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In this paper we present a numerical study investigating the effects of nozzle geometry on stability, shape and flow characteristics of micron-sized liquid jets, produced by injection molded gas dynamic virtual nozzles (GDVNs) operating in vacuum. The jet characteristics are described as a function of (i) capillary-to-orifice distance, (ii) nozzle outlet orifice diameter, and (iii) liquid feeding capillary angle. An experimentally verified numerical model of GDVN with laminar two-phase Newtonian compressible flow, based on finite volume method and volume of fluid interface tracking, is used to assess the changes. The study is performed for two sets of liquid flow rates while keeping the gas flow rate constant. It is observed that for each value of capillary-to-orifice distance and nozzle outlet diameter there is a minimum liquid flow rate below which the jet is unstable. We find that the nozzle outlet diameter has the biggest influence on the jet diameter, length and velocity, while liquid capillary angle has no observable effect on jet characteristic. Varying capillary-to-orifice distance does not affect the flow field around micro-jet. It is found that the liquid and the gas interaction near the meniscus primarily affect the jet stability and shape. [ABSTRACT FROM AUTHOR]

• Experimental characterization of droplet velocity emerging from gas-focused micro-jet breakup. • Validation of the numerical model of micro-jet in terms of its velocity and shape. • Improved capability in virtual design of micro-nozzles for sample delivery in femtosecond crystallography. Compressible multiphase numerical simulations of gas-focused micro-jets are compared with the experimental data obtained with the dual pulse imaging laser-induced fluorescence drop velocimetry. Such jets, originating from a 3D printed gas dynamic virtual nozzle into a low-vacuum (150 Pa) environment, are increasingly being used for sample delivery in serial femtosecond crystallography. The distance traveled by a detaching drop from the jet is measured between the two consecutive illumination pulses with a known time delay at the positions 200 µm and 450 µm from the nozzle. Additionally, the high-speed camera images are used to analyze the shape of the jet. An axisymmetric, compressible, Newtonian two-phase helium-water mixture model is numerically solved within the framework of the volume of fluid and the finite volume method. The experimental and the computational studies are performed with a constant volumetric liquid flow rate of 14 μl/min and the gas mass flow rate in the range from 4.6 mg/min to 20 mg/min. The related jet Reynolds number ranges from 120 to 220 and Weber number from 30 to 150. The maximum difference between the measurements and the results of the numerical model in terms of the droplet velocity and jet diameter is within 10 %. The study provides new information on the jet velocities for micron-sized gas-focused nozzles. The validated numerical model can be used as a design tool for the nozzles dedicated to the specific needs of the femtosecond crystallography experiments. [ABSTRACT FROM AUTHOR]