: the scCO2 slug could selectively dissolve apolar oil components, and the water slug provides stable water/scCO2 promoting interface, which compels the miscible oil/scCO2 slug out of the nanoslit. Furthermore, the synergistic effects of scCO2 slug and water slug were found to account for a decreased injecting pressure and an improved dissolution capability of scCO2 slug, which endows the CO2 WAG higher oil recovery efficiency than that of both scCO2 injection and water injection. Our work here provides a comprehensive interpretation of EOR mechanism of CO2 WAG, and the results could provide insights into the optimization of the engineering process.

In tight oil reservoirs, nanopore throat acting as the narrowest section of fluidic channel determines the oil transport performance; injecting CO2 is found to significantly promote the oil flow. Despite substantial efforts, the underlying transport mechanism of above phenomenon remains unclear. Employing molecular dynamics simulation, the oil transport through a nanopore throat is studied. A high energy barrier derived of conformation deformation, oil/pore interaction and Jamin effect is found to impede the oil transport. The CO2 activating effect for oil transport is present, and a dependence on CO2 amount is observed. The underlying mechanism was well documented from the aspects of oil swelling, interfacial tension and surface sliding. Our study provides fundamental insight into the oil transport across nanopore throat and CO2 activating effect; the results have promising applications in enhanced oil recovery in CO2 flooding.

Rational regulation of the size and uniformity of nanoparticles has drawn great interests and shown widespread application, but this cannot be simply achieved by the vapor route. In this work, by adopting a chemical vapor deposition approach, the growth process was intricately regulated to guide the reagent supersaturation, and the large-scale growth of uniform-sized In2O3 nanooctahedra was realized. A one-time nucleation and synchronous growth mode controlled by the reagent supersaturation ratio is proposed to be responsible for the uniformity of size. Furthermore, a series of comparative experiments were conducted to study the size dependence on reaction duration, and temperature difference between the heating and depositing zones. This study demonstrates a feasible approach to prepare uniform-sized nanoparticles through precisely controlling the crystal growth process, and the developed growth strategy could be generalized to synthesize uniform-sized nanostructures of other material systems.

Graphene oxide (GO), as an ultrathin, high-flux, and energy-efficient separation membrane, has shown great potential for CO2 capture. In this study, using molecular dynamics simulations, the separation of CO2 and N2 through the interlayer gallery of GO membranes was studied. The preferential adsorption of CO2 in the GO channel derived from their strong interaction is responsible for the selectivity of CO2 over N2. Furthermore, the influences of interlayer spacing, oxidization degree, and channel length on the separation of CO2/N2 were investigated. Our studies unveil the underlying mechanism of CO2/N2 separation in the interlayer GO channel, and the results may be helpful in guiding rational design of GO membranes for gas separation.

Design and fabrication of functional carbon nanostructures is a challenging but meaningful mission for scientists to propel the development of nanotechnology, such as nanopharmacology, nanobiology and nanofluidic manipulation. In order to fabricate the carbon nanostructures, using forced-field-based molecular dynamics simulations, we proposed a feasible method to obtain the carbon nanostructures through tailoring-induced self-scrolling of graphene flakes. And the shapes of the carbon nanostructures could be regulated by controlling the tailoring patterns. We also analyzed the mechanism for the tailoring-induced self-scrolling of graphene flakes. By analyzing the scrolling process in detail, it was found that the van der Waals interactions were responsible for the formation of the carbon nanostructures. In addition, we also found that the tailoring could also induce the scrolling of Boron Nitride flakes, and this indicates that the tailoring might provide an universal method for self-assembly of two dimensional materials. This work is expected to trigger further studies on the design of nanostructures and the applications of these nanostructures in functional nanodevices.

