Why gas-focused microjets are so fast: kinetically resolved, shear-driven flow focusing in vacuum
arXiv:2607.11802
The paper explains why gas‑focused liquid microjets used in serial femtosecond crystallography achieve speeds far above traditional pressure‑driven limits, showing that the jet is driven by shear stress from a hypersonic, rarefied gas whose expansion is resolved with a kinetic Shakhov‑BGK solver.
Abstract
Gas-focused liquid microjets -- the flow-focusing sample delivery on which serial femtosecond crystallography depends -- reach speeds several times the pressure-driven (Bernoulli) bound, unexplained by continuum, local-equilibrium models that do not resolve the rarefied, hypersonic expansion of the focusing gas. We resolve that expansion with a deterministic kinetic (Shakhov--BGK) solver and couple it to the slender liquid jet. The jet is \emph{shear-driven}, not pressure-driven: the tangential stress of the hypersonic gas supplies nearly all of the axial momentum, accounting for the anomalous speed. The gas does not become ballistic behind the near field -- its stress decays as a power law and it stays coupled -- and its constitutive regime is set by a single rarefaction parameter , the orifice diameter over the source mean free path, through the thermodynamic Deborah number (Knudsen times Mach), whose surface maps where the Newtonian-gas closure fails: the small- vacuum corner where crystallography jets operate. The kinetically computed surface stress is the input for the fully non-Newtonian (viscoelastic-liquid) sequel.
6 pages, 5 figures (18 plots)