Characteristics Of Fluid Transport On Grapheneengineered Surfaces

The fast transport of fluids on solid surfaces plays a critical role in micro-and nano-fluidics,energy conversion and a range of heat transfer applications,and is directly related to the efficiency of these processes.Adhesion between solid and liquid will lead to extremely low tangential velocities of fluids near the wall,and even zero at the solid-liquid interface,which becomes a key obstacle to efficient fluid transport.With the miniaturization of functional device and flow scale,the influence of solid-liquid interactions on mass transport has become more and more prominent.There are usually two strategies to reduce solid-liquid adhesion:modifying coatings with low surface energy on solid surface and decreasing solid-liquid contact area by constructing micro-and nano-structures.However,these methods usually suffer from weakening solid-solute interaction or failing due to water filling inside structures.It is still of great theoretical significance and practical applications to achieve fast and long-lasting fluid transport while hold the functionality of the solid surface,but it is also very challenging.This dissertation proposes a strategy of using monolayer graphene to regulate the solid-liquid interactions.Thtough the simple architecture design of graphene-covered solid surface,solid-liquid adhesion can be reduced while protecting specific property of the underlying substrate.The main results and conclusions are as follows:(1)Adhesion properties of graphene engineered surface based on silicon wafer substrate are investigated by means of water contact angle tests and solid-liquid friction measurements.The results show that graphene can reduce the threshold of adhesion force and,therefore,enhance the sliding of liquid droplets on smooth hydrophilic surface.The inherent contact angle hysteresis of silicon wafer can be changed from 45.9°to 36.3°by transferring a single layer of graphene on the surface.And the sliding angle of droplets with volume of 20?30?L can be reduced by 20°?30°.Measurements of solid-liquid friction demonstrate that the adhesion threshold of a droplet just as it happen to slide on a surface will be decreased by34.6%due to the presence of graphene,and thereby eliminating the transition stage from resting to sliding.Moreover,the kinetic friction of the droplet when it slides can be reduced by 12.2%.(2)The flow characteristics and electric double layer effects on graphene engineered surface based on silica substrate are investigated by molecular dynamic simulation.The results show that graphene can shield the short-range interaction between substrate and water molecules,but has little effect on the long-range Coulomb interaction between charged wall and ions dissolved in water.This graphene engineered interface allows for strong flow slip that commonly exists on hydrophobic surface,with slip length ranging from 4.5 nm to 7.6nm at 0?-70 m C m^-2 charge densities.On the other hand,there exist an electric double layer effect on this uniquely heterogeneous surface similar to that on graphene-free surface with the same charge density.It is shown by theoretical analysis that the efficiency of streaming potential in a nanochannel formed with such graphene engineered surface can be enhanced by a factor of 20?100 at different charge densities.(3)The wetting and condensation performance of graphene engineered surface based on structured hydrophobic substrate are investigated experimentally.Micro pillar arrays are structured by combing photolithography and template method,and then graphene was successfully transferred to the structured surface.It was demonstrated that graphene can inherit the hydrophobicity of underlying substrate while preventing water filling inside the valleys,thus avoiding the wetting transition from Cassie mode to Wenzel mode for liquid droplets.The sliding angle of this graphene-covered surface is also smaller than that of graphene-free one.In view of these properties,the condensation rate of vapor on the graphene engineered surface can be increased by 60%at a temperature difference of 40?.The nature of solid-liquid interface plays an essential role in fluid transport.The strategy to design the novel interfacial structure by inserting single atomic layer of graphene and the characteristics of fluid transport on such engineered surfaces are of great academic value and practical significance for modulation of interfacial molecular interactions and achieving fast and efficient fluid transport.