Mazzoni Stefano, Cerbino Roberto, Vailati Alberto, Giglio Marzio
CNR-Istituto Nazionale per la Fisica della Matenia and Dipartimento di Fisica, via Celoria 16, 20133 Milan, Italy.
Ann N Y Acad Sci. 2006 Sep;1077:351-64. doi: 10.1196/annals.1362.029.
It has been shown recently that diffusion processes exhibit giant nonequilibrium fluctuations (NEFs). That is, the diffusing fronts display corrugations whose length scale ranges from the molecular to the macroscopic one. The amplitude of the NEF diverges following a power law behavior proportional to q(-4) (where q is the wave vector). However, fluctuations of wave number smaller than a critical "rolloff" wave vector are quenched by the presence of gravity. It is therefore expected that in microgravity conditions, the amplitude of the NEF should be boosted by the absence of the buoyancy-driven restoring force. This may affect any diffusion process performed in microgravity, such as the crystallization of a protein solution induced by the diffusion of a salt buffer. The aim of GRADFLEX (GRAdient-Driven FLuctuation EXperiment), a joint project of ESA and NASA, is to investigate the presence of NEFs arising in a diffusion process under microgravity conditions. The project consists of two experiments. One is carried out by UNIMI (University of Milan) and INFM (Istituto Nazionale per la Fisica della Materia) and is focused on NEF in a concentration diffusion process. The other experiment is performed by UCSB (University of California at Santa Barbara) concerning temperature NEF in a simple fluid. In the UNIMI part of the GRADFLEX experimental setup, NEFs are induced in a binary mixture by means of the Soret effect. The diagnostic method is an all-optical quantitative shadowgraph technique. The power spectrum of the induced NEFs is obtained by the processing of the shadowgraph images. A detailed description of the experimental apparatus as well as the ground-based experimental results is presented here for the UNIMI-INFM experiment. The GRADFLEX payload is scheduled to fly on the FOTON M3 capsule in April 2007.
最近研究表明，扩散过程呈现出巨大的非平衡涨落（NEFs）。也就是说，扩散前沿呈现出波纹，其长度尺度范围从分子尺度到宏观尺度。非平衡涨落的幅度遵循与q(-4)成正比的幂律行为而发散（其中q是波矢）。然而，波数小于临界“滚降”波矢的涨落会因重力的存在而被抑制。因此可以预期，在微重力条件下，由于缺乏浮力驱动的恢复力，非平衡涨落的幅度应该会增大。这可能会影响在微重力环境下进行的任何扩散过程，比如盐缓冲液扩散诱导蛋白质溶液结晶的过程。GRADFLEX（梯度驱动涨落实验）是欧洲航天局（ESA）和美国国家航空航天局（NASA）的一个联合项目，其目的是研究在微重力条件下扩散过程中出现的非平衡涨落。该项目包括两个实验。一个由米兰大学（UNIMI）和意大利国家材料物理研究所（INFM）进行，聚焦于浓度扩散过程中的非平衡涨落。另一个实验由加利福尼亚大学圣巴巴拉分校（UCSB）进行，涉及简单流体中的温度非平衡涨落。在GRADFLEX实验装置的UNIMI部分，通过索雷特效应在二元混合物中诱导出非平衡涨落。诊断方法是全光学定量阴影图技术。通过对阴影图图像的处理获得诱导出的非平衡涨落的功率谱。这里给出了UNIMI - INFM实验的实验装置以及地面实验结果的详细描述。GRADFLEX有效载荷计划于2007年4月搭载在FOTON M3太空舱上飞行。
