Materials Science Institute and Department of Physics, University of Oregon, Eugene, Oregon 97403, USA.
Phys Rev E. 2017 Oct;96(4-1):042606. doi: 10.1103/PhysRevE.96.042606. Epub 2017 Oct 18.
A thermal colloid suspended in a liquid will transition from a short-time ballistic motion to a long-time diffusive motion. However, the transition between ballistic and diffusive motion is highly dependent on the properties and structure of the particular liquid. We directly observe a free floating tracer particle's ballistic motion and its transition to the long-time regime in both a Newtonian fluid and a viscoelastic Maxwell fluid. We examine the motion of the free particle in a Newtonian fluid and demonstrate a high degree of agreement with the accepted Clercx-Schram model for motion in a dense fluid. Measurements of the functional form of the ballistic-to-diffusive transition provide direct measurements of the temperature, viscosity, and tracer radius. We likewise measure the motion in a viscoelastic Maxwell fluid and find a significant disagreement between the theoretical asymptotic behavior and our measured values of the microscopic properties of the fluid. We observe a greatly increased effective mass for a freely moving particle and a decreased plateau modulus.
悬浮在液体中的热胶体将经历从短暂的弹道运动到长时间的扩散运动的转变。然而，弹道运动和扩散运动之间的转变高度依赖于特定液体的性质和结构。我们直接观察到自由漂浮示踪粒子在牛顿流体和粘弹性 Maxwell 流体中的弹道运动及其向长时间状态的转变。我们研究了自由粒子在牛顿流体中的运动，并展示了与稠密流体中运动的公认 Clercx-Schram 模型高度一致的结果。弹道到扩散转变的函数形式的测量值提供了对温度、粘度和示踪剂半径的直接测量。我们同样测量了粘弹性 Maxwell 流体中的运动，发现理论渐近行为与我们对流体微观性质的测量值之间存在显著差异。我们观察到自由运动粒子的有效质量大大增加，平台模量降低。
