In Situ Study On Phase Transition Of Water In Environmental Scanning Electron Microscope

The microscopic understanding of materials is relied on the development of microscopy.Historically,the technological evolution from optical microscope to electron microscope have directly stimulated a bazillion discoveries of new physical and chemical fundamentals as well as advanced materials.Scanning electron microscope(SEM)is,undoubtedly,the most widely used tool for microscopic characterization in modern materials science and engineering.Especially,the incorporation of a gas pressure around the sample area of conventional SEM,namely,the environmental scanning electron microscope(ESEM),has opened up new possibilities in observing samples(insulating and wet)in their natural states,leading to unprecedented chances for studying the details of biological,organic and hydrated samples.In this decade,in situ study with ESEM actually plays an important role in the field of material research.Considering the most commonly used gas environment is water,the discussions on in situ observations in ESEM,can be classified into three parts:(1)Observations on phase changes of water(morphological changes during phase transition of water,wetting,water transportation,and so on).(2)Observations on material changes with the participations of water(swelling,deliquescence,metallic corrosion,fabrication of nanostructure,hydration process of cements,and so on).(3)Observations on dynamic processes in ESEM(reactions in batteries,growth of twodimensional materials and nanomaterials,crystallization of oxides by high-temperature ESEM and so on).Here,we focus on microscopic changes induced by phase transitions of water,referring to the first category as mentioned above.The main research contents are listed as following:Firstly,we focus on the effect of the microstructures of materials on the gas-liquid phase transition and use in situ ESEM observation to explore the microscopic growth dynamics of water droplets on adaxial and abaxial surfaces of Bambusa multiplex leaf(BML)during wetting.Our results show that,due to the dramatic structural differences between adaxial and abaxial surfaces,the growth of water droplets on the two sides is quite different;and the wetting behavior on both sides of BML is quantitatively analyzed,in terms of contact angles and sizes of water droplets as a function of time.This conceptual study demonstrates a straightforward pathway to understanding the wetting behavior,and the results may pave the way for further research on bio-inspired materials.Then,by in situ observation of ice formation from water vapor via ESEM,we revealed that hexagonal ice crystals are developed by a step-by-step pathway in a supersaturated water vapor environment;we also discerned that such steps came from two different origins,which are screw dislocations and initial steps;the relationship between the edge-length(of hexagonal ice crystals)and the growth time was quantitatively studied at controlled temperatures and pressures by experimental data fitting.This study shows that qualitative and quantitative observations of ice formation can be made with simple setups,and it should inspire future investigations toward important physicochemical processes using ESEM,especially those that simultaneously involve two or more phases.