【作者】 王玉亮；

【导师】 赵学增； BharatBhushan；

【作者基本信息】 哈尔滨工业大学 ， 机械电子工程， 2010， 博士

【摘要】 固液界面边界条件是流体力学研究中的一个基本内容。近年来研究显示,在固液接触面上流体相对于固体墙的运动速度并不总是零,这种现象称为固液界面边界滑移,由滑移长度来表征。边界滑移的存在将减小流体与固体墙之间的拖曳摩擦力,对基于微纳米流体流动的生物芯片的应用具有重要的意义。理论分析和实验结果显示,存在于固液界面上的纳米气泡是产生边界滑移的主要原因。本文应用原子力显微镜(AFM)在聚苯乙烯样品表面研究了固液界面纳米气泡与样品基底的相互作用,改进了的接触模式AFM边界滑移测量方法并提出采用敲击模式AFM法测量滑移长度,应用这两种方法对不同润湿性样品表面进行了测量,初步建立了纳米气泡尺寸及分布密度与边界滑移之间的关系。采用改进了的微悬臂夹持器在去离子水中实现了聚苯乙烯样品表面纳米气泡成像并对纳米气泡进行表征,在纳米尺度上揭示了沿固液气三相接触线的线张力对纳米气泡接触角影响机理并计算得到线张力数值。在应用敲击模式进行纳米气泡成像过程中,发现除增加扫描载荷外,降低探针的扫描速度同样会造成纳米气泡聚合。通过实验观察和对具体的纳米气泡进行跟踪,揭示了纳米气泡聚合的过程。纳米气泡的聚合同时伴随着纳米气泡的移动,在外界扰动下,小的纳米气泡首先被移动,与较大纳米气泡发生聚合,产生新的纳米气泡。提出采用AFM的力调制模式曲线分析的方法区分样品表面固体颗粒和纳米气泡。研究了纳米气泡在聚苯乙烯薄膜上产生纳米凹痕的作用机理。将聚苯乙烯薄膜浸入去离子水中后,在薄膜上产生大小不同的纳米气泡。随着浸入时间的增加,在较大尺寸纳米气泡周围产生聚苯乙烯环状结构,而小尺寸纳米气泡逐渐消失,在样品表面留下纳米凹痕结构。结合理论分析和实验现象,建立了纳米气泡的存在对聚苯乙烯薄膜影响的模型。模型显示,纳米气泡较高的内部压强与固液气三相接触线上表面张力沿竖直方向分量共同作用引起聚苯乙烯薄膜发生变形,从而产生纳米凹痕结构。针对纳米气泡易于移动的问题,从提高纳米气泡稳定性的角度出发,提出了纳米气泡非移动性概念。首先采用AFM的力-距离曲线分别在进给和收缩运动中测量了作用在探针上的表面张力沿竖直方向分力,验证了接触角滞后理论对于纳米气泡仍然成立。在此基础上,应用接触角滞后理论计算在纳米凹痕结构和疏水材料岛状结构上移动纳米气泡所需的水平初始力,建立了这两种结构提高纳米气泡非移动性的理论模型。随后,分别在具备纳米凹痕结构和疏水岛状结构的样品表面上施加较高的扫描载荷,实验验证了以上两种结构能够提高纳米气泡的非移动性。针对当前采用接触模式AFM测量边界滑移时无法消除静电力和表面粗糙度对测量结果影响的问题,采用双低速逼近法和虚拟固液接触面法分别确定静电力引起的微悬臂偏转分量和修正实验测量得到的固液接触面位置,改进了接触模式AFM边界滑移测量方法。利用它在不同逼近速度下和不同润湿性样品表面测量滑移长度,探索了样品逼近速度和表面润湿性对边界滑移的影响。针对接触模式测量中无法消除微悬臂偏转对分离距离和逼近速度影响的问题,又引入敲击模式AFM法在不同润湿性样品表面测量其滑移长度,并比较了两种测量方法的优劣。结合测量得到的滑移长度和AFM液体中成像得到的纳米气泡,初步建立了纳米气泡尺寸和分布密度与滑移长度之间的关系。更多还原

【Abstract】 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.更多还原