Study Of Nanobubble And Substrate Interaction And Measurement Of Slip Length At Solid Liquid Interfaces

The boundary condition at the solid-liquid interface is a fundamental problem in fluid dynamics. Recent studies have shown that at solid-liquid interfaces, the fluid velocity is not always equal to that of solid surfaces, a phenomenon called boundary slip. The degree of boundary slip is evaluated by a slip length. The existence of boundary slip will reduce drag friction force between fluid flows and soild walls, which is in particular significant for micro/nanofluidics based biosensor applications. Theoretical and experimental studies suggest that at the solid-liquid interfaces, the presence of nanobubbles is responsible for the existance of the boundary slip. In this paper, atomic force microscopy (AFM) is applied to study nanobubbles-substrate interaction at solid-liquid interfaces on polystyrene surfaces. The traditional contact mode AFM method of slip length measurement is modified and a new method based on tapping mode AFM is developed. Using these two methods, slip lengths are measured on hydrophilic, hydrophobic, and superhydrophobic surfaces. Based on nanobubble imaging and slip length measurement, the relationship between nanobubbles and boundary slip is developed.In this paper, the tip holder commonly used for imaging in air is modified to improve nanobubble imaging on polystyrene surfaces in water. The influence of line tension force along solid-liquid-gas three phase contact line on contact angles of nanobubbles is investigated and the line tension force is measured on nanoscale. Influence of scan speed and scan load on nanobubble imaging is investigated. The detailed process of nanobubble coalescence is studied by experimental observation. With external disturbance, small nanobubbles are firstly moved and merged into large nanobubbles during coalescence. Cantilever tip-nanobubble interaction is performed using force modulation curves, which can be used to distinguish nanobubbles and solid objects at solid-liquid interfaces.In this paper, the mechanism of nanobubble induced nanoindents on polystyrene films is investigaed. The evolution of nanobubbles and polystyrene films are studied after the films are immersed into DI water. With time, rim stuructures appear and gradually grow up around larger nanobubbles. Smaller nanobubbles gradually disappear leaving nanoindents. Based on the theoretical analysis and experimental observation, a model is set up to explain the phenomenon. The high inner pressure and decomponent force of surface tension force in the direction normal to the sample surfaces are thought to be the reason for the generation of nanoindents.From aspect of improving nanobubble stability and reducing friction force between solid-liquid interfaces, a concept of nanobubble immobility is proposed. The force-distance curves in contact mode AFM are firstly applied to study solid-liquid-gas three phase contact properties for nanobubbles. By comparing the decomponents of surface tension forces along vertical direction for both approaching and retracting movements, it is verified that contact angle hysteresis is still valid for nanobubbles. Based on the concept of contact angle hysteresis, a model is developed to evaluate nanobubble immobility on nanoindents and island structures by calculating the force needed to slide nanobubbles. The model is then verified through experiments by applying higher scan load on surfaces with nanoindents and island structures, respectively.In the study of boundary slip, two very low approach velocities are first applied to determine the cantilever deflection component generated by electrostatic force. In order to eliminate the influence of surface roughness on the measured slip length, the mean plane of sample surfaces are taken as virtual solid-liquid interfaces. Through above treatment, the contact mode AFM method is improved. The method is then used to measure slip lengths using different approach velocity on surfaces with different wettabilities. The effect of approach velocity and wettability on boundary slip is studied. To simplify the process of slip length measurement, tapping mode AFM is applied to measure slip lengths on surfaces with different wettability. With the measured slip length and nanobubble images, the relationship between nanobubbles and boundary slip is developed.