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Surface Tension and Viscosity Measurements

It is difficult to measure the surface tension and viscosity of molten metals or alloys by traditional measurement techniques due to the high temperature. Another difficulty is that the sample in liquid phase tends to be contaminated easily. Electrostatic levitation (ESL), has been used by several groups [2, 3] to measure the thermophysical properties of molten alloys, which eliminates contamination by a container. Since the sample is positioned and heated by two independent systems, and the electrostatic system does not generate any heat, a wide temperature range can be achieved. The levitated sample can be undercooled or superheated and sustain the state long enough to complete the measurement.

Measurements of surface tension and viscosity share the same oscillating drop method of containerless processing. In this work, the oscillating droplet technique was adopted to measure the surface tension and viscosity of liquid zirconium. The levitated sample can be melted and oscillated in a force-free environment as a high-quality mechanical oscillator. The surface tension is related to the resonant frequency of the oscillation. The resonant frequency of the liquid metal sphere is related to the surface tension of the liquid metal sphere, which can be evaluated by Rayleigh’s equation [4]:

where f is the cycle frequency of oscillation; у is the surface tension of the metal droplet; l is the oscillation mode number which is 2 in this case. M is the droplet mass, which was scaled before and after processing and show small mass loss (Table 1). Thus, the sample evaporation during the process was neglected.

In this method, the levitated liquid sample is excited to oscillate with an additional alternating electrostatic field near the natural frequency of the sample. The oscillating sample finds its natural frequency as it dampens freely to its equilibrium state. The surface tension is determined by the oscillation frequency of the sample.

Table 1 Sample mass loss in ESL

Composition

Samples

Pre-test

(mg)

Post-test

(mg)

Mass

loss

(%)

57 ppm

#062

44.514

44.459

0.124

#069

44.511

44.492

0.043

#067

43.288

43.273

0.035

1560 ppm

#073

43.266

43.256

0.023

#071

48.769

48.753

0.033

#044

47.263

47.226

0.078

2.42%

#048

44.427

44.398

0.065

#050

46.964

46.930

0.072

#058

46.101

46.069

0.069

Average

0.060

1 Standard deviation

0.029

The measurement of surface tension can be affected by the Marangoni flow on the surface of the liquid sample. For example, in ESL experiments, when the period of recirculation of the flow loops is comparable to that of oscillation, the sample can resonate and self-oscillate. This phenomenon increases errors for surface tension measurements.

 
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