Mechanical Mechanisms Of Active Control Of The Dynamic Wetting Of Nanofluid

Nanofluid refers to the new liquid medium by evenly dispersing nanostructures like nanoparticles,nanowires,nanotubes to the base fluid such as water,oil,alcohol etc.Due to the excellent thermal conductivity,electrical conductivity,fluidity,friction and drag properties,nanofluid have been extensively stuied in fields of heat transfer,energy,chemical,drug transport etc.One can actively obtain various amazing nanofluid properties via simply changing the nanoparticle charaterizations,which makes the nanofluid a kind of "smart fluid".In recent years,with the rapid development of nanotechnology,nanofluid and its wetting phenomena are becoming more and more significant in areas like micro-fludics,nano-electronical systems,micro-nano device design,DNA self-assembly.Dynamic wetting of nanofluids is an important branch of the solid-liquid interface wetting.The nanofluids have more unique wetting properties compared with base liquids due to the addition of nanoparticles,which have a large surface area,and their interactions with liquid molecules and solid walls in nanoscales.Nanoparticles can adjust the physical properties of nanofluid like viscosity,surface tension and rheology proeperties,which will affect the nanofluid wetting properties such as viscous diffusion,equlibrium contact angle and evaporation.Besides,nanoparticle motion,accumulation,self-pinning and formation of polycrystalline structures will change various forces,like disjoining pressure,surface tension,local viscosity and friction force at the contact line region,as well as the competition between them.As a result,the effects of nanoparticles on both the bulk and local wetting properties of nanofluid are the focuses of researches on nanofluid wetting mechanisms.Especially the dynamic regulation of the contact line motion by leads to various depostiotion patterns of nanoparticles,which is important in applications like inkjet printing and coating.At the microscopic scale,the surface effect is highlighted.Surface force,Van der Waals force,electrostatic force,viscous force and the disjoining pressure will become the dominant factors in the dynamic wetting of nanofluid,thus it is necessary to study the wettig mechanisms and active control of nanofluid from a molecular scale.In this paper,four key problems,that respectively are the nanoparticle-tuned dynamic wetting,nanoparticle-regulated contact line motion,nanofluid droplet evaporation with multi-rings depositions by nanoparticles and the directed movement of droplets on a conical substrate are studied.The dynamic wetting of nanofluid droplet was studied through molecular dynamics simulations.The inhibition effect of nanoparticles on the nanofluid wetting was analyzed based on the spreading law,which describles the power law relation between the contact radius and the spreading time.With the increase of nanoparticle volume fraction and hydrophilicity,the spreading mechanism of nanofluid droplet was changed from disjoining pressure and surface tension dominated spreading to viscous force dominated spreading.This achieved the active control of nanofluid wetting and provides theory basis for understanding the physical and mechanical behaviors of nanofluids on solid substrates.In order to further explore the effect of nanoparticles on the dynmiac wetting of nanofluids,this paper studied the active control of three contact line motion forms using a single nanoparticle:slip without hysteresis,alternate pinning-depinning and complete pinning.The excess free energy,which is caused by the deformation of droplet from equilibrium state,overcome the potential energy barriers and provides driving forces for the contact line and nanoparticle.Two originations of energy barrier,those are the adhesion of nanoparticle and liquids to the substrate,and their effect factors were analyzed.The distribution theory of excess free energy was proposed to expound the effect of substrate wettability on the dirving forces on nanoparticle.Bsed on the excess free energy driving menchamism and the exces free energy distribution theory,the nanoparticle-regulated pinning and de-pinning criteria of contact line was proposed.Furtherly,the evaporation of nanolfuid droplet on substrate with alternated hydrophilic and hydrophobic stripes,as well as the multi-ring depositons of nanoparticles were studied.The effect of the substrate wetting feature size(the stripe length)on the multi-ring structures are explained from two aspects,those respectively are the initial pinning mechanism of contact line and the forces evolution mechanisms of contact line during the stick-jump movement.The competition among the capillary driving force,the nanoparticle-exerted hysteresis force as well as the substrate-exerted hysteresis force on the contact line was analyzed based on the substrate wetting feature size.Based on the obtained results,the structures of self-assembly of nanoparticles and the design maps of multi-rings depositons are proposed.Above studies on the nanofluid droplet spreading,controlling of contact line motion and nanoparticle depositions analyzed the regulation mechanisms of nanoparticles on the dynamics wetting of nanofluid.On this basis,this paper aims at furtherly manipulate the dynamics wetting and movement of nanofluid droplet on complex substates with geometrical,physical or chemical inhomogeneity.The mechanism of spontaneouly directed transport of a droplet on conical substrate sthrough both Molecular Dynamics simulation and Surface Evolver simulation.The curvature ratio,that is,the ratio of the local curvature of the substrate to the on-it droplet radius,results in three movement stages of droplets on the conical substrate from the tip side to the base side.The Laplace driving force and the contact angle hysteresis force of each stage were explained by examining the effect of curvature ratio on the droplet morphology,and the stagnation of droplet when excessing the critical curvature ratio was expounded.It enables manipulation of droplets self-driven by means of control the substrate curvature gradient,which provides important basis and exploits thinking for regulation of nanofluid wetting on complex substrate or microchannels.