Gaseous Flow Focusing II

Serial femtosecond crystallography (SFX) is the most remarkable application of gaseous flow focusing. In this application, the gaseous stream driving the jet is normally discharged into a vacuum chamber, which makes the gas current reach the sound velocity in the nozzle orifice. We devote the first part of this chapter to analyzing this transonic version of flow focusing. Gaseous flow focusing can be employed to produce fibers and films when the liquid phase is a viscoelastic fluid. The second part of this chapter describes these interesting applications of flow focusing. The large strain rates produced in the tip of the tapering meniscus of flow focusing can trigger the coil-stretch transition of the polymers dissolved in the liquid, which leads to the building of large axial viscoelastic stresses. This can fundamentally change the behavior of weakly viscoelastic flow focusing, increasing the stability of both the tapering meniscus and the emitted jet. This chapter closes by analyzing this novel and interesting phenomenon. Attention is paid to this viscoelastic transition and the superstability of the jets emitted when that transition occurs.