Measuring viscosity at extreme pressures*

Viscosity plays a role in the mixing and transport of fluids at high pressures and temperatures, which can be found in applications ranging from planetary interiors to inertial confinement fusion. Experimental measurements of viscosity of materials at high energy-density (HED) conditions---that is, pressures greater than 100 GPa---are limited in parameter space, and theoretical calculations of viscosity are inconsistent in the warm dense matter regime. Here we present estimates of the dynamic viscosity of epoxy, a proxy for a hydrocarbon, using the acceleration of embedded bluff bodies (Al'tshuler et al., 1986). The epoxy was shock-compressed with laser ablation to a pressure of 240 GPa at the OMEGA laser facility. The sample was embedded with stainless steel microspheres, and the displacement of the spheres was imaged with x-ray radiography. The trajectories of the particles was corroborated with an unsteady forcing model for the shock-particle interaction to estimate the epoxy's dynamic viscosity from the history force.