Study Of Surface Wettability On The Capture Efficiency Of Micro-sized Droplet And Water Vapor

Water is the source of life and the cornerstone of economic development and social stability.However,water scarcity is becoming increasingly acute worldwide,with the shortage of freshwater being particularly pronounced.How to obtain clean freshwater resources in a simple and efficient way is a key issue that needs to be solved urgently in today’s society.At present,researchers mainly obtain freshwater resources through water vapor adsorption,seawater desalination,wastewater recycling and treatment,and fog collection.However,the water vapor adsorption method provides limited water capacity,and wastewater processing and reuse often requires huge energy inputs,which makes the cost of freshwater access higher and is not friendly to underdeveloped areas.Seawater desalination technology that is based on clean solar energy resources and fog collection technology that aggregates and collects a large number of small droplets wrapped in dense fog can provide a large amount of clean freshwater without the need for additional energy supply,making them important means to alleviate the shortage of freshwater resources in local areas.Efficient continuous fog collection is mainly composed of three processes:(1)the capture of small droplets in the fog flow from the fog collection point on the surface of the material,(2)the rapid movement and fusion of the captured small droplets on the surface of the fog collection,and(3)the rapid detachment of large droplets formed by the aggregation of small droplets on the surface of the fog collection.Regulating the surface properties of the material,so that the three processes of water collection are simultaneously highly efficient and synergistic,which is the key to efficient water collection.Previous studies have focused on the movement,fusion and detachment of droplets on the surface of the material,and the lack of systematic research on the initial stage,that is,the efficiency of droplet capture,is not conducive to the improvement of the overall water collection efficiency of fog.On the other hand,the surface wettability of the material affects the spread and evaporation of the droplets on it,and further affects the condensation behavior of the droplets.Therefore,it is necessary to regulate the surface wettability and maximize the droplet condensation capture efficiency.In addition to the collection of fog,solar seawater desalination strategy is an important way to alleviate freshwater resources.The strategy consists of two processes:brine evaporation and saturated water vapor condensation.At present,most studies are focused on regulating evaporator/water,and the properties of the evaporator/air interface to improve the efficiency of water evaporation,while there is little regulation of the condensation behavior of water vapor which directly produces usable freshwater.In this context,we focus on the wettability of material’s top surface,and mainly carry out two works:to explore the influence of surface wettability on the fog collection efficiency,and to reveal the mechanism of surface contact angle affecting the condensation efficiency of tiny droplets during this process;to explore the influence of surface wettability on water vapor condensation in seawater desalination system,and to optimize surface design,improving the overall efficiency of seawater desalination.Detailed research contents are as follows:1)Design super-lubricated surfaces with varied water contact angles but sharing the same small sliding angles,and study their fog collection performance.According to the single variable principle,all the related parameters in the system remain the same except for the different surface contact angles.Parameters of fog flow also keep the same.We firstly constructed Nepenthes-inspired slippery liquid infused porous surfaces(SLIPS)with minimal sliding angles.Through regulating types of the lubricant oil,which are infused to the hydrophobic surface with silicon nanowires(Krytox-101,DMS-T21,PDM-7040,Hydroxy-PDMS,with similar lubricant viscosity,different surface tension),we obtained surfaces with apparent water contact angles ranging from 75~o to 120~o,and sliding angle less than 10~o.Then,we recorded the water collected over prolonged time of four kinds of surfaces under high and low speed of fog flow.Results under two situations show consistent results:the hydrophilic surfaces are better than the hydrophobic surfaces as regards to the fog collection,and the smaller the contact angles,the greater the amount of water collected in the same time.Since sliding angles of the surfaces are very small,effects of droplets’fusion and shedding differences on the surfaces can be ignored,so the difference in the total water collected can be attributed to the difference in the efficiency of initial fog capture.Further,in order to explore mechanism of surface contact angles’influence on the droplet capture,first,we compared the droplet collisions on four surfacesthe,and it was found that the phenomenon of tiny droplet emission was easy to occur when the droplets collided on the relatively hydrophobic surface,but this phenomenon did not occur on the hydrophilic surface.This phenomenon has an impact on the amount of fog captured on different surface.At the same time,we compared the condensation-growth status of droplets on the four kinds of surface in the first-round and multiple-round fog collection.Taking consideration of the temperature change of different surfaces in the fog collection process,it is proposed for the first time that the droplet condensation upon phase-change accompanies with droplet deposition without phase-change in fog collection process,and the difference in the efficiency of droplet condensation is the root cause of the difference in the droplet capture efficiency on the surfaces with different contact angles.This study clarifies mechanism of influence of surface wettability on droplet capture during fog collection process for the first time,and provides a theoretical basis for the design of an efficient fog collection system.2)Design a passive radiative cooling surface with directional droplet gathering capability for efficient condensation of water vapor and rapid transfer of water droplets.We firstly constructed polymer surfaces with excellent passive radiative cooling properties to improve the condensation efficiency of water vapor during desalination.Further,hydrophobic/hydrophilic patterned radiative cooling surface and surface with gradient wettability are constructed,which were able to induce rapid convergence and shedding-off of condensed droplets,to improve renewal rate of the water collector surfaces during process of seawater desalination,further enhance the efficiency of water vapor capture,thereby promote the efficiency of seawater desalination.This work optimizes the seawater desalination system from the perspective of improving the efficiency of droplet condensation,and provides new ideas for using seawater desalination technology to alleviate the shortage of freshwater resources in the world.