| 摘要: | 在微觀尺度下，液體是一個強耦合多體系統，且具有既豐富及複雜的行為。在最近十年，微觀液體的動力行為已成為一個重要的議題。但大部份之前的研究都著重在均向系統。非均向液體也將是一個有趣的議題，但一直為人忽略。二加一維液體是當中最簡單的系統。簡而言之，它是由一束柔韌的鏈所組成的系統。在該系統中，沿著鏈(垂直)方向的耦合遠比橫切(水平)面上的耦合強。故此，水平面與垂直方上的結構與運動特性都不同。因為缺乏實驗觀察，它的微觀行為還沒有透徹瞭解。 微粒電漿液體是由一群微米級帶負電的粒子懸在低壓弱液離氣體中組成，當中的粒子經由庫倫作用能自我組織成液體狀態。當粒子直徑大於3微米，在垂直方，從上往下的離子流尾跡場效應將提供額外的耦合作用，同時使粒子排列成鏈狀結構。故稱之為二加一維鏈狀微粒電漿液體。因其獨特的時間及空間尺度，能夠利用光學顯微鏡直接觀察，故能用作一個很好的平台來比擬及瞭解液體在微觀尺度下的動力學行為。 在本論文中，藉由自行開發的立體掃描微粒追踪系統，我們探討了二加一維鏈狀液體之微觀結構與運動。我們發現，在一個水平面上，它的結構及運動都與一般二維液體相同。又當觀測時間短於系統之鬆弛時間，粒子可以表現出集體運動，同時伴隨著異常擴散。水平面上的集體運動可分為縱向及橫切兩種。沿著鏈方向，由於粒子的垂直翻轉被抑制，粒子只能有橫切方向的集體運動。經由它們的不同組合，本系統將呈現不同的，特有的二加一維集體運動，例如鏈束的抖動(wiggling)，鏈束的傾斜－直化(tilting-restraightening)，鏈束的扭轉－直化(twisting-restraightening)與斷鏈－重連等。此外，水平微結構和立體聲集體運動之間的關連性，亦透過機率關連分析法得到佐證。 The microscopic liquid can be treated as a strongly coupled many-body system, which exhibits vast and complicated micro-dynamics. It has been a hot topic in the past decade. Nevertheless, the previous studies are mainly limited to the system with isotropic coupling. It is very interesting to extend the investigation to the liquid with anisotropic coupling. 2+1D liquids composed of bundles of flexible chains is one of the simplest examples, where the coupling along the chain direction is stronger than that in the transverse plane. Due to the lack of direct experimental observation, the micro-picture of 2+1D liquid remains elusive. The dusty plasma liquid formed by negatively charged dust particle suspended in low pressure gaseous discharges, where the dust particle interacts with each other through the Coulomb interaction, provides us a platform to mimic and understand the generic microscopic dynamical behaviors of liquids at the kinetic level because of the capability of direct visualization. The wake field of downward ion wind on particles at larger diameter (> 3 mum) provides extra vertical coupling and alignment, and induces the chain bundle structure. When the chain length is long (40 particle per chain), the chains are flexible. The 2+1D dusty plasma liquid is then formed. In this work, the micro-structure and motion in the cold 2+1D dusty plasma liquid were experimentally investigated through our stereo scanning dust tracking system. It is found that the horizontal structure and motion are similar to those of the 2D liquids. When the observation time is shorter than the relaxation time of the system, particles can exhibit collective motion, which is also associated with anomalous diffusion. Horizontally, the collective motion can be classified to the longitudinal and the transverse types. Along the vertical chain, under the suppression of the vertical flipping, only the transverse type collective motion is allowed. The cooperation of horizontal and vertical collective motion leads to the special collective stereo excitations, such as straight vertical chains with small amplitude wiggling, chain tilting-restraightening, bundle twisting-restraightening, and chain breaking-reconnection. The demonstration of the first direct visualization of those basic excitations was conducted in this work. In addition, horizontal micro-structure and the stereo collective excitations are found to be statistically correlated through the measurement of correlation probability. |
