Mechanical Mechanisms Of Wetting At Solid-liquid Interfaces And Mass Transport On The Nanoscale

Wetting of solid-liquid interfaces and mass transport at the nanoscale have attracted wide attention in academia and industry in recent years,due to their scientific significance and important application prospects in materials,energy,environment and other fields.On one hand,microscopic intermolecular interactions are the essence of wetting problems and reflect the macroscopic wetting properties.Understanding the basic principles of wetting at the microscale helps to develop wetting theories,explore the interface science,and lay a theoretical foundation for regulating macroscale wetting properties and mass transport behaviors.On the other hand,with the development of nanotechnology,many nanostructures can be manufactured,such as nanopores,nanochannels,graphene capillaries and micro-patterned surfaces.Liquids on these nanostructured surfaces manifest special wetting behaviors such as fast and unidirectional transport,super-hydrophobicity and wetting transition,which can be applied to realize certain functions like self-assembly,self-cleaning,power generation,water desalination,microfluidic control and enhanced oil recovery.Therefore,the mechanical mechanisms of wetting and mass transport at the nanoscale are not only the theoretical bases of interfacial science but also essential for industrial production.The contact line refers to the area where three phases meet.It plays an important role in determining the wetting and transport behaviors.This dissertation focused on the effect of contact line on wetting and transport properties and discussed three aspects,including microscopic origin of capillary force balance at contact line,evaporation-driven liquid flow through nanochannels and molecular mechanism of viscoelastic polymer enhanced oil recovery in nanopores.A theoretical model was proposed to describe and quantify the capillary force on the liquid in coexistence with its vapor phase.The analysis was based on the decomposition of the solid-liquid and solid-vapor interface tensions into three terms,either of which has a clear physical meaning.The proposed model was verified by molecular dynamic simulations over a wide contact angle range.Following the same approach,Young's equation was also validated at the nanoscale from a view point of mechanical interpretation.Then this theoretical model was extended to more general situations including on rough solid substrates and in two-liquid systems.Surface roughness of solid substrate was found to reduce the mobility of liquid and alter the contribution strategy of solid-liquid perpendicular interaction to the capillary force.A new method of contact angle prediction was proposed based on Young's equation.Evaporation-driven liquid flow through nanochannels was investigated using molecular dynamics simulations.The evaporation flux from the solid-liquid interface was found to be higher than those from the middle region of the channel or the liquid-vapor interface,resulting in the size dependence of total flux.Based on Gibbs free energy analyses,the driving mechanism for evaporation was proposed to explain the effect of channel width,channel wall wettability and relative humidity on evaporation flux.The energy conversion analysis indicated that the effective pressure gradient exerted on a liquid flow by evaporation depends on the channel length.Evaporation-driven liquid flow through nanochannels could be modeled quantitatively using this knowledge.The detailed process of a viscoelastic polymer displacing oil trapped in a dead-end nanopore was investigated using molecular dynamics simulations.The interactions between polymer chains and oil were found to provide an additional pulling effect on extracting the oil droplet,which plays a key role in increasing the oil displacement efficiency.The results also demonstrated that the oil displacement ability of polymer can be reinforced with the increasing chain length and viscoelasticity.The results in this dissertation not only provide new physical insights into the interface wetting and mass transport phenomena at the nanoscale,but also have important scientific significance in the application fields of micro-nano fluidic chip design,nanostructure self-assembly,water desalination,enhanced oil recovery,etc.