Giavazzi Fabio, Fornasieri Alessandro, Vailati Alberto, Cerbino Roberto
Dipartimento di Biotecnologie Mediche e Medicina Traslazionale, Università degli Studi di Milano, via F.lli Cervi 93, 20090, Segrate, Italy.
Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133, Milano, Italy.
Eur Phys J E Soft Matter. 2016 Oct;39(10):103. doi: 10.1140/epje/i2016-16103-9. Epub 2016 Oct 31.
When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near-critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature T above the upper critical solution temperature T, in the wide temperature range [Formula: see text] C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents [Formula: see text], [Formula: see text] and [Formula: see text], which are found in agreement with the 3D Ising values. We then study the system in the two-phase region (T < T ). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature T, is rapidly brought to a temperature T > T . During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity I , of the crossover wave vector q and of the diffusion coefficient D during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.
当二元混合物中存在宏观浓度梯度时，由于该梯度与自发平衡速度涨落之间的耦合，会出现长程非平衡浓度涨落（NCF）。当混合物接近临界点时，也可能观察到长程平衡浓度涨落（ECF）。在此，我们研究了在垂直浓度梯度存在下，近临界混合物苯胺/环己烷中NCF与临界ECF之间的相互作用。为此，我们利用一台商用光学显微镜和一个简单的定制控温样品池，以获取关于涨落的同时静态和动态散射信息。我们首先在高于上临界溶解温度(T_{uc})的固定温度(T)下，在宽温度范围([公式：见原文]) (^{\circ}C)内表征临界ECF。在此范围内，我们观察到散射强度和质量扩散系数均呈现预期的临界标度行为，并确定了临界指数([公式：见原文])、([公式：见原文])和([公式：见原文])，发现它们与三维伊辛模型值一致。然后我们在两相区域（(T < T_{uc})）研究该系统。特别地，我们表征了当初始处于温度(T)的混合物迅速升温至(T'> T_{uc})时，ECF与NCF之间的相互作用。在瞬态过程中，存在垂直扩散质量通量，这导致了NCF的出现，随着通量趋于零，其幅度随时间消失。我们还研究了扩散过程中平衡散射强度(I_e)、交叉波矢(q_c)和扩散系数(D)的时间依赖性，发现所有这些量都表现出受扩散动力学支配的指数弛豫。
