Interfacial Properties Of Nanobubbles At The Solid/Liquid Interface Studied By Advanced Nano-imaging

The nanobubbles and micropancakes at the solid-liquid interface are newdiscoveries in the field of interfacial physics in recent two decades. The existence ofnanobubbles has been basically confirmed by many experiments and they havebecome a hot topic of research in the interfacial science. It is found that nanobubbleshave broad impacts on surface science, fluid mechanics, biomedicine, nanotechnologyand some application fields. For example, they can lead to protein folding, cause largesliding of the interface, affect the efficiency of flotation and biological chip, and alsobe used as detergent and mask, promote the growth of plants, fishes and mice.Therefore, it is of great theoretical and practical significance to study the interfacialnanobubbles.Currently, the central issues on nanobubbles researches are focused on twopoints: the stability mechanism and contact angle of nanobubbles. Firstly, the lifetimeof nanobubbles predicted from the classical wetting and gas dissolution theories is atthe level of μs. However, they are experimentally observed to be stable for hours oreven days,10~11orders of magnitude longer than the theoretically prediction.Besides, the contact angle of nanobubbles at the solid-liquid interface is much largerthan that on the same surface at macroscale. The traditional Young’s equation and linetension corrected Young’s equation both can not explain this phenomenon. Tounderstand the above puzzles, deeply studies are urgently needed on the physicalproperties of nanobubbles (e.g. mechanical properties and surface tension) and on theeffects of the surrounding environment variations on the interfacial properties ofnanobubbles (e.g. surfactants, temperature). More importantly, new techniques shouldbe used to obtain the interfacial information and inner chemical components.Based on the above aims, we have established a method to investigate themechanical properties of an individual nanobubble and/or micropancake byPeakForce quantitative nano-mechanics (PF-QNM) mode atomic force microscopy(AFM). We have measured the stiffness of nanobubbles and micropancakes, andstudied the effects of ethanol solutions on their interfacial behaviors. Besides, basedon the third generation synchrotron radiation technique, e.g. scanning transmissionsoft X-ray microscopy (STXM), the stability and inner information of nanobubblesare preliminarily investigated. The main research contents of this thesis can be divided into the following four parts, constituting the overall framework of this thesistogether with sections of introduction, summary and outlook:(1) Methodology establishment of measuring the mechanical properties ofinterfacial nanobubbles by PF-QNM.PF-QNM is a newly developed mechanical imaging mode AFM, whose keyadvantage is to be able to obtain the high resolution imaging of the mechanicalproperties of samples at the nanoscale. This makes it possible to measure the surfacetension of nanobubbles. In this part, we have established the method to measure themechanical properties of interfacial nanobubbles by using PF-QNM. The stiffness ofinterfacial nanobubbles in water is given out for the first time, which is close to thesurface tension of water and is size dependent where small bubbles have largestiffness. Generally, the surface tension of the gas-liquid interface of nanobubbles isconsidered to be closely related to the stability of interfacial nanobubbles. The smallerthe surface tension, the smaller the additional pressure and so the more stable thenanobubbles. Our results indicate that surface tension is not the main factor toinfluence the stability of interfacial nanobubbles in water.(2) Effects of ethanol aqueous solutions on the interfacial properties ofnanobubbles.Generally, surface tension is assumed to affect the contact angle at macroscale.So, based on the first part of work, we study in detail the effect of ethanol on thecontact angle and surface tension of nanobubbles. It is found that the contact angleand surface tension of nanobubbles decrease with the increasing of the ethanolconcentration, and the surface tension of nanobubbles is similar to that of thesolutions at different ethanol concentrations. Although the contact angle ofnanobubbles and that at macroscale are very different, they both can be linearly fitted.According to the mathematical relationships, we propose a contact angle modifiedYoung’s equation.(3) Investigation on the interfacial behaviors of micropancakes in water and inethanol aqueous solutions by PF-QNM.Micropancakes are another state of gas accumulation at the solid-liquid interfacediffered from nanobubbles, which have great significance for understanding theabnormalities of the stability and contact angle of interfacial nanobubbles. PF-QNMhas sensitive mechanical resolution and is expected to provide some information ofmicropancakes that can not be obtained by TM-AFM. Thus, in this part of work, PF-QNM is used for the first time to measure the nanomechanical properties ofmicropancakes. Monolayer and bilayer micropancakes and their composites withnanobubbles are observed and their stiffness is measured. It is found that for bilayermicropancakes the bottom layer micropancakes are stiffer than the upper layer ones.Furthermore, the effects of ethanol aqueous solutions on the micropancakes areinvestigated, showing that the stiffness of micropancakes reduces with the increasingof the ethanol concentration. The micropancakes can not be observed above a criticalconcentration, but the micropancakes reappear again in water. The stiffnessmeasurement of PF-QNM provides a sensitive technique for detecting the existenceand number of layer of micropancakes, and for distinguishing between micropancakesand nanobubbles.(4) The interfacial properties of nanobubbles studied by synchrotron radiationtechniques.AFM is presently the main technique in the study of the interfacial nanobubbles,having high spatial resolution, easily nanobubbles production method, but withoutcomponent chemical resolution, only obtaining the surface morphology ofnanobubbles. The synchrotron radiation scanning transmission soft X-ray microscopy(STXM) images the morphology/structure, quantitative distributions (maps) ofchemical components in2D/3D, molecular orientation, as well as obtains electronicstructure of the materials via the spatially-resolved near edge X-ray absorption finestructure (NEXAFS) spectroscopy, for a wide range of samples from the fields ofmaterials sciences, environmental/earth sciences, and life sciences at high spatialresolution (sub30nm), high spectral resolution (<0.05eV) and in variable sampleenvironments. In this part of work, we have established a STXM based experimentalplatform for nanobubbles research. We have obtained the STXM imaging ofsub-microbubbles for the first time and studied their stability. The distribution of thechemical components in nanobubbles has also been explored. Meanwhile, apreliminary study on the adsorption behaviors of Kr on the surfaces of differentparticles is performed by using X-ray fluorescence adsorption spectroscopy and X-rayfluorescence imaging.