Numerical Simulation Of The Inhomogeneous Structures And Fluidic Properties Of Nano-confined Flows

With the recent development of micro-chemical technology and membrane science,the nanoflows become ubiquitous in chemical engineering.However,theoretical understanding about the nanoflows is still scarce when facing with the failure of traditional hydrodynamics.The missing of theoretical models hinders the application of nanoflows.Thus,further development of nanofluidics is being required to solve the problems such as the deviation between nanoflows and traditional theories,and the regulation of nanoflows.This work is aimed at providing a systematic investigation of nanofluidics,targeting at the confined fluidic structure,the pressure-driven flow mechanism,the spontaneous flow mechanism and the flow within grafted-macromolecules.It is expected to unravel the nanofluidic mechanisms,improve the theoretical models and develop multi-scale simulation methods with both accuracy and efficiency.The primary findings are listed as follows.(1)DPD simulation is employed to investigate the nano-confined behaviors of fluid,especially its heterogeneous structure and enhanced transport.It is demonstrated that nanoflows are closely correlated to the heterogeneous boundary region.Resulted from the heterogeneous density,the heterogeneous viscosity directly lead to a distortion of velocity profile and an enhancement in transport flux.Meanwhile,the resistance composition of a nano-channel is also analysed by DPD simulation.A funnel(hydrophilic)-channel(hydrophobic)structure is designed to optimize the flow resistance.(2)Involving the heterogeneity of nanoflows,theoretical models are derived for the transport flux and apparent slip length.The flux model is verified by DPD simulation and previous experiments.It is demonstrated that the model,on one hand,can be applied to nanofluidic systems to make precise predictions,and on the other hand,can be simplified to traditional hydrodynamic models when enlarging the channel width.The new model sheds a light on the nanofluidic mechanism and bridge the gap between the nano-and macro-fluidics.The model for apparent slip clarifies the increasing tendency of slip length with descending channel width.(3)Based on the membrane of layered graphene and carbon nanotube,and on the capillary and evaporation processes of water within the membrane,MD simulation is employed to study the spontaneous water transport in nanochannels.The capillary imbibition flux of water shows non-monotonic variation with the channel width,which results from a complex competition among surface tension,slip length,confined density,etc.Thus,an optimal width can be assigned to a nanoscale capillary.The Lucas-Washburn equation that is widely used for capillary flows is modified to correctly reproduce the water imbibition within nanochannels.For the capillary-evaporation process,the spontaneous water flow is subjected to different controlling steps according to the channel length.In short channels,the process is controlled by the evaporation,which can be accelerated by hydrophilic modification on the external suface.In long channels,however,the process is controlled by the capillary process,which can be promoted by hydrophilic modification on the internal surface.Besides,the hydrophilicity gradient on the internal surface of nanochannels can act as a fluidic diode.(4)A coupled MD-DPD multiscale simulation method is developed to achieve accurate and efficient calculation.The MD-DPD method is verified in simple flows,where it provides accurate Poiseuille and Couette flow fields compared with analytical solutions.When applied to flow with bio-molecule or polymer grafted surfaces,the method can reproduce the results from MD simulation while significantly reduces the simulation time.With the aid of the MD-DPD method,the flow within grafted macro-molecules is investigated.Two kinds of models can be used to depict such flows:one is the slip length model and the other one is the coupled Brinkman-Stokes model.