Narayanan Theyencheri, Chèvremont William, Zinn Thomas
ESRF - The European Synchrotron, 38043Grenoble, France.
J Appl Crystallogr. 2025 Mar 7;58(Pt 2):535-542. doi: 10.1107/S1600576725001244. eCollection 2025 Apr 1.
The high brilliance of fourth-generation synchrotron sources coupled with advanced X-ray detectors enables a wide range of dynamic studies of colloids and other soft-matter systems. In particular, the higher fraction of coherent flux provided by these new sources is a major boost for X-ray photon correlation spectroscopy (XPCS). As a result, not only can equilibrium dynamics be accessed but also relatively fast out-of-equilibrium processes can be investigated by XPCS. This article briefly recalls the statistical properties of coherent scattering and then demonstrates a case study of non-equilibrium fluctuations in a driven colloidal system. A simple example is the resuspension of colloids by vigorous shaking, where the inhomogeneous flow generates local variations in number density of particles leading to strong velocity fluctuations. The Brownian motion of the particles homogenizes the suspension with time and the system gradually returns to pure diffusive dynamics. On the other hand, in a uniformly sheared suspension of particles, such concentration gradients do not form and upon cessation of shear the return to Brownian dynamics is rapid. These transient non-equilibrium effects can inadvertently influence micrometre-range particle size measurement by means of dynamic scattering methods.
第四代同步辐射源的高亮度与先进的X射线探测器相结合，使得对胶体和其他软物质系统进行广泛的动态研究成为可能。特别是，这些新光源提供的相干通量比例更高，这对X射线光子相关光谱学（XPCS）是一个重大推动。因此，XPCS不仅可以研究平衡动力学，还可以研究相对快速的非平衡过程。本文简要回顾了相干散射的统计特性，然后展示了一个受驱动胶体系统中非平衡涨落的案例研究。一个简单的例子是通过剧烈摇晃使胶体重新悬浮，其中不均匀的流动会导致颗粒数密度的局部变化，从而产生强烈的速度涨落。随着时间的推移，颗粒的布朗运动会使悬浮液均匀化，系统逐渐恢复到纯扩散动力学。另一方面，在颗粒的均匀剪切悬浮液中，不会形成这种浓度梯度，并且在剪切停止后，恢复到布朗动力学的速度很快。这些瞬态非平衡效应可能会无意中影响通过动态散射方法进行的微米级颗粒尺寸测量。
