Crossover of 2- and 3-dimensional critical phenomena on mercury-sapphire interface

众所周知，在环境条件下，液态汞很难润湿玻璃或蓝宝石等非金属物质。而汞在液-气临界点（1478℃，167 MPa）附近的高温高压条件下润湿蓝宝石衬底，润湿膜厚度在压力-温度平面上呈直线不连续跳跃。这是一阶相变，称为预润湿（PW）转变。本文采用光学反射率、光学发射率、静态光散射和动态光散射等方法研究了润湿膜的静态和动态特性，试图揭示二维和三维临界现象之间的交叉，从静态光散射的角度，我们观察到，当存在润湿膜时，漫散射出现在镜面反射光斑周围。有趣的是，这种漫散射沿着垂直方向很强。 关于我们 沿水平方向沿着较弱。这一结果表明，重力对润湿膜中光散射体的结构起着重要的作用。在PW过渡区，厚膜和薄膜之间存在接触线，重力使接触线沿水平方向沿着排列，导致沿垂直方向沿着发生强漫散射。从动态光散射中，我们观察到散射强度的间歇性波动，这些波动被解释为由接触线穿过激光光斑的移动引起。我们估计了接触线的速度，发现接触线主要沿沿着方向移动。作为相关的研究，我们研究了硒-铊混合物在石英衬底上的润湿现象。该体系随铊浓度的增加从半导体向金属转变，并与汞类似，随密度的降低表现出金属-非金属的转变。令人惊讶的是，观察到“临界点脱湿”现象：润湿膜不是在临界温度附近形成，而是在较低温度区域形成。这与Cahn预测的“临界点润湿”完全相反，是非常有趣的现象。少

It is well known that liquid mercury hardly wets non-metallic substances such as glasses or sapphire under ambient conditions. However, mercury wets sapphire substrate under high temperature and pressure conditions near the liquid-gas critical point (1478℃、167MPa), and the thickness of the wetting film jumps discontinuously at a line on the pressure-temperature plane. This is the 1st order phase transition called the prewetting (PW) transition. In the present work, we studied the static and dynamic properties of the wetting film by using optical methods such as (1) optical reflectivity, (2) optical emissivity, (3) static light scattering and (4) dynamic light scattering, and tried to unveil the crossover between the 2-and the 3-dimensional critical phenomena.From the static light scattering, we observed that diffuse scattering appears around the specularly reflected light spot when the wetting film is present. Interestingly, this diffuse scattering is strong along the vertical directio … More n, and weak along the horizontal direction. This result indicates that the gravity plays important role on the structure of the light scatterer in the wetting film. In the PW transition region, there exist contact lines between the thick and the thin wetting films, and the gravity makes the contact lines aligned along the horizontal direction, resulting in the strong diffuse scattering along the vertical direction. From the dynamic light scattering, we observed intermittent fluctuations in the scattering intensity, and these fluctuations are interpreted to be caused by the movement of the contact line across the laser light spot. We estimated the velocity of the contact line, and found that the contact line moves mainly along the vertical direction.As a related study, we studied the wetting phenomena of selenium-thallium mixtures on a quartz substrate. This system transforms from semiconductor to metal with increasing thallium concentration, and has a similarity to mercury, which shows metal-nonmetal transition with decreasing density. Surprisingly, "critical point dewetting" phenomena was observed : the wetting film is not formed near the critical temperature but is formed in the lower temperature region. This is completely opposite to the Cahn's prediction of "critical point wetting" and is very interesting phenomena. Less