Ojha R P, Lemieux P-A, Dixon P K, Liu A J, Durian D J
Department of Physics and Astronomy, University of California, Los Angeles, California 90095-1547, USA.
Nature. 2004 Feb 5;427(6974):521-3. doi: 10.1038/nature02294.
Characterization of the microscopic fluctuations in systems that are far from equilibrium is crucial for understanding the macroscopic response. One approach is to use an 'effective temperature'--such a quantity has been invoked for chaotic fluids, spin glasses, glasses and colloids, as well as non-thermal systems such as flowing granular materials and foams. We therefore ask to what extent the concept of effective temperature is valid. Here we investigate this question experimentally in a simple system consisting of a sphere placed on a fine screen in an upward flow of gas; the sphere rolls because of the turbulence it generates in the gas stream. In contrast to many-particle systems, in which it is difficult to measure and predict fluctuations, our system has no particle-particle interactions and its dynamics can be captured fully by video imaging. Surprisingly, we find that the sphere behaves exactly like a harmonically bound brownian particle. The random driving force and frequency-dependent drag satisfy the fluctuation-dissipation relation, a cornerstone of statistical mechanics. The statistical mechanics of near-equilibrium systems is therefore unexpectedly useful for studying at least some classes of systems that are driven far from equilibrium.
表征远离平衡态的系统中的微观涨落对于理解宏观响应至关重要。一种方法是使用“有效温度”——这样一个量已被用于混沌流体、自旋玻璃、玻璃和胶体，以及诸如流动的颗粒材料和泡沫等非热系统。因此，我们要问有效温度的概念在多大程度上是有效的。在这里，我们在一个简单的系统中通过实验研究这个问题，该系统由放置在向上气流中的细筛网上的一个球体组成；球体由于其在气流中产生的湍流而滚动。与难以测量和预测涨落的多粒子系统不同，我们的系统没有粒子间相互作用，其动力学可以通过视频成像完全捕捉。令人惊讶的是，我们发现球体的行为与受简谐束缚的布朗粒子完全一样。随机驱动力和频率依赖的阻力满足涨落耗散关系，这是统计力学的基石。因此，近平衡态系统的统计力学对于研究至少某些类别的远离平衡态驱动的系统出人意料地有用。
