Shock trains in liquid jets and their effect on protein crystals and protein molecular structure*

 Shock wave trains in liquid jets are a counterpart of the long-known Mach patterns in supersonic gas jets, but were only discovered recently during X-ray free-electron laser (XFEL) ablation experiments. These shock trains have a surprisingly rich physics. As in the gas case, they form through oblique shock reflections at the jet boundary, but also exhibit unique cavitation phenomena, and realize a system where a maximal intensity sound is generated, with sound pressure levels above 270 dB in water. The formation of shock trains is a self-stabilizing phenomenon, and can also be realized using nanosecond optical lasers.

These shocks can affect megahertz serial experiments at the XFELs, by damaging or changing the samples before they are probed. In an XFEL pump-probe experiment, we used the pump X-ray pulse to launch shoch trains, and the probe to record X-ray diffraction from previously shocked protein crystals. For both lysozyme and hemoglobin protein crystals, the diffraction quality degraded, indicating damage to the crystalline lattice. For lysozyme, a 40 MPa shock pressure threshold for crystal damage could be estimated. While the lysozyme proteins reverted to their initial structure after the shocks, the hemoglobin proteins had a remanent change. These initial studies show how the shock trains liquid jets can become a versatile tool to study cavitation in liquids and shock damage in soft matter.