Surface tension measurements on liquid metals in microgravity
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INTRODUCTION
INTEREST in the surface tension of liquid metals and alloys stems both from scientific considerations, because understanding these properties is essential to validating theories of the liquid state, and from practical considerations, such as their applications in processing, chemical, and metallurgical industries.[1] The surface tension is critical in casting, molding, crystal growth operations, smelting, refining, brazing, sintering, zone refining, and fiber formation.[2] The dependence of surface tension on temperature is also important because it leads to Marangoni convection, which is particularly important in such processes as welding and crystal growth.[3,4] With the advent of powerful mathematical modeling techniques for metallurgical phenomena, there is renewed interest in reliable data on the physical properties of liquid metals. Presently, the knowledge and accuracy of these data are the limiting factors in the models.[5] The property database for liquid metals is incomplete and the conventional measurement techniques are not entirely satisfactory. The electromagnetic levitation technique is a widespread method for containerless processing of liquid metals. A major advantage of the method is the avoidance of sample contamination through the use of high-purity processing gas and contact-free measuring techniques.[3] The oscillating drop technique involves excitation of the surface oscillations of a levitated droplet about its equilibrium shape and observation of the frequencies of these oscillations. These are related to the surface tension, which serves as the restoring force for the oscillations.[6] There has been some skepticism about the results obtained by the oscillating drop technique, because this method tends to yield higher values for the surface tension than those from conventional methods. Although some auIVAN EGRY, Professor, GERD JACOBS, Graduate Research Assistant, and GEORG LOHOEFER, Research Scientist, are with the Institute for Space Simulation, German Aerospace Research Establishment, D-51170 Cologne, Germany. ELLIOT SCHWARTZ, Research Scientist, is with The Gillette Company, Boston, MA, 02127. JULIAN SZEKELY, formerly Professor, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is deceased. PETRA NEUHAUS, Research Scientist, is with Dornier GmbH, D-88039 Friedrichshafen, Germany. Manuscript submitted March 9, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
thors have attributed this to the better purity of the sample’s surface, it may also be a systematic error due to the presence of the electromagnetic levitation field, which produces a magnetic pressure at the surface and leads to an apparent increase of the surface tension. Using a correction formula derived by Cummings and Blackburn,[7] Sauerland et al. obtained surface tension values for gold which were independent of sample mass and in agreement with values derived by conventional techniques.[8] This work demonstrated that the oscillating drop technique
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