In situ viscometry of high-pressure melts in the Paris-Edinburgh cell: application to liquid FeS

Knowledge of the viscosity of high-pressure melts is important in various domains of research, such as geosciences or materials science. Experimentally, the viscosity of liquids can be determined using the falling sphere technique. This method has been developed at high pressures and high temperatures at Beamline ID27 of the European Synchrotron Radiation Facility (ESRF), combining a Paris-Edinburgh cell with in situ X-ray radiography. The press is interfaced to Soller slits and imaging systems to measure high-quality diffraction patterns and high-resolution X-ray images of the sample. The viscosity of the liquid is derived from the velocity of a dense sphere falling through the pressurized melt and the Stokes law. An optimized two-circle diffractometer allows for moving the press upside down in order to perform a series of measurements on a single sample. The simultaneous collection of X-ray diffraction data on liquids offers the unique opportunity of investigating the relations between viscosity and the structure of melts. The potential of this new equipment is illustrated on the example of FeS melt viscosity and its implications for the Earth's core.