In their supercritical state simple fluids are generally thought to assume a homogeneous phase throughout all combinations of pressures and temperatures, although various response functions or transport properties may exhibit anomalous behavior, characterizing a state point as either more gas-like or liquid-like, respectively. While a large body of results has been compiled in the last two decades regarding the details of the supercritical phase in thermodynamic equilibrium, far less studies have been dedicated to out-of-equilibrium situations that nevertheless occur along with the handling of substances such as carbon dioxide or Argon. Here we consider successive compression-expansion cycles of equal amounts of Argon injected into a high-pressure chamber, traversing the critical pressure at two times the critical temperature. Due to expansion cooling, the fluid temporarily becomes sub-critical, and light scattering experiments show the formation of sub-micron-sized droplets and nanometer-scale clusters, both of which are distinct from spontaneous density fluctuations of the supercritical background and persist for a surprisingly long time. A kinetic rate model of the exchange of liquid droplets with the smaller clusters can explain this behavior. Our results indicate non-equilibrium aspects of supercritical fluids that may prove important for their processing in industrial applications.

在超临界状态下，简单流体通常被认为在所有压力和温度组合中呈现均相，尽管各种响应函数或传输特性可能会表现出异常行为，分别将状态点表征为更像气体或更像液体. 虽然在过去的 20 年里已经汇编了大量关于热力学平衡中超临界相细节的结果，但很少有研究专门研究在处理碳等物质时仍然发生的失衡情况二氧化硫或氩气。在这里，我们考虑将等量氩注入高压室的连续压缩-膨胀循环，在临界温度的两倍处穿过临界压力。由于膨胀冷却，流体暂时变为亚临界状态，光散射实验表明形成了亚微米级液滴和纳米级簇，这两者都不同于超临界背景的自发密度波动，并持续了出奇的长。液滴与较小簇交换的动力学速率模型可以解释这种行为。我们的结果表明超临界流体的非平衡方面可能对其在工业应用中的处理很重要。这两者都与超临界背景的自发密度波动不同，并且持续时间长得惊人。液滴与较小簇交换的动力学速率模型可以解释这种行为。我们的结果表明超临界流体的非平衡方面可能对其在工业应用中的处理很重要。这两者都与超临界背景的自发密度波动不同，并且持续时间长得惊人。液滴与较小簇交换的动力学速率模型可以解释这种行为。我们的结果表明超临界流体的非平衡方面可能对其在工业应用中的处理很重要。