Investigation Of The Highly Efficient Production And Properties Of Nano Gas Layers And Nanobubbles At The Solid-liquid Interface

Interfacial and nano gas layers are two unique structures of the gas phase at the solidliquid interface.In recent decades,their abnormal properties and application prospects attracted attention and inspired elegant studies in the field of interface science.Nanobubbles and their technologies provided novel ideas to solve challenges in many fields,such as electrochemical,mineral flotation,wastewater treatment,agriculture,drug delivery,medical imaging,etc.The nano gas layers with the quasi-twodimensional state may widely affect some physical and chemical reactions at the interface.Solving fundamental problems of the interfacial gas phases is signification because of their application potential.In the past decades,numerous investigations focused on explaining the existence and the abnormal physical characteristics of interfacial nanobubbles that were beyond the expectation of classical theories.Besides,the stabilization of the nano gas layer is unknown.The controversies between the observations and theoretical predications brought challenges and opportunities for this field.The gas nature of the observed interfacial gas domains has also been questioned.In fact,distinguishing soft nanobubbles and nanodroplets only by the morphologic characterizations is difficult,as well as recognizing gas layers and insoluble layers.Nowadays,more and more methods are able to identify interfacial nanobubbles,and its gas essence has been widely accepted by the scientific community.Abnormal physical properties of nanobubbles have gradually been reasonably explained by theoretical innovations.The gas nature of the nano gas layer is still suspicious due to the absence of direct evidence.Nowadays,researches of nano gas layers mainly focus on the relationship between nano gas layers and interfacial nanobubbles.Scholars added the gas layer into the models to explain the stabilization of nanobubbles.However,the properties of the nano gas layers remain many open problems that deserve to contemplate.For example,classical thermodynamics is unable to resolve the stabilization mechanism of nano gas layer,which needs theoretical innovations.In addition,an efficient preparation method of nano gas layers is absent in the experiment,which causes those deep investigations of their properties to be achieved hardly and further hinders the progress of application research and theoretical innovation.In order to develop a highly efficient and controllable method to produce nano gas layers,they need to understand their nucleation mechanisms and conditions.Based on the above challenges in the research,this work firstly verified the gas nature of nano gas layers.Considering that there are no qualitative techniques currently available with both the high spatial resolution and the high chemical sensitivity to acquire reliably direct evidence,we evaluated the existence of nano gas layers by the in-situ degassed contrasting and force curve analysis.Besides,the transformation from gas layers to nanobubbles also supported the homology of two gas domains.In addition,founded on the recent investigations of the interfacial water structure,we presented a hypothesis on the stabilization of the gas layer.To develop the efficient and controllable production of nano gas layers,we paid attention to the substrate hydrophobicity which is a crucial factor in the nucleation of interfacial gas domains.The highly Oriented Pyrolytic Graphite(HOPG)is widely applied as a substrate in research on interfacial gas phases.Varying hydrophobicity of the HOPG surface and the mechanism behind were studied and discussed in the paper.Results showed that the surface hydrophobicity of graphite surface in air or pure water increases over time.The adsorption of hydrocarbons from the environment may cause the rise of surface hydrophobicity.The uneven adsorption of hydrocarbons on the HOPG surface was observed by the surfaceenhanced infrared spectrum and the surface potential.The effect of substrate hydrophobicity on nucleation of interfacial gas domains can be in-situ studied by using this property of the graphite surface.Next,we studied the synergy effect of the substrate hydrophobicity and gas oversaturation innovatively by controlling the aging time and gas content of water.Results showed that more nanobubbles were produced on the HOPG surface with the increasing aging time,and the higher gas saturation of water induces a bigger volume of bubbles.Nano gas layers and their coexistence were observed frequently on the aged surface by controlling the oversaturation.A novel method of producing nano gas layers efficiently was presented.In addition,we also analyzed the differences between the new method and the alcohol-water exchange in creating the interface gas domains and illustrated the inherent differences between the two methods,which further explains the advantage of the method.Using this new method,we measured the morphologic transformation of the coexistence in a gas oversaturated environment.The observations imply that the coexistent system may contain complex transferring of gas.It also proves that the nanobubble should be standing over the gas layer rather than pining at the surface.The dynamics between nano gas layers and nanobubbles are signification for understanding the relationship between the two interfacial gas domains and the behaviors of gas molecules at the interface.In sum,this paper focused on the challenges in the research on interfacial gas domains,especially for the fundamental and essential problems of studies of nano gas layer.We innovatively developed a new method for the efficient and controllable production of nano gas layers.Meanwhile,the relationship between nano gas layers and nanobubbles is discussed.Our results and findings deepen the understanding of the dynamics of the aggregation,nucleation,growth,and stabilization of gas domains at the solid-liquid interface,and promote the development of research on the interfacial gas domains.