Study Of Nanobubbles At Solid-Liquid Interface And The Influence Of Nanobubbles On Boundary Slip Of Fluids

When hydrophobic substrates are immersed in water, surface nanobubbles are often formed spontaneously at the solid-liquid interface. Studies show that surface nanobubbles can help to increase the slip length and thus to decrease the drag of fluid flow. With the development and application of micro/nano electro mechanical systems and microfluidic systems, how to reduce the drag of fluid flow in micro pipe becomes a hot issue of research. As the main form of gas at solid-liquid interface, the influence of surface nanobubbles on the boundary slip has drawn great attention. In this paper, atomic force microscope(AFM) was used to study the morphology and to explore the mechanism of the stability of nanobubbles on polystyrene(PS) surface, by controlling the size and distribution of nanobubbles, the slip length measurement experiments were completed using colloid probe contact mode AFM on PS surface with different coverage rate of nanobubbles, and then the relation model of nanobubbles and slip length was established.The effects of surface roughness of substrates on the size and the distribution of nanobubbles were studied, the bigger micro bubbles were imaged on the rougher PS surfaces and the possible reasons for the nucleation of the bigger micro bubbles were analyzed. The size dependence of contact angle was also studied in a lager size scale(100nm~ 13μm), which is helpful to investigate the relationship between nanobubbles and boundary slip.Based on the analysis of interfacial gas enrichment(IGE), the influence of line tension and contamination on the contact angle, the reason why the contact angle of nanobubbles is so smaller than the Young contact angle was analyzed. By measuring the force-distance curves on highly ordered pyrolytic graphite(HOPG) surface before and after alcohol-water exchange, the coexistence of IGE and nanobubbles was verified; based on the model of Das, the influence of contamination on the contact angle of nanobubble was discussed; the line tension of nanobubble within size of 100nm~13μm was calculated; according to the modified Young equation, the mechanism of line tension reducing the contact angle was analyzed and investigated.Coalescence phenomenons of different sized nanobubbles were studied, the concept of “relative effect of three phase contact line” was porposed; through analyzing the effect of surface properties and the effect of contact line on bubble coalescence, the uncertainty for the movability and coalescence of nanobubbles was revealed. The detailed merging process of two nanobubbles was imaged for the first time, based on which the quantity of gas molecules, before and after coalescence, was calculated. The possible reasons for the gas increase after coalescence, such as the electrostatic effects, the impurities, the tip-bubble intraction, and line tension, etc. were analyzed and discussed, and IGE was thought to be responsible for the gas increase after bubble coalescence.The influence of set-point on the morphology of nanobubbles with different size was studied, and the larger nanobubbles were found to be softer than the smaller ones. In the temporal evolution experiment, the hypothesis of “three-phase contact line of bubbles kept pinning” of Weijs was confirmed; comparing the theoretical lifetime and experimental lifetime of nanobubbles within incubation time, the validity of the contact line pinning theory was verified and discussed. Based on the model of tip-bubble interaction, the reason of the faster gas diffusion from the over-sized bubbles after 12 hours incubation was analyzed. By considering the effect of the contact line pinning, the gas influx near the contact line from an IGE, a thin ‘contaminant film’ around the gas-liquid interface, the electrostatic effect and line tension, a viable stability mechanism for surface nanobubbles was established.The availability of measuring slip length using contact mode / dynamic mode atomic force microscope method was discussed when there were nanobubbles on the solid-liquid interface, the relation model between nanobubbles and slip length was established considering the size and the distribution of nanobubbles. The size and the distribution of surface nanobubbles can be controlled by changing the surface roughness of PS substrates and the gas concentration in water, the slip length measurement experiments were carried out on PS substrates with different coverage rate of bubbles, and the influence of nanoubbles on the slip length was revealed, by comparing the theoretical and experimental results, the availability of the new model was verified and discussed.