The shock was not the whole story.
Helium at one-quarter of the nitrogen pressure still produced a stronger h-BN exfoliation signal. Following the particles through the nozzle explains why gas acceleration matters more than the spectacle of a shock.
Advanced Engineering Materials · 2026 · Md Farhadul Islam, Saeed Arabha, Ray Hixon, Reza Rizvi
The force hidden inside the flow
Compressible-flow exfoliation sends layered powders through a nozzle at high speed. Predicting the mechanism matters because stronger shock conditions do not automatically mean that the material layers receive enough force to separate.
Which part of the gas flow initiates layer separation, and can that force be related to the experimentally observed exfoliation yield?
Putting the nozzle and the shock to the test
The authors paired converging–diverging nozzle and shock-tube experiments on hexagonal boron nitride with quasi-one-dimensional particle-laden-flow analysis. They compared measured optical absorbance across gas and pressure conditions with calculated gas velocity, shock position, and thrust on the sheets.
The experiments used h-BN powder in open and closed shock tubes and a commercial converging–diverging nozzle. The collected material was dispersed and centrifuged before UV–visible measurement. Absorbance is the experimental indicator used here; it is not a direct mass-yield percentage. The nozzle comparisons report three independent trials with error bars of one standard deviation. A quasi-one-dimensional gas model, followed by a particle-motion calculation, connects that signal to velocity, density, and force.
Key findings
Initiation mechanism
Open-versus-closed shock tubes and the modeled force history identify gas acceleration as the key to starting layer separation.
Gas dynamics
Helium and nitrogen develop different velocity, density, and particle-force histories; a cutoff near 30 μN links modeled work with absorbance ratios.
Product comparison
The lower-pressure helium dispersion has the stronger 300 nm absorbance signal in the tested nozzle comparison.
Design around the force the layers receive
The combined flow analysis and experiments identify gas-driven thrust—not the shock by itself—as the practical trigger to tune when designing this exfoliation process.
The practical outcome is a mechanism that connects an operating choice to the product signal: accelerate the gas, generate thrust on the particles, and compare the resulting dispersion. The lower-pressure helium result is especially useful because it shows why pressure alone cannot rank candidate conditions. The quasi-1D model and optical signal support that interpretation within the tested setup; they do not establish a universal force threshold for every material or nozzle.