High-pressure viscosity of liquid Fe and FeS revisited by falling sphere viscometry using ultrafast X-ray imaging

The viscosity of liquid Fe and FeS has been extensively studied, yielding results differing by almost a factor of ten (2.4-23.7. mPa s for liquid Fe, 3.6-17.9. mPa s for liquid FeS, and 7.4-35.6. mPa s for the Fe-S eutectic composition), possibly due to the low resolution of slow cameras previously employed (typically 30-60. frames/s) in falling sphere measurements using X-ray radiography. Here we revisit the viscosity of liquid Fe and FeS up to 6.4 GPa using recently developed ultrafast X-ray imaging. In this study, we imaged the falling spheres at a rate of 500. frames/s, which is around 10 times faster than previous viscosity measurements. Our measurements showed that terminal velocity is achieved only in a limited region of falling distance, and that substantial oversampling, using sufficiently high-speed X-ray imaging, is essential to accurately determine the terminal velocity and the resulting viscosity. We obtained a viscosity of 6.1-7.4. mPa s for liquid Fe and 4.7-5.7. mPa s for liquid FeS at pressures to 6.4 GPa along their respective melting curves. The viscosity of liquid FeS is about 25-35% lower than that of liquid Fe between around 2 and 6.4 GPa along their respective melting curves. After correction for the effect of different melting temperatures between Fe and FeS on viscosity, we found that viscosity of liquid FeS is 31-42% lower than that of liquid Fe at 1800. °C and 1-6 GPa, suggesting that sulfur markedly decreases viscosity in liquid Fe.