Effects Of Temperature And Surface Roughness On Water Evaporation Rate And Kinetic Behavior At The Nanoscale:A Theoretical Study

Water is the source of life and plays an important role in human survival and ecological sustainability.Water evaporation is not only one of the most important links in the water cycle,but also an important way of water resource regeneration.Water evaporation is a very common phenomenon and widely exists in nature,industry and human life activities.Since the 17^th century,scientists have started a series of studies on evaporation.While observing water evaporation by experimental means,they have also put forward many theoretical models related to evaporation and its influencing factors.With the continuous improvement of the research level and the shrinking of the research scale,a large number of studies have found that the evaporation of interfacial water under microscopic conditions has a significant difference from the macroscopic water.The interfacial water will be affected by the interaction from the surface and the interaction between water molecules at the nanoscale,which makes the evaporation of water will be affected by more factors,thus the evaporation of water becomes more complex.In addition,an in-depth study of the dynamics of nanoscale water?such as thermal noise in the water and the rotational behavior of water molecules?is not only helpful to understand the evaporation properties of nanoscale water molecules,but also plays an important role in understanding various biochemical processes.There are many factors that can affect the evaporation of interfacial water,such as temperature,surface hydrophilicity and hydrophobicity,surface heat conductivity,surface roughness,the composition of evaporation solution and so on.In this thesis,the effect of temperature on the evaporation of nanoscale water on a uniformly complete wetting surface has been studied by applying molecular dynamics?MD?simulations.For bulk water,the rate of evaporation varies exponentially with temperature.However,we find that with the increase in temperature,the growth of the water evaporation rate becomes slow,which is slower than the exponential change of bulk water.Compared to a thicker water film,the evaporation rate of a monolayer water film is much slower.Analyses show that the hydrogen bond?H-bond?lifetimes and orientational autocorrelation times of the outermost water film decrease slowly with the increase in temperature.This can be attributed to the constriction from the substrate,which slows down the decrease of the H-bond lifetimes and the orientational autocorrelation times.Water molecules need a longer time than expected to break the hydrogen bond network and evaporate,so the evaporation rate increases slowly.When the thickness of the water becomes thinner,the H-bond lifetimes and orientational autocorrelation times of water are much longer.This suggests that the lower evaporation rate of the monolayer water film on the uniformly complete wetting surface mainly comes from the obvious limit to the rotation of water from the uniformly complete wetting surface.This study is important for alleviating drought disasters and avoiding quick water loss of vegetation in high-temperature areas.Essentially,water evaporation is the result of competition between the thermal noise and hydrogen bonding networks.In order to further explore the influence mechanism of temperature on water evaporation at the nanoscale,we analyze the influence of temperature on the thermal noise,hydrogen bond and rotational properties of water molecules in water.In statistical thermodynamics,the reference temperature of the system can determine the speed of the molecule Brownian motion in the system,which will affect the system collisions between molecules.However,the autocorrelation time of thermal noise is associated with the speed of the collision of molecules.Thus the temperature of the system can be reflected through the thermal noise of the system.The hydrogen bond and rotation characteristics of water molecules change with the temperature increases,which affects the evaporation of water.The finite autocorrelation time of thermal noise is closely related to the directional transportation in the molecular scale.Therefore,we can further explore the mechanism of temperature affecting the evaporation of a small amount of water evaporation by studying the correlation characteristics of the thermal noise at different temperatures.In this work,we use molecular dynamics simulations to compare the autocorrelation behaviors of the thermal noise,hydrogen bonds,and molecular rotations found in water.We found that the intrinsic picosecond autocorrelation time for thermal noise is caused by finite molecular rotation relaxation,in which hydrogen bonds play the role of a bridge.Furthermore,the simulation results show that our method of calculating the autocorrelation of thermal noise,by observing the fluctuating force on an oxygen atom of water,provides additional information about molecular rotations.Our findings may advance the understanding of the abnormal kinetic behavior of water molecules evaporation under different temperatures at the nanoscale,and the applications of terahertz technology in measuring the structural and dynamical information of molecules in solutions.In addition,most solid surfaces are not as flat as ideal in the real world,they are usually rough and uneven.Surfaces with roughness usually exhibit different characteristics at the nanoscale.The effect of surface roughness on the evaporation of nanoscale water on the hydrophilic rough surface has been investigated using molecular dynamics simulation.The results show that the evaporation rate of a nanoscale water film on the hydrophilic rough surface first increases and then decreases with the surface roughness increases.We find that the surface roughness could affect the water distribution.When the surface roughness is low,a lot of water is distributed on the convex and concave surface.The lower potential barrier of water results in the accelerated evaporation.When the roughness increases,water molecules are mainly distributed on the concave surface,the increasing total potential barrier with roughness attributes to the decreasing evaporation rate.As the roughness further increases,water molecules are steadily distributed on the concave surface,the increase of viscous losses caused by the more curved meniscus of the outermost water results in the linear decrease of water evaporation rate.In addition,our further study finds that decreasing the surface charge which reducing the hydrophilicity of the surface does not change the nearly linear decreasing trend of evaporation rate.This finding is helpful in understanding the evaporation on biological surfaces,designing the surface of water evaporation enhancement,or water harvesting and many other applications.