The experimental observations of high-velocity particle-laden flow interactions have been rare, given the challenge of creating high-velocity flows with numerous particles. These observations serve a vital role in comprehending various natural events, ranging from planetary formation to cloud interactions.
microjetThat is, until today. In studies performed at the Omega Laser Facility at the University of Rochester’s Laboratory for Laser Energetics (LLE), researchers from Lawrence Livermore National Laboratory (LLNL) have displayed sequences for the first time X-ray radiography pictures of two interacting tin ejecta microjet.
The study has been published by Physical Review Letters and chosen as an Editor’s Suggestion with LLNL physicist Alison Saunders acting as the primary author.
“These interactions had never been seen previously, and so we didn’t really know what to anticipate,” Saunders said. “Surprised to observe the lower-density jets passing past each other fully unaffected by the lower shock pressure This may be seen as a stream of dispersed particles colliding with one other.”
Saunders said it also was a surprise to see the greater-density jets from the higher shock pressure interact significantly.
“We dubbed it the ‘water-hose experiment’ because it appeared like we were spraying two water hoses at each other and seeing them splash as they struck each other,” she added.
Colliding tin ejecta microjets
The researchers captured the first radiography photos of clashing tin ejecta microjets at two different shock pressures. Ejecta microjets are micron-scale jets of microscopic particles that fly at incredible velocities (velocities over several kilometers per second, or many thousands of miles per hour) (velocities above several kilometers per second, or several thousands of miles per hour). The researchers observed two regimes of interaction behavior as a function of shock pressure. At a shock pressure of 11.7 gigapascals, the jets move at 2.2 km/s and pass past each other unattenuated, however at a pressure of 116.0 gigapascals, the now higher-density jets travel at velocities of 6.5 km/s and contact violently, generating a corona of material surrounding the interaction zone.
“We also use a simplified collisional model in a radiation hydrodynamics code to model the interactions and find that the model is incapable of reproducing the exact interaction behavior we observe, suggesting that more experiments are needed to understand the physics driving ejecta microjet interaction behavior,” Saunders said.
The researchers employed OMEGA Extended Performance (EP) with its short-pulse capabilities to photograph the jet interactions. Two long-pulse lasers drive shocks into two tin samples imprinted with triangle grooves on their free sides. As the wonders break out from the free surfaces, the groove features invert to generate planar microjets of material traveling toward each other.
Later, the EP short-pulse beam impacted on a microwire provides a brilliant burst of X-rays that enables the scientists to obtain an X-ray radiograph of the jets as they meet. The X-ray radiograph also offers quantitative information on the jets pre- and post-collision, such as jet densities and particle packing inside the jets.
“The study gives the first photos of ejecta microjet interactions and with that, raises a lot of intriguing concerns about the physics controlling the collisional behavior,” Saunders said, adding that tin is a material that is known to melt above the shock pressures studied in this experiment. “We have reason to suspect that the lower-pressure jets may include more solid material than the jets from the high-pressure shock drives.”
Saunders said this raises the issue of whether the variation in contact behavior found between the two examples is a consequence of the change in material phase or other jet properties, such as density, velocity, or particle-size distributions. The collisions occur with microscopic particles moving at great velocities and entail very high strain-rate mechanics.
The team hopes to clarify some of the physics ambiguities and understand what is driving the discrepancies seen in contact dynamics: density, material phase, particle-size distributions, the elasticity of collisions, or a combination of all of these. As a part of that, the team intends to broaden the diagnostic capabilities to include new tests that could assess some of these features directly.