Dynamic light scattering measurements of high-pressure viscosity utilizing a diamond anvil cell

Abstract

Dynamic light scattering (DLS) is a widely used and powerful technique for studying the dynamics of fluids. We demonstrate here the first use of DLS in the diamond anvil cell. Using 91 nm polystyrene spheres suspended in methanol in the chamber of a diamond anvil cell, we show that the viscosity of the fluid can be accurately measured using this technique. Measurements up to 2.90 GPa are reported. Previous reports of DLS measurements have used large‐volume pressure cells that limited the data to moderate pressures, but with the diamond anvil cell no such limitations exist. However, a significant difficulty in DLS measurements with the diamond cell comes from the small size of the pressure chamber. Not only does this lead to a weak scattered intensity, but scattering from the sample is combined with partly coherent scattering from the diamond/air and diamond/sample interfaces, thus complicating data interpretation. We show how this heterodyne component can bias the measured diffusion coefficients and then demonstrate a straightforward procedure by which this heterodyne component can be minimized. The resulting diffusion coefficients (ranging from 10⁻⁸ to 10⁻¹⁰ cm² s⁻¹), recalculated as viscosity, are found to have an average precision of 2.5% and are in excellent agreement with literature values.