Viscosity Measurements in Shock-compressed Epoxy*

Viscosity determines momentum transport in a system and plays a crucial role in mixing and growth of hydrodynamic instabilities. Viscosity measurements in High Energy Density (HED) states are particularly important to accurately develop hydrodynamic models and to bridge the gap between simulations and experimental results of complex systems such as Inertial Confinement Fusion (ICF). Inclusion of viscous dissipation in the modelling of ICF implosions has led to a better understanding of hot spot turbulence, demonstrating the pressing need for developing empirical viscosity diagnostics.

We measured viscosity in dynamically compressed Stycast 1266 (CH, 1.1 g/cc) by tracing the acceleration of particles embedded in the target. The OMEGA-60 laser facility was used to drive a shock (peak⁓248 GPa) through the CH target, which was embedded with stainless steel (7.8 g/cc) and titanium (4.56 g/cc) microspheres that were accelerated by the flow behind the shock. The particle positions were recorded with time-resolved X-ray radiography. The velocities of the particles and CH were used to determine the viscous and inviscid force contributions acting on the particles using a shock-particle forcing model. From the forces, we calculated the dynamic viscosity of CH to be less than 10 Pa-sec. A Bayesian inference analysis on the force model was performed to determine the probability distribution of viscosity for a given range of input variables.