Mechanism Of Fluid Transport In Nanochannels:Molecular Dynamics Study

The transport behavior of fluid in a nanochannel is closely related to human's production and living,so study of fluid flow in a nanochannel is of great value and necessity.However,it is difficult to directly observe the fluid flow in a nanochannel by means of current experiment methods due to the limitations of the existing experimental conditions and means.Therefore,understanding the behavior and the mechanism of fluid flow in nanoscale confined spaces at the molecular level by means of computer simulation technology is of great importance for the development and application of nanofluid technology.In this paper,the flow behavior and the corresponding mechanism of fluid in a serious of rock nanochannels and the interlayer nanochannels of graphene-oxide membranes were studied by molecular dynamics simulation.The main contents of this paper include the following five aspects:?1?The dynamical mechanism of oil transport in SiO₂ and CaCO₃ nanochannels are studied.The results show that the interaction between oil and SiO₂/CaCO₃nanochannels plays an important role in the migration of oil in the nanchannels.Due to the different interaction energies between oil and SiO₂ channels of various width,the displacement of central of mass?COM?of oil in a 6 nm SiO₂ channel is over 30times larger than that in a 2 nm channel after 2 ns simulation,and due to the different interaction energies between the SiO₂ channel surface and the oil molecules at different locations,the diffusion coefficient of oil molecules in the middle of the 6 nm channel is almost twice that of the oil molecules near the channel surface.Because the electrostatic interaction between polar oil molecules and SiO₂ channel surface is much larger than that between nonpolar oil molecules and the channel surface,phenol and pyridine molecules are more difficult to migrate in the nanochannel compared with C18.It is also found that the channel component has an important effect on the migration of C18 in nanochannel.For example,the COM displacement of C18 in the gold channel is much smaller than that in the SiO₂ nanochannel and the CaCO₃nanochannel under the same conditions,due to the different interaction energies between C18 and various nanochannel surfaces.Finally,the effects of surface roughness on the transport velocity and the flow characteristics oil are studied.The deeper the cavity is,the smaller the COM displacement is.These results will provide useful theoretical guidance for the study of residual oil recovery in nanchannels.?2?The detachment process of oil molecules from the SiO₂ channel surface by the use of surfactant cetyltrimethylammonium bromide?CTAB?aqueous solution,and the migration process of the oil droplet in SiO₂ nanochannels during water injection are studied.The results show that surfactant molecules help to promote the formation of water channels between the oil droplets and the SiO₂ channel surface during the detachment process,which is crucial for reducing the interaction between oil droplets and SiO₂ channel surfaces;and the surfactant is beneficial to prevent the oil molecules from readsorbing to the nanochannel surface.The pressure gradient has a great influence on the migration of oil droplets.The greater the pressure gradient is,the faster the oil droplets migrate.Due to the large interaction between the polar oil molecules and the SiO₂ channel surface,the phenol droplet is difficult to detach from the surface.Under the same pressure gradient,the COM displacement of the phenol droplet is one-eighth of the displacement of the C12 droplet.Due to the strong hydrophilicity of the CaMg?CO₃?₂ surface,the C12 droplet is easy to be detached,while at the same time the water molecules near the CaMg?CO₃?₂ surface are strongly adsorbed by the channel surface.The COM displacement of the C12 droplet in the CaMg?CO₃?₂ nanochannel is one-seventh of that in the SiO₂ nanochannel.Due to the strong interaction between the graphene and the oil molecules,the C12 droplet are difficult to detach from the graphene surface by use of the surfactant aqueous solution.These results provide a scientific theoretical basis for enhancing oil recovery in the low-permeability reservoirs.?3?The extraction and migration process of dodecane in a graphene slit with scCO₂ are studied.The results show that some C12 molecules can be dissolved from the graphene surface by scCO₂,15.1 percent of the C12 molecules in the first adsorption layer and 69.8 percent of the C12 molecules in the second adsorption layer can be dissolved from the graphene surface,then the interaction energy between oil and graphene surface is reduced,thus the oil recovery can be enhanced.The increase of temperature is beneficial to the dissolution of the oil molecules.The number of C atoms in the first adsorption layer and the second adsorption layer are reduced by 17.2%and 33.3%respectively,as the temperature increases from 323 K to 373 K.Pyridine is easy to be detached from the graphene surface by scCO₂,which leads to the decrease of the interaction energy between the pyridine molecules and the graphene surface by25%.The displacement efficiency of light oil is higher than the alkanes.These results will provide a useful theoretical guidance for the study of scCO₂ flooding in shale.?4?The migration process and its mechanism of hydrogen/methane mixture?H₂/CH₄?in the interlayer nanochannel of GO are studied.The migration characters of gas molecules in the interlayer nanochannel depend on the interaction between the molecules and the GO sheets,as well as the size of the molecules.The interaction between the CH₄ molecules and the GO sheets is much greater than that of H₂molecules.The strong adsorption of GO sheets on CH₄ molecules inhibits the migration of CH₄ molecules in the interlayer nanochannel.Simulations show that the degree of oxidation of GO and the interlayer space have great influence on the migration of gas molecules.The higher the degree of oxidation is,the lower the permeability of H₂ is.The greater the interlayer space is,the higher the permeability of CH₄ is.Under the influence of CH₄,the penetration of H₂ first increases and then decreases.It is also found that the increase of temperature and the initial pressure can greatly increase the permeability of H₂.The above results reveal the potential mechanism of gas migration and separation in the nanochannel of graphene oxide,and propose a suitable theoretical structure of GO separation membrane for H₂/CH₄,which will provide a theoretical basis for developing effective layered separation membrane materials.?5?The migration mechanism of water molecules in the interlayer nanochannel of graphene oxide and the desalination performance are studied.The simulation results show that the migration of water molecules in the interlayer nanochannel is affected by the interaction between the nanochannel and the water molecules.The desalination mechanism of the GO membrane can be attributed to the steric hindrance effect and the adsorption of the nanochannel surface.The interlayer space,the pressure difference and the proportion of oxygen-containing functional groups in graphene oxide layer have significant effects on the permeability of water molecules and the salt rejection rate.With the increase of the interlayer space,the permeability of water increases gradually and the rejection of salt ions gradually decreases.As the interlayer distance of the graphene oxide increases,the water permeability gradually increases.The higher the hydroxyl content on the GO sheets is,the smaller the permeability of water molecules is,and the higher the rejection of salt ions is.