An improved method of noncontact measurement of the surface tension of a molten material has been developed, partly to overcome the disadvantages of contact measurement techniques as described in the preceding article, "Noncontact Measurement of Resistivity of Molten Material" (NPO-20369). This method also overcomes the primary disadvantage of an older noncontact method in which a nonrotating levitated sample is set into vibration, the frequencies of vibrational resonances are measured, and the surface tension is determined from the known relationship among the surface tension, frequencies, and other relevant quantities. The validity of the older method is limited to viscosities less than about 1 poise. The present method works over the full range of viscosities encountered in the thermal processing of metals, glasses, and metallic glasses.
Like the method of measuring resistivity described in the preceding article, the present method of measuring surface tension involves electrostatic levitation and noncontact heating of the sample in a vacuum chamber, plus the use of a rotating magnetic field to apply torque to the sample. In this case, the application of torque is metered and timed to introduce a predetermined amount of angular momentum. The magnetic field is then turned off and the sample allowed to settle into a steady state, in which it rotates as a rigid body. The shape and the frequency of rotation of the sample are measured in the steady state. Then by use of a computational model of a rotating liquid drop, the surface tension is computed for the measured shape and frequency. This method has been verified experimentally on electrostatically levitated molten drops of aluminum and tin.
This work was done by Won-Kyu Rhim and Takehiko Ishikawa of Caltech forNASA's Jet Propulsion Laboratory. NPO-20367