Quirks of bullet-fast microjets better understood through experiments

When a shockwave hits a material, it can launch jets of material from irregularities on the surface 10 times faster than a speeding bullet. While generally an increase in the shock strength leads to an increase in jet mass, in 2020, simulations counterintuitively predicted that this trend breaks down near the transition from continuously solid jest to release of jets of molten material.

Bober et al. tested these predictions for the first time using a thin tin plate with a groove cut on top. The researchers struck the plate with a speeding projectile, causing the groove to invert and launch a tin jet.

The jets were studied with an array of high-speed X-ray cameras to analyze their velocity and mass. Using a range of impact pressures, the researchers investigated how the jets changed when the pressure was high enough to cause the tin to melt.

The results showed stronger shocks eject more massive jets. However, near the pressure boundary between liquid and solid ejecta, this correlation was less pronounced and depended more on the groove size and shape. The findings, which confirmed the simulations, suggest crossing the melting threshold can either increase or reduce the jets based on the surface characteristics.

“Now that we have experimentally confirmed the microjet behavior predicted by our computations, I am excited to dive deeper and explore its origins,” said co-author David Bober.

The work will help researchers better understand microjets, which are important for studies of planetary formation, asteroid impacts, explosives, and condensed matter shock physics. Already, the researchers are working on follow up experiments to study the local phase of the jets and material parameters that might affect the jets.

Source: “Understanding the evolution of liquid and solid microjets from grooved Sn and Cu samples using radiography,” by David B. Bober, Kyle K. Mackay, Minta C. Akin, and Fady M. Najjar, Journal of Applied Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0056245 .