  The cavitation threshold of polydimethylsiloxane (silicone) oils was studied
using the planar impact of flyer plates to generate large transient negative
pressures within the liquids. The plate-impact experiments used a 64-mm-bore
gas-gun to launch thin sabot-supported flyer plates onto liquid capsule targets
in which a thin Mylar diaphragm formed a free surface at the back of the
sample. The shock wave driven into the target capsule by the flyer impact
placed the silicone oil in tension upon reflection from the rear free surface,
eventually causing the sample to cavitate. The spall strength, or critical
tension which cavitates the liquid, was determined by monitoring the
free-surface velocity using a photonic Doppler velocimetry system. This study
explored the effect of viscosity and loading strain rate on a system of three
silicone oils having vastly different viscosities (4.8$\times$10$^{-2}$ Pa s to
2.9$~\times$10$^{1}$ Pa s), but otherwise similar properties. The spall
strength was found to remain constant over the ranges of strain rate and
viscosities probed in this work. A comparison of the experimental results to
models for the cavitation threshold of liquids suggested that homogeneous
nucleation of bubbles was the dominant mechanism for tension relief at the
onset of cavitation.
