Investigation Of The Gaseous Nature And Stabilization Mechanism Of Nanobubbles In Water

Soft matter is a whole new subject which across the boundaries of condensed matter,material science and biological macromolecules.Unlike the gases,simple liquids and highly-ordered solids,softer matter has a variety of forms and numerous novel properties,its prospects in a wide range of applications have attracted extensive research interest both at home and abroad.Bubbles in water,especially nano-scale bubbles on the surface,were found with a lot of strange properties which are totally unexpected for a liquid-solid system containing only gas,they have received continuous attention among soft matter research in the past 20 years.It is highly likely that nanoscale bubbles exist on various surfaces in liquid and can have certain impacts,thus study on such small bubbles would help understanding many inexplicable phenomena in condensed matter and also their puzzling existence.The existence of nano-sized bubbles on the solid-liquid surface is contrary to the predictions by classical theories.According to Laplace equation,the pressure inside such nano-sized bubbles would reach tens or even hundreds of atmospheric pressures.Under such a large pressure the bubbles should disappear within milliseconds,but the experimentally observed nanobubbles can survive for a long time(hours or even days);another fact is that their wetting behavior differs to their macroscopic counterpart,the contact angles of surface nanobubbles(10-35 °)are tens of degrees less than the complementary angles of droplets on the same surface,usually hydrophobic ones.These puzzling properties have made nanobubbles controversial from the beginning of discovery,but also attracted much attention.Although these doubts are still clouding fundamental research,nanobubbles-related applications are emerging,including cleaning,mineral flotation,drug delivery,promoting aquaculture and wastewater purification.With the deepening of research in recent year,more and more evidence indicated that nanobubbles lying near the hydrophobic surface would strongly affect the long range hydrophobic attraction,boundary slip and could play a role in anesthesia induced by inert gases,also their existence might reduce the efficiency of hydrogen production via catalysis,capacity of porous membrance filtration and the lifetime of a propeller blade by causing damage.Due to the limitations of acquiring chemical information inside nanobubble by previously developed techniques,our knowledge on these small bubbles remains in their morphology,distribution and mechanical-responds,little is known about the gas species and the state of matter inside nanobubbles and also their surrounding liquid environment.Therefore,it is urgent and necessary to accurately measure the gas sate inside nanobubbles,and then establish an excise and reasonable model by proper theories and computer simulations.This would be important,at one hand,help elucidate the many surprising facts involving nanobubbles like superstability;at another hand,would shed light on the solving the inefficiency caused by absorbed nanobubbles and promote further applications of nanobubbles in industry and our daily life.Research in this dissertation is divided into three parts.Firstly,concerning that nanobubble formation on solid surface are quiet irregular and sparsely distributed.Many current methods of nanobubble production are not very efficient and might easily introduce unwanted contaminations which are difficult to distinguish from bubbles.A one-step,contamination-free,easy-to-operate and highly efficient nanobubble production method from ultra-pure water system was introduced.Based on the “textbook” principle of higher gas solubility in liquid at lower temperature,the cooling water was preserved a long-time at cold environment and directly added to the substrate surface.A gas oversaturation would be acquired near the liquid-solid surface during the steady equilibrium of temperature and that would generate surface nanobubble efficiently.This method is clean and convenient,easy to implant to other surface like mineral surface,thin films and also biological system like cell membrance.Furthermore,in order to obtain the state of internal gas inside bubbles,the nanosized bubble generated by the previously developed cooling water method and water splitting method are investigated by synchrotron based soft X-ray microscopy.Surface nanobubbles are located and further the near-edge X-ray absorption fine structure of gas inside nanobubble and the surrounding water environment are acquired and analyz-ed.Results suggested that confined in nanobubbles,such a small space in which the height is less or close than the mean free path of gas molecules in atmospheric air(68nm),the gas inside bubble exhibit a novel “aggregated state” with the density of gas at least tens of times higher that atmospheric air(1.25kg/m3),which could not be explained by Laplace pressure.The observed nanobubbles have extraordinary stability under X-ray irradiation,and the water environment are oversaturated with gas molecules which are hundreds of times exceed the values in water at thermodynamic equilibrium.All these discrepancies indicated that gas aggregation behavior near the hydrophobic surface in a confined space are very different from common expectations,new theory should be developed to clarify this “dense state”.Finally,using the popular method of molecular dynamic simulation in exploring theoretical description of soft matter research,I hope to establish a reasonable theoretical model of nanobubbles.The behavior of gas accumulation at surfaces with different hydrophobicity were studied,effects of initial gas oversaturation,gas type,solvent condition on the formation and stability were investigated.The diffusion behavior of gas molecules inside nanobubble and in water are statistically calculated and compared,and further in attempting to uncover the mechanism of stunning “dense state” aggregation on hydrophobic surfaces at molecular level.