【作者】 徐学锋；

【导师】 雒建斌；

【作者基本信息】 清华大学 ， 机械工程， 2007， 博士

【摘要】 液体的蒸发现象在自然界和人类活动中普遍存在,并在制冷、印刷、涂层和新材料制备等领域得到广泛利用。研究表明,液滴靠近接触线的微观区域在液滴的传热、传质过程中起着极为重要的作用。由于系统分辨率的限制,关于液滴接触线附近区域液体流动的实验研究较少。本文的主要工作之一是研制了一台微流体观测系统。系统由荧光显微镜、CCD及图像处理软件等组成,采用颗粒示踪测速(Particle Tracking Velocimetry,PTV)技术,并以荧光纳米颗粒作为示踪颗粒,实现了对微小区域内流体的速度测量。该系统具有较高的分辨率和测量精度,系统测量误差主要来源于示踪颗粒的布朗运动。应用该系统,对蒸发水滴中的Marangoni流动进行了观测和分析。结果表明,液气界面上存在一个驻点,表面流动、表面张力梯度和表面温度梯度在该点改变方向,这种现象在以往的文献中很少报道。本文通过引入液滴接触线附近区域的热传导模型,解释了液滴表面温度的这种非单调变化。液滴接触线附近区域的液体流动,与液滴微观区域(Micro-region)中的蒸发和散热有着紧密联系。实验发现,在液滴附着且Marangoni流动存在时,接触线附近宏观区域中液体外向流动速度与位置无关,并随时间增大。分析表明,液体外向流动速度和外向流动区域的高度有关。光干涉膜厚测量技术的测量结果表明,外向流动区域高度随时间的变化与本文的分析相吻合。颗粒浓度低时,液滴接触线平稳解附,随着颗粒浓度的增加,出现突然解附和多次附着现象。分析表明这和颗粒沉积引起的解附势垒增高有关。实验发现Marangoni流动导致了颗粒在液滴表面的聚集,不同的接触角产生不同的聚集状态。此外,通过观察垂直射流中纳米颗粒与固体表面的碰撞、吸附和解吸现象,模拟了固体表面的材料去除过程,为研究化学机械平坦化(Chemical Mechanical Planarization, CMP)中材料去除机理提供了新的手段。更多还原

【Abstract】 The evaporation of the liquid is ubiquitous in nature and human activities, and has a wide application in different industries including refrigeration, printing, coating, and production of novel materials. Previous studies show that the micro-region plays a key role in the heat and mass transfer of the liquid droplet. Due to the limitation of the resolution, experimental investigations on the liquid flow in the region near the contact line are scarce.In this study, a microfluid measurement system has been established by adopting the PTV technique. The system includes a fluorescent microscope, a CCD camera, and an image processing software, and utilizes fluorescent nanoparticles as tracing particles. The error analysis of the system shows that the Brownian motion of tracing particles plays a significant role in the accuracy of velocity measurements.By using the system, the Marangoni flow in the evaporating water droplets has been observed. The flow pattern of the droplet indicates that a stagnation point where the surface flow, the surface tension gradient, and the surface temperature gradient change their directions exists at the droplet surface. The deduced non-monotonic variation of the droplet surface temperature, which is different from that in some previous works, is explained by a heat transfer model considering the adsorbed thin film of the evaporating liquid droplet.The movement of the liquid in the region near the contact line of the droplet is closely relative to the evaporation and the heat transfer in the micro-region. The experimental results indicate that the velocity of the outward flow in the macro-region near the contact line is independent of the position, and increases with time. Analysis shows that the velocity of the outward flow relates to the height of the outward region. The result of the height measurement of the outward region using optical interference technique exhibits a consistency with the analysis in this work. A lower particle concentration in the liquid droplet results in a smooth de-pinning of the contact line while a higher concentration leads to an abrupt one. This is because that the deposition of the particles will increase the potential energy barrier for de-pining. As a result of the Marangoni flow, the collection of the particles on the droplet surface can lead a radial deposition.In addition, by observing the collision, the adsorption and the desorption of nanoparticles in a normal impacting liquid jet, the material removal process of the solid surfaces is simulated. It can provide means for investigating the material removal mechanism in the Chemical Mechanical Planarization.更多还原