Mercury wetting film on sapphire

Abstract

We have measured optical properties of a mercury wetting film on sapphire under high temperature and high pressure near the liquid-gas critical point of mercury by using a newly developed 45 degrees reflection technique. We have analyzed the experimental data to deduce the density, the thickness, and the coverage of the wetting film quantitatively as functions of pressure and temperature. As a first approximation, we have assumed a slab model for the density profile of the wetting film, and found that the density of the wetting film dslab is much smaller than that of bulk liquid at the liquid-vapor coexistence curve. This result is consistent with the Lifshitz theory, from which we may predict that the sapphire substrate prefers wetting film with density lower than the metal-nonmetal transition. When the temperature is close enough to the prewetting critical temperature Tpw(c), the effective slab density dslab shows a sharp decrease as the pressure approaches the liquid-gas coexistence. This indicates that the slab model is not sufficient to describe the shape of the wetting film, and a smooth variation of the density has to be taken into account. In the prewetting supercritical region, two anomalies are observed in the reflectances. Possible mechanisms of these anomalies are discussed.

Dynamics of a mercury prewetting film near the prewetting critical point

“…The PW line merges tangentially to the LV-curve at T w , and is terminated at the prewetting critical point (CPW, T cpw = 1468°C and P cpw = 158.6 MPa). Detailed optical reflectivity measurements with a 45°reflection geometry indicate that the density of the prewetting film is much smaller than that of the bulk liquid at the liquid-vapor coexistence curve [4]. This result is consistent with the Lifshitz theory, from which we may predict that the sapphire substrate prefers a prewetting film of non-metallic character [5].…”

“…The existence of the film is confirmed unambiguously by the optical reflectance measurement with a 45°reflection geometry [4]. This is an example of the critical point wetting phenomena [3].…”

Thermal radiation method: proposal of a new technique for measuring interfacial or surface fluctuations

“…Thickness fluctuations Ál reach a few nm and the lateral correlation length k hundreds of nm. These values are huge compared to the thickness l of the Hg film, which is estimated to be about 10 nm by optical reflectivity measurements (Ohmasa et al, 2001). It would be very interesting to elucidate how Ál and k vary with temperature and pressure, or how these fluctuations relate to the prewetting critical fluctuations.…”

“…We also measured the reflectivity by using a He-Ne laser (wavelength 633 nm) as a light source and a photomultiplier (PM) as a detector. Further details of the experimental setup are shown in other papers (Ohmasa et al, 2001;Kajihara et al, 2003). The measurements were carried out over wide temperature and pressure ranges up to 1803 K and 200 MPa including the prewetting supercritical region by decreasing (or increasing) the pressure at constant temperatures.…”

“…The order parameter is film thickness (strictly speaking, coverage) and thickness fluctuations, which indeed have not been reported, are naturally expected in the supercritical region like density fluctuations for a liquid-gas transition. Hg on sapphire is one of such prewetting systems and we have studied the average properties of the film by optical reflectivity measurements before (Ohmasa et al, 2001). However, it is very difficult to study the fluctuations in the film by Xray, neutron or light scattering methods, because the prewetting critical point of the system is located at extreme conditions of temperature T CPW = 1741 K and pressure 159 MPa.…”