Global and local instability of flow focusing: The influence of the geometry

Abstract

In the flow focusing technique, a liquid flow rate Q is injected through a microcapillary to form a meniscus attached to its edge. The meniscus is stretched until a thin jet tapers from its tip due to the action of a gas stream driven by a pressure drop ⌬p. Both the liquid jet and the gas stream cross the orifice of a plate located in front of the capillary at a distance H. In the present work, the stability of both the tapering liquid meniscus and the emitted jet is analyzed experimentally. Three regimes are identified: ͑i͒ the steady jetting regime, where the liquid meniscus is stable and the jet is convectively unstable; ͑ii͒ the local instability regime, where the liquid meniscus is stable and the jet is absolutely unstable; and ͑iii͒ the global instability regime, where the liquid meniscus is unstable. The mechanisms responsible for the transitions between those regimes are described. The experiments show the existence of a minimum value Q min of the flow rate Q below which flow focusing is globally unstable independent of the pressure drop ⌬p applied to the gas stream. The dependence of the stability threshold Q min with respect to the capillary-to-orifice distance H is analyzed considering different liquids. If the rest of the geometrical parameters are fixed, there is an optimum value H opt of the capillary-to-orifice distance H for which the stability threshold Q min is minimum. We also determine the dependence of H opt and the corresponding minimum flow rate Q opt with respect to the capillary diameter. In addition, we find that Q min diverges as the capillary-to-orifice distance H decreases and approaches a certain critical value, at which the transition from flow focusing to "flow blurring" takes place. We confirm our interpretation of the experimental results by conducting numerical simulations for the aforementioned three regimes.

Ceramic micro-injection molded nozzles for serial femtosecond crystallography sample delivery

“…When using GDVNs to deliver precious protein samples in SFX experiments, one of the most important parameters is the minimum liquid flow rate, Q min , as it determines the rate of sample consumption and the smallest achievable jet diameter (further discussed in the next paragraph). The study of Vega et al 23 found empirically that the optimum minimum flow rate to produce a jet for low viscosity fluids is given by the relationship…”

“…In previous flow-focusing studies, the energy supplied by the gas flow to the system was described in terms of the gas pressure. 19,23,28 However, we will base our analysis on mass flow because this quantity is conserved throughout the nozzle, regardless if the flow is choked at the exit gas aperture.…”

“…17,18 Early experimental and theoretical investigations of flowfocusing were carried out by the group of Gañán-Calvo. [19][20][21][22][23] In many of their studies, flow-focusing was achieved by drawing air through a small orifice in a flat metal plate (flat-plate apparatus), and forming a jet from a capillary precisely positioned above the orifice. 24 The original apparatus was later miniaturized, replacing the plate with a surrounding outer borosilicate capillary that was flame polished at one end to produce a smooth convergent aperture.…”

“…21 This can result in nonoptimal gas flow streams, causing decentering of the liquid supply capillary relative to the gas aperture, and a different optimum distance of the capillary orifice to the nozzle exit-all of which have been shown to be important in the performance of flow-focusing nozzles. 20,21,23 To improve nozzle reproducibility while reducing fabrication complexity, we investigated ceramic micro-injection molding to mass-produce high precision outer capillaries for GDVNs. Micro-injection molding can fabricate items of varying length scales, including features with dimensions of a few micrometers on parts which are centimeters in length.…”

Parallelization of microfluidic flow-focusing devices

“…In this regime, the dynamics of drop breakup is entirely controlled by the flow rate of the 25 outer phase, 8 such that the drop size depends on the flow rate of the outer phase, 6, 9 its viscosity and surface tension, 10 and the channel geometry. 7,11,12 However, the necking process, which eventually leads to drop formation, is slow, limiting the maximum drop 30 generation frequency and hence the throughput. The drop generation frequency can be increased if an additional thin orifice is added after the crossjunction.…”