A computational investigation of microjet focusing with secondary fluid and compressible gas

Double-flow focusing nozzles (DFFNs) are used to produce liquid microjets for sample delivery in serial femtosecond crystallography (SFX), where crystallized samples are dispersed in a primary liquid jet and delivered into the x-ray beam path. Such jets are generated by focusing a primary liquid jet with a coaxial secondary liquid and outer gas. Precise control of microjet stability and morphology is crucial for efficient sample delivery. This study presents a comprehensive numerical model of double-flow focused liquid jets from a DFFN, where the liquid phase consists of water and ethanol, and the focusing gas is helium. An axisymmetric, Newtonian, compressible, two-phase gas–liquid and two-component water–ethanol model is formulated and solved using the finite-volume method and volume-of-fluid framework. The water–ethanol mixing and resulting changes in density, viscosity, and surface tension as a function of ethanol concentration and temperature are simulated using the Jouyban–Acree model. The jet characteristics as a function of material mixture properties, focusing-gas compressibility, and temperature are investigated numerically. A parametric analysis is performed over the typical SFX operating range, varying the water and ethanol flow rates from 0 to 20 μl/min and the helium gas flow rate from 0 to 20 mg/min. Jet stability, diameter, length, and velocity are analyzed. The influence of local mixture properties, mapped onto the Ca–We plane, reveals transitions between viscous-dominated and inertia-driven jetting regimes. The predicted jet diameters are consistent with the Young–Laplace equation and scaling expression. The findings provide a physical insight required for the virtual design of DFFNs.