Why gas-focused microjets are so fast: kinetically resolved, shear-driven flow focusing in vacuum

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 $\delta=D/\ell_0$, the orifice diameter over the source mean free path, through the thermodynamic Deborah number $De_\theta\simeq K\!n\,M$ (Knudsen times Mach), whose $De_\theta=1$ surface maps where the Newtonian-gas closure fails: the small-$\delta$ vacuum corner where crystallography jets operate. The kinetically computed surface stress is the input for the fully non-Newtonian (viscoelastic-liquid) sequel.