Water Transport Performance Of Inhomgenous Nanoconfined System

Water is the source of life and the most important resource on the earth.It plays an extremely important role in human survival and production and life.The research on water has always been a hot topic in scientific research.With the deepening of the research on nanoscale,confined water at this scale has also attracted extensive attention.Confined water generally exists in the restricted environment of biological channels and synthesis,and shows great differences with bulk water in terms of structure,thermodynamics,electrochemistry and kinetics.Due to its atomically smooth wall surface and ultra-long slip length,graphene and its derived materials have great application prospects in the fields of seawater desalination,ion screening,drug transport,etc,and are usually used as materials for nano-transport channels.Therefore,the study of the transport properties and related mechanisms of confined water is helpful to the study of the mechanism of micro-nano scale water and has guiding significance for the preparation of nanochannels.In this paper,the molecular dynamics simulation method was used to systematically study the water transport mechanism,droplet movement and water phase transformation in graphene nanochannels based on the molecular interaction force and coulomb interaction at the solid-liquid interface.The specific research content is divided into the following parts:Firstly,the transport behavior of water confining in an inhomogeneous nanochannel is studied.At 300K,the confined water in the nanoscale channel presents the coexistence of single,double and triple layers of liquid to adapt to the inhomogeneous confinement.With the increase of density,the confined water is transformed into a two-dimensional ice crystal with regular arrangement of oxygen atoms,and the change rule of the density of water structure transition with height is found.The change of water structure leads to the change of friction coefficient,which affects the transport performance of water.When pressure is applied to the water molecules,the confined water at different densities produces three velocity distributions.When the water is in low density(0.8 g/cm~3)or high density(1.4 g/cm~3),the water confined in the large layer spacing move faster then the water confined in the smaller layer spacing.At intermediate density,the velocity distribution of water molecules between different layer spacing tends to be consistent.The average velocity of water decreases first,then increases and then decreases with the increase of overall density.Secondly,the droplet movement in nanoscale is discussed.The addition of Cu substrate to graphene makes the graphene become hydrophobic.At this time,when the droplet is placed at the opening of the wedge-shaped graphene space,it realizes directional drive due to the action of capillary force and Van Der Waals force.When the droplet moves from the open end to the narrow end region,the droplet will gradually slow down,and then slowly enter the parallel graphene region.After a period of equilibrium,the phase state of the water in the parallel region will also change at a certain height.The height of the parallel region has a great influence on the phase state of water.When the height is between 6(?) and 7(?),water transforms from liquid to form a stable tetragonal ice crystal structure.When the height is greater than 7(?) and less than 9(?),a double-layer water structure is formed;when the height is greater than 9(?),a liquid bridge similar to the bulk water is formed.The influence of solid-liquid interaction parameters on droplet movement is also studied.The critical surface energy parameters that water can enter parallel regions are found.When the Cu substrate is removed,the droplets are ejected again from parallel regions due to the hydrophobicity of graphene.The results of this study enrich the phase diagram of confined water,provide a new idea for the movement control of nanoscale droplets,and have certain guiding significance for the fluid transport in micropores,the permeation behavior of nanofiltration membrane,drug transport and water collection.