Owing to the outstanding and extraordinary wetting properties, heterogeneous surfaces have extensive and intriguing applications, such as microfluidic systems, inkjet printing and protein hydration. In this work, a heterogeneous surface is constructed by adding one hydrophilic patch at the center of a hydrophobic surface, and the dynamical adsorption process of nanoscale water droplets with different sizes on the heterogeneous surface is investigated adopting molecular dynamics simulations. The detailed adsorption processes and microscopic parameters including contact angle, density distribution profile, interaction energy, etc. are analyzed. The results present that as the droplet size increases, the water droplet would experience spreading, restricting, vibrating and slipping on the heterogeneous surface. In the process of spreading, all the water molecules spread on the hydrophilic patch under the hydrogen-bond interactions. As the droplet size increases, the pre-adsorbed water molecules play the role of an “anchor”, promoting the adsorption of the rest of the water molecules and restricting the water droplets within the hydrophilic patch. By increasing the droplet size further, some water molecules would get rid of the restriction from the pre-adsorbed water molecules and vibrate at the boundary of the hydrophilic patch. Finally, during the process of slipping, the wetting behaviors of the water droplet are dominated by the hydrophobic surface. The wetting behaviors discussed here are helpful for comprehensive exploration of the wetting behaviors of various solutions on the heterogeneous surface at the molecular level, and the present research might trigger further studies on the applications of the homogeneous surfaces.

•We find a special phenomenon called “water molecules spread on precursor film” during the motion of droplet.•We investigate the microscopic mechanism for the motion of nanoscale water droplet.•The Ebind and Ewater are analyzed to unveil the driving and resisting forces.•The narrow modified segment and large wetting gradient can promote the motion of the nanodroplet.In this work, molecular dynamics simulations were employed to investigate the motion behavior of water nanodroplet on wetting gradient surface. Research exhibited that the water nanodroplet could move from the hydrophobic segment to the hydrophilic segment, spontaneously. Detailed observation revealed that the motion of nanodroplet included spreading and shrinking processes, and a precursor film forming on the −NH2 segment played a pivotal role on the transportation of nanodroplet. The microscopic motion mechanism was discussed, and the intricate and intriguing effects of Ebind and Ewater were analyzed to unveil the driving and resisting forces. Furthermore, the modified segment width and wetting gradient were also discussed, and results unraveled that the narrow modified segment and large wetting gradient promoted the motion of the nanodroplet. Our work provides fundamental insights into the microscopic motion mechanism of nanodroplet, and the results are expected to facilitate the design and fabrication of wetting gradient surface.

Stimuli-responsive self-assembly is playing an increasingly important role in emerging applications, ranging from smart materials to biosensors. However, obtaining essential information for further development, such as molecular arrangement and interaction, is still experimentally challenging. A molecular-level understanding of the stimuli-responsive self-assembly is needed. Azobenzene-containing (azo-containing) amphiphiles organize into photo-responsive assemblies because of the cis–trans isomerization triggered by the irradiation of ultraviolet (UV) and visible light. In this study, we applied a coarse grained (CG) molecular dynamics (MD) simulation, with the necessary potential parameters fitted from theoretical calculation data, to study the photo-induced self-assembly of 4,4′-bis(hydroxymethyl)-azobenzene (AzoCO), a simple azo-containing amphiphile. An unusual “chaotic micelle” and “monolayer phase” were obtained with cis- and trans-AzoCO molecules, respectively. The structural information and formation mechanism were studied. The “chaotic micelle” possesses a chaotic but not a pure hydrophobic interior as commonly understood. Through comparative simulations, we found that the azo (–NN–) group of azobenzene plays a crucial role in the formation of the “chaotic micelle”. The “monolayer phase” is arranged by abreast rod-like trans-AzoCO molecules; the axial symmetry of the trans-AzoCO molecule drives the formation of this structure. The novel “chaotic micelle” and “monolayer phase” have potential applications in nanotechnology and bioengineering. This work is expected to trigger further studies on stimuli-responsive phenomena and materials.

In this work, molecular dynamics simulations were employed to investigate the influence of inhibitor concentration on inhibition performance. The adsorption configuration, adsorption process, and inhibition performance were studied. We found that an ordered self-assembled inhibitor film can form on a metal surface, which plays a key role in corrosion inhibition. When the concentration reaches a critical value, inhibitor molecules organize into a micelle in aqueous solution. Further increasing the concentration did not change the inhibitor film on the metal surface, so that the inhibition efficiency remained at a constant level. Our analyses demonstrate that the number of water molecules displaced by inhibitor molecules, the diffusion of water molecules in the inhibitor film, and the stability of the inhibitor film play key roles in the process of corrosion inhibition. This work provides a thorough molecular-level understanding of inhibition behavior and the role of inhibitor concentration.