Research On The Structure-functionalized Micro/nano Fluid-solid Interface For Regulating Fluid Transport Characteristics And Its Mechanisms

With population growth,environmental pollution,and climate change,the shortage of freshwater resources is gradually becoming a serious factor threatening human life,health,and industrial development.Fog collection and desalination have attracted considerable attention as economical and environmentally friendly ways of obtaining fresh water.Fog collection includes three stages:fog capture,droplet transport,and removal.The efficient transport of droplets is crucial since the detained droplets or residual water layer on the surface will decrease the fog collection efficiency.However,droplet transport on solid surfaces is still plagued by problems such as contact line pinning and long-distance lossless self-transport of multiscale droplets.As for the desalination of reverse osmosis,the selective transport of saltwater is also limited by the permeability-selectivity trade-off caused by the nanoscale pore size of reverse osmosis membranes,making it difficult to achieve efficient controllable fluid transport.To address the above problems,this thesis proposes the graphene composite surfaces and cross-hatch textured cone through micro/nano structure design and graphene coating from the perspective of micro/nano structural and functionalized design at the fluid-solid interfaces.The fluid transport characteristics on the structure-functionalized surfaces are investigated,and the influence and underlying mechanism of the structure-functionalization design of micro/nano fluid-solid interface on fluid transport.The main research contents and results are as follows:(1)The competition between the driving force on droplets and energy dissipation during motion,control of droplet motion stability,and the pinning effect caused by surface defects have always been key issues in the droplets transport on micro/nano-structured surfaces.Based on the wetting transparency and ultra-low fluid-solid interfacial friction of monolayer graphene,the monolayer graphene-covered gradient textured composite surfaces are proposed to realize the ultrafast unidirectional self-transport of water droplets.For monolayer graphene-covered gradient nanopillared composite surface,the droplet can move spontaneously from the sparsest to densest regions of pillars while a wettability gradient is created by the gradient distribution density of pillar matrix relying on the wetting transparency of monolayer graphene.In particular,the gradient short pillared texture triggers an opposite self-transport regularity in which the water droplet moves from the densest to sparsest regions of pillars,intrinsically because the gradient short-pillared surface leads to a wetting transition from hydrophobic to hydrophilic since the monolayer graphene can be adsorbed into the sparsest short-pillared texture.For the monolayer graphene-covered copper nanocone(GNC),the water droplet transport spontaneously from the tip to the base of the GNC.The droplet self-transport depends on the Young-Laplace pressure difference and the fluid-solid Interfacial friction,and the fluid-solid interfacial friction is proportional to the droplet velocity and the contact area between the droplet and the GNC.The rule of energy change during the droplet self-transport process indicates that the potential energy of the droplet and the interaction energy between the droplet and the GNC undergo cooperation and competition successively,resulting in the droplet first speeding up and then slowing down to a steady moving state.For the monolayer graphene-covered wedge-shaped copper composite surface,the droplets can realize the directional and ultrafast spontaneous transport from the tip of the wedge-shaped substrate to the wide end by wetting gradient and curvature gradient.The law of energy variation during the whole droplet self-driving on the composite surface indicates that there is a competitive relationship between the potential energy of water droplet and the interaction energy between the droplet and the composite surface,i.e.the interaction energy between water droplet and the composite surface is partially converted into the potential energy of water droplet.(2)In response to the challenges of slow transport speed,short distance,poor integrity and robustness faced by droplet self-transport,a novel cross-hatch textured cone(CHTC)with gradient microchannels and circular grooves is proposed to realize ultrafast directional long-distance self-transport of multi-scale droplets,whose self-transport velocity is about one to four orders of magnitude faster than natural or biomimetic structures with single curvature gradient.The CHTC triggers two modes of fluid transport:Droplet transport by Young-Laplace pressure difference and suction pressure-induced fluid transfer in microchannels on cone surface.The gradient microchannels connected by circular grooves ensure that the residual water layer and the water droplet detained on the CHTC can still be spontaneously transported to the base.A theoretical formula on the suction pressure is established to elucidate the driving force responsible for the self-transport of both detained water droplet and the residual water layer.Notably,the fog collection efficiency of the CHTC will be about twice as high as the cone without microchannels.These findings will open the door to achieve long-distance directional ultrafast self-transport of droplets and will provide tremendous inspiration for the design of water collection apparatus.(3)The permeability-selectivity trade-off rule is one of the main bottleneck problems of ion selective transport in reverse osmosis process.In this work,based on the temporal selectivity mechanism proposed by our group,a rotating porous graphene composite reverse osmosis membrane model is constructed by combining the ultra-low friction characteristic of the graphene-fluid interface to explore the temporal ion-selective transport behavior and mechanism.The molecular dynamics simulation results show that the permeability of rotating porous GCu M increases up to 156L/cm~2/day/MPa with a salt rejection larger than 90%,which is 2.6 to 1000 times larger than the commercial and other advanced reverse osmosis membranes.The boundary slip velocity between the saltwater-GCu M interface inhibits the passage of salt ions,resulting in high salt rejection even in the case of 2-4 nm diameter pores.The stability of the ionic hydration shell and the energy barrier of the ions passing through the pores increase with the increase of the angular velocity,which explains the influence of the boundary slip velocity on the salt rejection from the molecular mechanism.The geometric relation that governs the temporal selectivity principle on porous rotating GCu M has been further derived to explain pore size,membrane thickness,and hydrated ion mass on high permeability and salt rejection.