Study Of The Droplet Transport Characteristics And Phase Transition Processes On Nanostructured Surfaces

Faced with the increasingly serious energy,environment and atmosphere issues,developing the highly-efficient energy utilization technologies is crucial to the stable and sustainable development of our country's economy and society.Liquid-gas phase transition as a common and efficient energy transfer process,has been widely applied to the industrial equipment involving heat and mass transfer.Enhancing heat and mass transfer performance of liquid-gas transition can improve the energy utilization efficiency of industrial producing processes,and further ensure the realization of our country's energy-saving and emission reduction targets.Solid surface as a carrier of liquid-gas transition process,its properties significantly affect the heat and mass transfer efficiency.Therefore,designing and manufacturing the engineering micro/nano-structured surfaces with enhanced heat and mass transfer performance are of great research and application value.The present dissertation aims at the droplet transport processes and liquid-gas transition processes on nanostructured surfaces,studies their details and internal mechanism,and systematically explore the regulating effects of surface physicochemical properties.The main results and conclusions are as follows:(1)Surface rough structures can reduce the nanodroplet spreading speed during dynamic spreading phase,moreover,with the decrease of surface solid fraction,the static wetting mode of nanodroplet transforms from the suspended Cassie state to the partially wetted state,and finally to the wetted Wenzel state,and the static contact angle of nanodroplet correspondingly increases first and then decreases.(2)In the impact processes of nanodroplets on solid surfaces,the maximum spreading time is described as a power law of impact velocity,and the relation of maximum spreading factor with impact velocity or the Reynolds number is exponential.Furthermore,decreasing surface solid fraction not only reduces the maximum spreading factor,but also increases droplet rebound speed.At last,through analyzing and modifying the estimation of viscous dissipation and each energy term,we propose an improved model describing the relationship between maximum spreading factor and impact velocity,which shows greater accuracy for impinging nanodroplets dynamic behaviors.(3)Nanodroplets undergo coalescence in the inertially limited-viscous regime: the liquid bridge radius is described by a power law of spreading time.Nanoscale droplets may also spontaneously bounce off surfaces after coalescence,which is dependent on the physical and chemical properties of structured surfaces: the weakening of surface wettability and the decrease of solid fraction reduce the solid-liquid attraction force,and the adhesion work exerted by surfaces correspondingly decreases,as a result,the merged nanodroplets prefer to jump away from surfaces with higher speed.Additionally,the increase of droplet number and droplet initial size or the decrease of droplet initial size ratio result in the decrease of nanodroplet jumping speed and energy conversion efficiency.(4)Nanodroplets successively exhibit constant contact angle(CCA),constant contact radius(CCR),and mix mode during evaporation on structured surfaces,and the coupling effect of pinning force and depinning force cause triple-phase contact line to move,shrink or adhere to surface in each evaporation mode.The evaporation-induced CCA-CCR transition,in essence,is caused by the droplet Cassie-Wenzel wetting transition,which is remarkably dependent on structure size and surface intrinsic wettability: reducing structure spacing or enhancing surface hydrophilicity postpones the CCA-CCR transition and improves the evaporation rate of nanodroplet.(5)In the steam condensation processes on nanostructured surfaces,vapor molecules tend to nucleate at the positions with lower potential energy,the droplet growth patterns mainly include natural growth and coalescence growth: the size of nucleus increases linearly with time during natural growth;in coalescence growth stage,the coalescence between small droplets can influence the wetting mode of resultant large droplets and induce the occurrence of dewetting transition.Moreover,it is found that the cooperation between spatially ordering nucleation and dropwise growth endows the hybrid structured surfaces with better mass transfer performance compared with homogeneous structured surfaces,and the condensation rate increases with the increase of solid fraction.The results show that appropriately reducing structure gap can not only restrain the formation of Wenzel droplet,but also enhance the heat and mass transfer efficiency of surface.This dissertation selects some common physic phenomena in living and manufacturing processes,and respectively investigates the droplet transport processes and liquid-gas phase change processes on nanostructured surfaces,the results not only deepen the understanding of internal details and mechanism of these processes,but also provide guidance for the design and optimization of structured surfaces,which has positive research significance.