Identification Of Surface Nanobubbles And Resolving The Stiffness Of The Gase-liquid Interface

The study of the surface nanobubbles has been increasing continuously in the past 20 years,which has become a highlight in colloidal science and interface physics researches.The application prospects of nanobubbles in industry,medicine,environment and other fields are constantly emerging,so it is of great practical significance to study the basic mechanical properties of surface nanobubbles.In order to deeply study the surface nanobubble,researches on the identification of surface nanobubble from nano-objects should be firstly carried out.In this paper,a comparative investigation on the topographic features and nanomechanical responses of surface nanobubbles,polymeric nanodroplets and solid microparticles probed by atomic force microscopy in different operating modes.(1)The study found that under the standard tapping mode,these microscopic objects exhibit similar topographies,but it is difficult to distinguish them effectively whether they are spherical or ellipsoidal.However,distinct difference,which is caused not only by their different mechanical properties but also by different cantilever tip-sample interactions that affected by the tip wettability,was observed in the successive topographic imaging with controlled scanning forces and in the nanoindentation tests,allowing for the identification of surface nanobubbles.(2)The successive topographic imaging with controlled scanning forces shows that the width and the height of surface nanobubbles decrease with the increasing of the scanning force.When the scanning force increased from 0.3 n N to 5.0 n N,surface nanobubbles will disappear completely from the height image,while the surface nanobubbles will restore their origin when the loading force decreases to0.3 n N again.(3)In addition,in the nanoindentation tests,it is found that the interaction between the nanobubbles and the hydrophilic probe is similar to that of the spring,and both follow Hooke's law.Therefore,based on the force-distance curves obtained by the indentation tests,we further studied the stiffness of surface nanobubbles and found that it decreases with the increase of bubble curvature radius,and the theoretical model established by energy analysis and force analysis can reasonably explain the size-dependent of stiffness.We determined the interface tension of nanobubble by the stiffness model,we found that it is much lower than the surface tension of water.(4)Finally,we also explored the applicability of the nanobubble stiffness model to nanocavitation.The study found that although consistent with the trend of nanobubbles stiffness,due to the lack of deformation equations describing the deformation of gas-liquid interface of the cavitation,the stiffness model of nanocavitation cannot be transplanted into the stiffness model of nanobubbles.In this thesis,the research on the identification of the surface nanobubbles and the stiffness of gas-liquid interface will help to further understand some peculiar physical properties and phenomena of surface nanobubbles and accelerate the application of surface nanobubbles in the real industries productions.