Measuring the dynamic viscosity of HED fluids

The mixing and transport of fluids at high pressures and temperatures can be found in applications ranging from planetary interiors to inertial confinement fusion. However, despite the role that viscosity has on the evolution of hydrodynamic instabilities and energy transfer in turbulence, experimental measurements of viscosity of materials at high energy-density (HED) conditions are limited in parameter space. Calculating viscosity from theory is also challenging, particularly in the warm dense matter regime. In this talk, we present preliminary results from experimental and modeling campaigns at OMEGA and EP that use two methods to infer the dynamic viscosity of dynamically-compressed fluids. In the first technique, we use x-ray radiography to measure the displacement of accelerating microspheres in solid hydrocarbon epoxy (CH). The microsphere trajectory is corroborated with an unsteady forcing model to obtain the epoxy's dynamic viscosity from viscous force contributions. The viscosity is estimated to be less than 10 Pa-s. In the second technique, we use VISAR to measure the velocity of a shock front in a fused silica sample machined with a sinusoidal perturbation. Viscosity is expected to modify the evolution of the decaying shock front at different perturbation wavelengths. Refinements and limitations to the application of these viscometry platforms to HED fluids will also be discussed.