Droplet Dynamic Behaviors And Condensation Heat Transfer Characteristics On Micro/Nanostructured Superhydrophobic Surface

As the rapid development of microelectro-mechanical system,the scales of mechanical equipments decrease constantly,the solid-liquid interaction has significant effect on the performances of microfluidic mechanical equipments.In addition,the heat flux increases rapidly with the miniaturization of microelectronic devices,the heat dissipation has become a severe challenge for microelectronic equipments.Recently,micro/nanostructured superhydrophobic surfaces show the capabilities for regulating droplet dynamic behaviors and improving dropwise condensation,which could improve the performances of microfluidic mechanical equipments and offers a new path to realize the miniaturization of heat exchanger and enhancement in performances of thermal management system for devices with high heat flux.Based on above background,this paper systematacially studies the droplet dynamic behaviors and condensation heat transfer characteristics on micro/nanostructured superhydrophobic surfaces,mainly including the fabrication and wettability of micro/nanostructured hydrophobic and superhydrophobic surfaces,coalescence-induced droplet jumping on hierarchical microgrooved superhydrophobic surface,coalescence-induced aqueous ethanol droplet jumping with low mass fractions on micro/nanostructured superhydrophobic surfaces and condensation heat transfer characteristics of steam on microgrooved hydrophobic and hierarchical microgrooved superhydrophobic surfaces.The hierarchical microgrooved superhydrophobic,CuO nanostructured flat superhydrophobic,microgrooved and smooth hydrophobic surfaces were fabricated by machining and chemical oxidation methods.The wettability of all fabricated surfaces was measured by optical contact angle measuring device.The results showed that the stable Cassie state of droplet was observed on hierarchical microgrooved and CuO nanostructured superhydrophobic surfaces,but droplet presented metastable Cassie state on microgrooved hydrophobic surface.The microgrooves hindered lateral wetting of droplet,resulting in the anisotropic wettability on microgrooved hydrophobic and hierarchical microgrooved superhydrophobic surfaces.The contact angle perpendicular to groove direction was larger than that parallel to groove direction.In order to explore the mechanism of microgrooved structures regulating the hydrodynamics during coalescence-induced droplet jumping,a visual experimental apparatus for coalescence-induced droplet jumping was constructed to comparatively study droplet jumping on hierarchical microgrooved and flat superhydrophobic surfaces.The work showed that the confined microgrooves played a key factor on tailoring droplet coalescence hydrodynamics.On hierarchical microgrooved superhydrophobic surface,the self-jumping of the deformed droplet in microgroove was driven by Laplace pressure difference with maximum surface-to-kinetic energy conversion efficiency of8%.The energy conversion efficiency(? ? 46%)for coalescence between deformed and undeformed droplets was 7.2× higher compared to coalescence-induced droplet jumping on flat superhydrophobic surface,which clarified that the enhancement in coalescence-induced droplet jumping velocity and energy conversion efficiency depended on the surface curvature and momentums in jumping direction of droplet.Furthermore,this study confirmed that the protruding microstructures between coalescing droplets enhanced coalescence-induced droplet jumping by redirection of momentum in coalesced droplet.However,the grooved structures between coalescing droplets reduced coalescence-induced droplet jumping due to weakened impacting effect between coalescing droplet and surface.The aqueous ethanol with 8% and 16% mass fractions and deionized water were used as test liquids to study the effect of liquid surface tension on coalescence-induced droplet jumping on flat and hierarchical microgrooved superhydrophobic surfaces.The results indicated that coalescence-induced jumping of aqueous ethanol droplets with surface tension larger than 40 m N/m and viscosity less than 2 m Pa·s on flat superhydrophobic surface was dominated by inertial-capillary regime,the influence of viscous effect could be neglected.The decreased liquid surface tension only caused the decrease of droplet jumping velocity,the energy conversion efficiency maintained constantly in the range of 5.6%?6%.On hierarchical microgrooved superhydrophobic surface,due to the decrease of liquid surface tension,the increased surface adhesion resulted in deformed aqueous ethanol droplet in microgroove only moving upward and suspending on microgroove instead of self-jumping by driving of Laplace pressure difference.Furthermore,the increased surface adhesion changed morphological transition of aqueous ethanol droplets during coalescence,in turn affecting the momentums transfer in coalesced droplet,resulting in decreased droplet jumping velocity and energy conversion coefficient.With liquid surface tension decreasing,the enhancement in coalescence-induced droplet jumping caused by confined microgrooves and protruding microstructures between coalescing droplets reduced,and the reduction effect caused by grooves between coalescing droplets enhanced.Based on above studies,a visual experimental system for steam condensation heat transfer was constructed to study dropwise condensation heat transfer characteristics on smooth hydrophobic,microgrooved hydrophobic and hierarchical microgrooved superhydrophobic surfaces with different groove direction.The effects of micro/nanostructures and anisotropic wettability on droplet dynamic behaviors and condensation heat transfer performances were analyzed.The results showed that coalescence-induced jumping of multiple small droplets(< 100 ?m)at small surface subcooling(?T < 5 K)and forced jumping of large deformed droplets(400?500 ?m)in microgrooves at a broad range of surface subcooling(?T < 12 K)simultaneously emerged on hierarchical microgrooved superhydrophobic surface.Furthermore,large deformed condensed droplets in microgrooves could spontaneously move upward and suspend on the microgrooves by driving of Laplace pressure difference,resulting in the formation of hierarchical condensation with enhancement in condensation heat transfer,which suppressed flooding condensation at large surface subcooling.Various droplet dynamic behaviors for surface renewal including coalescence-induced droplet jumping and jumping droplet induced sweeping at small surface subcooling as well as coalescence-induced sweeping and suspended droplet departure at large surface subcooling enabled the hierarchical microgrooved superhydrophobic surface to possess the best condensation heat transfer performance and eliminate the effect of anisotropic wettability of microgrooved structures on dropwise condensation.However,the anisotropic wettability of microgrooves significantly affected the dropwise condensation on microgrooved hydrophobic surface.The increased effective heat transfer areas,liquid columns sweeping droplets on adjacent plateaus and combination of liquid columns sliding and departure of large droplets spanning grooves enabled obviously higher condensation heat transfer performance on vertical microgrooved hydrophobic surface than smooth hydrophobic surface.But the horizontal microgrooves eliminated sliding down of liquid columns in microgrooves and impeded departure of large droplets spanning microgrooves,resulting in the condensation heat transfer performance deteriorating on horizontal microgrooved hydrophobic surface compared to smooth hydrophobic surface.