The phenomenon of surface freezing has been studied by second harmonic and plasma generation measurements in the entire composition range of liquid Ga-Bi alloys. Surface freezing was observed for the first time for all alloys with compositions between the eutectic (x Bi ¼ 0.0022, T eut ¼ 29.48 C) and monotectic point (x Bi ¼ 0.085, T mono ¼ 222 C). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. On melting of these films a clear hysteresis behaviour is found, characterizing this type of transition as a first order phase transition. The maximum difference of 20 K between the liquidus temperature and the melting temperature of the surface freezing films was observed for the eutectic alloy. The line of surface freezing temperatures merges with the liquidus approaching the monotectic point. An estimate of the thickness of the Bi-rich surface freezing films from the interfacial free energies yields a value of the order of 10 nm. The correlation of the surface freezing and wetting transition occurring in the Ga-Bi system as well as the thermodynamics of surface freezing are qualitatively discussed.

No abstract

No abstract

We report a detailed optical investigation of the wetting phenomena in fluid metalrich KxKCl1−x solutions for temperatures up to 820 °C and x≥0.86. To this end the fluid sample–sapphire interface has been probed by second harmonic generation (SHG) with continuous variation of temperature T at various x. At coexistence a wetting transition near Tw≊500 °C is signaled by a clear change in the temperature dependence of the SH intensity. This observation is in agreement with previous ellipsometric measurements. The precise location of the transition in these systems is strongly influenced by the presence of oxygen impurities segregating at the interface. This is manifested in a strong enhancement of the SH signal in the vicinity of 450 °C. In the homogeneous metal-rich phase the SH intensity shows a pronounced maximum as a function of temperature for T≳Tw. The occurrence of these maxima is explained consistently by assuming a salt-rich microscopic wetting film in between the metallic bulk fluid and the sapphire window whose thickness changes from thick to thin with increasing temperature. The line connecting these maxima positions in the x–T plane exhibits all the characteristics of a prewetting line. In particular, it is curved away from the coexistence curve as has been predicted theoretically for systems with screened Coulombic interactions, but is found here for the first time experimentally.

We report the first study of interfacial wetting at the liquid-vapour surface of metallic gallium-bismuth alloys approaching the miscibility gap. Ellipsometry has been used to probe the interface continuously with increasing temperature along the coexistence curve. Below the monotectic (eutectic) temperature Tmono a liquid Ga-rich phase is in equilibrium with solid bismuth and the real part epsilon 1 of the complex dielectric function measured at the liquid surface clearly reflects Ga-like behaviour. Above Tmono the second liquid Bi-rich phase becomes stable and epsilon 1 changes from Ga-like to Bi-like behaviour. We conclude that the Bi-rich phase, which has the higher mass density, now covers the low-density phase. This can be consistently interpreted as complete wetting of the interface between the vapour and the Ga-rich phase by a coexisting Bi-rich layer.