In situ measurement of interfacial tension of Fe–S and Fe–P liquids under high pressure using X-ray radiography and tomography techniques

Interfacial tension is one of the most important properties of the liquid iron alloy that controls the core formation process in the early history of the Earth and planets. In this study, we made high-pressure X-ray radiography and micro-tomography measurements to determine the interfacial tension between liquid iron alloys and silicate melt using the sessile drop method. The measured interfacial tension of liquid Fe–S decreased significantly (802–112 mN/m) with increasing sulphur content (0–40 at%) at 1.5 GPa. In contrast, the phosphorus content of Fe had an almost negligible effect on the interfacial tension of liquid iron. These tendencies in the effects of light elements are consistent with those measured at ambient pressure. Our results suggest that the effect of sulphur content on the interfacial tension of liquid Fe–S (690 mN/m reduction with the addition of 40 at% S) is large compared with the effect of temperature (∼273 mN/m reduction with an increase of 200 K). The three-dimensional structure of liquid Fe–S was obtained at ∼2 GPa and 1373–1873 K with a high-pressure tomography technique. The Fe–S droplet was quite homogeneous when evaluated in a slice of the three-dimensional image.