Tetra Point Wetting at the Free Surface of a Binary Liquid Metal

1. Tetra Point Wetting at the Free Surface of a Binary Liquid Metal Patrick Huber, Oleg Shpyrko, Peter Pershan, Holger Tostmann*, Elaine DiMasi**, Ben Ocko**, Moshe Deutsch*** Department of Physics, Harvard University, Cambridge MA, U.S.A. *University of Florida, Gainesville, FL, ** Brookhaven National Lab, Upton, NY, *** Bar-Ilan University, Tel Aviv Abstract We present x-ray reflectivity measurements from the free surface of a gallium-bismuth (Ga-Bi) alloy over a temperature range from T = 200°C up to T=280°C. We found a continuous formation of a wetting film at the surface driven by the phase transition of first order in the bulk at the monotectic temperature TM = 222°C. The observed wetting scenario is closely related to triple point wetting known from one component systems and properly described as complete wetting at a solid-liquid-liquid-vapor tetra point [1,2]. Our measurements of the microscopic structure of the wetting film in combination with the known bulk thermodynamics allow calculations of liquid-liquid interfacial tensions and the extraction of information on the surface potential. X-ray reflectivity measurements Microscopic View on Tetra Point Wetting • Bulk thermodynamics • Binary liquid metal with miscibility gap and monotectic point • Phase diagram measured with calorimetric methods by P. Predel [3] • Free Energy G(c,T) available from CALPHAD project [4] Bulk structure  surface structure regime III: Gibbs adsorbed monolayer of pure Bi. regime II: Thick wetting film of the heavier Bi-rich phase intrudes between the low density, Ga-rich phase and the vapor phase in defiance of gravity. [5,6] regime I: Gibbs adsorbed monolayer of pure Bi. [6,7] • Transition pinned at bulk first order transition (monotectic point). • Correctly described as complete wetting at a solid-liquid-liquid-vapor tetra point. [2] (Phenomenon related to well known triple point wetting for one component systems. [1]) • Experimental data indicate film structures dominated by density gradients. - In contrast to the frequently used “homogeneous slab” models. - but in agreement with density functional calculations for wetting in binary systems at hard walls. [7] • Preliminary analysis suggests: short-range, screened Coulomb interactions + long-range, van-der-Waals like dispersion forces are necessary to explain evolution of profiles confirming modern treatments of interactions in metals. [11] • Wetting scenario in Ga-Bi analogous to behavior in Ga-Pb system. [12] Gradient Theory for the liquid-liquid interface [8] References [1] R. Pandit, M. E. Fisher, Physical Review Letters 51, 1772 (1983) [2] S. Dietrich and M. Schick, Surface Science 382, 178 (1997) [3]P. Predel, Zeitschrift für Physikalische Chemie Neue Folge 24, 206 (1960) [4] L. Kaufman, H. Bernstein, Computer Calculation of Phase Diagrams, Academic Press, NY (1970) [5] D. Nattland, S. C. Muller, P. D. Poh, Freyland W., Journal of Non-Crystalline Solids 207, 772 (1996) [6]H. Tostmann, E. DiMasi, O. G. Shpyrko, P.S. Pershan, B.M. Ocko, M.Deutsch, Physical Review Letters 84, 4385 (2000) [7]N. Lei, Z. Q. Huang, and S. A. Rice, Journal of Chemical Physics 104, 4802 (1996) [8] H.T. Davis, Stastical Mechanics of Phases, Interfaces, and Thin Films, Wiley-VCH, NY (1996) [9] H. Kreuser and D. Woermann, Journal of Chemical Physics 98, 7655 (1993) [10] M. Merkwitz, J. Weise, K. Thriemer, Hoyer W., Zeitschrift Fur Metallkunde 89, 247 (1998) [11] N. W. Ashcroft, Philosophical Transactions of the Royal Society of London Series a 334, 407 (1991) [12] P. Wynblatt and D. Chatain, Berichte der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 102, 1142 (1998)