Wetting transition in liquid Ga-Bi alloys: light scattering study of surface energy and entropy

We have studied the surface energy, surface entropy and, partly, have received an estimate of the viscosity of liquid Ga-Bi alloys at compositions mainly in the Ga-rich phase region and at temperatures up to 620 K, Measurements have been performed by the noninvasive method of capillary wave spectroscopy which has been applied for the first time to investigate the surface properties of a liquid alloy at high temperatures. Analysis of the concentration dependence of the surface energy according to the Gibbs adsorption equation yields that in the composition range of 10(-2) less than or similar to x(Bi) less than or similar to 10(-1) and at temperatures of 450 K less than or similar to T less than or similar to 500 K a surface excess of Gamma(Bi) similar to 1.36 x 10(-5) mol m(-2) resides at the liquid-vapour interface corresponding to pure Bi. At lower x(Bi) a reduction of Gamma(Bi) occurs which is indicative of a prewetting transition. The variations of surface energy and entropy with composition are not consistent with a description by a simple monolayer model as was deduced from X-ray reflectivity results, Instead it is found that a multilayer model qualitatively accounts for the characteristic change of the surface quantities. The thickness of the multilayer interfacial region is estimated from the change of the relative surface entropy. This yields values between 10 and 20 A with a trend for an increasing number of surface layers towards the complete wetting transition at the monotectic point, All isopleths of the surface energy exhibit clear kinks at temperatures 10-20 K above the liquidus curve for compositions below the monotectic point. The corresponding discontinuity of the surface entropy is consistent with a first order transition of surface freezing reported recently for the Ga-Bi system. (C) 2002 Elsevier Science B.V. All rights reserved.