A surface tension measurement method based on the maximum bubble pressure (MBP) method was developed in order to precisely determine the surface tension of molten silicates in this study. Specifically, the influence of viscosity on surface tension measurements was quantified, and the criteria for accurate measurement were investigated. It was found that the MBP apparently increased with an increase in viscosity. This was because extra pressure was required for the flowing liquid inside the capillary due to viscous resistance. It was also expected that the extra pressure would decrease by decreasing the fluid velocity. For silicone oil with a viscosity of 1000mPa·s, the error on the MBP could be decreased to +1.7 % by increasing the bubble detachment time to 300s. However, the error was still over 1 % even when the bubble detachment time was increased to 600s. Therefore, a true value of the MBP was determined by using a curve-fitting technique with a simple relaxation function, and that was succeeded for silicone oil at 1000mPa·s of viscosity. Furthermore, for silicone oil with a viscosity as high as 10000mPa·s, the apparent MBP approached a true value by interrupting the gas introduction during the pressure rising period and by re-introducing the gas at a slow flow rate. Based on the fundamental investigation at room temperature, the surface tension of the SiO 2–40 mol % Na 2O and SiO 2–50 mol % Na 2O melts was determined at a high temperature. The obtained value was slightly lower than the literature values, which might be due to the influence of viscosity on surface tension measurements being removed in this study.
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