摘要 在奈米科學中，介觀液體在狹縫中的微觀動力行為與其流變反應， 一直都是個很重要的議題。然而，由於測量尺度的限制，大多數的科學 家只能透過電腦模擬或者在實驗上，量測其表面力與速度等巨觀特性。 其微觀結構與運動行為，卻鮮少人能直接測量並探討研究。 在微粒庫倫系統中，我們將微粒丟入充滿電漿的二維長形空腔中。 由於電子有較高的遷移率，故空腔與微粒會帶負電，透過庫倫力的交互 作用，進而形成二維長形庫倫結構。藉由調控背景擾動，並輔以光學顯 微鏡，我們不僅能透過追蹤粒子的運動軌跡直接觀察到微粒庫倫液體的 狀態並且能觀測了解其微觀結構排列行為。 我們利用平行反向的雷射光束施加剪力在介觀庫倫液體的兩側，在 粒子間的相互影響，邊界效應，背景擾動，與外加剪力的交互作用，我 們發現其平均速度分布會分隔成兩區: 兩側高速度區夾著中間低速度 區。這種現象稱做”剪力帶的形成” 。透過一些統計工具的計算分 析(例如: 速度分布?結構變化率?以及一些時空關聯函數等等…)，我 們深入探討介觀液體形成剪力帶的起源並加以比較與其他系統(如: 玻 璃態的物質等...) 的異同特性。 Abstract We investigate experimentally the generic microscopic dynamical behavior of a dust Coulomb liquid confined in a narrow mesoscopic channel and sheared by two parallel counter laser beams along the opposite boundaries, at the low stress limit where the shear induced average speed is comparable to the thermal speed. A liquid confined in a narrow channel with width down to the molecular scale usually exhibits anomalous behavior in structure and motion deviating from the bulk liquid, under the e®ect of discreteness, finite boundary, and lager thermal fluctuations. It is an interesting and important issue for nanoscience and technology. Nevertheless, the rheology studies have been mainly limited to the macroscopic force and velocity measurement on the confining boundary because of the lack of microscopic measures at the small atomic scale, which is insu±cient to construct an obvious microscopic picture for the sheared flow in a narrow channel. A dust Coulomb liquid formed by micrometer sized dust particles charged and suspended in a low pressure weakly ionized discharge background is an heuristic system to mimic and understand the generic dynamical behaviors at the kinetic level because of the capability of directly visualization. In this work, through monitoring the spatio-temporal evolution of micro-motion and correlating with the local structure rearrangement, we demonstrate the following findings: 1. The mean velocity profile exhibits shear bands with high mean shear rates in the outer region sandwiching a center zone with low shear rate. 2. Stress enhanced avalanche type cooperative topological rearrangement associated with the vortex type cooperative hopping involvi ing a cluster of particles are observed in the shear band, which are responsible for the observed enhanced longitudinal and transverse velocity fluctuations and the higher structural relaxation rate in the shear bands. It screens the external drive through relaxing the local stress and leaves a weakly perturbed center zone. 3. We also point out that the nonlinear threshold type stick-slip hopping after accumulation of local stress is the origin for shear thinning and shear banding which have also been observed in glassy systems under the slow dynamics. Unlike in the glassy system, the finite temperature e®ect leads to the absence of a finite yield stress.
