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

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 Delta 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: (i) the steady jetting regime, where the liquid meniscus is stable and the jet is convectively unstable; (ii) the local instability regime, where the liquid meniscus is stable and the jet is absolutely unstable; and (iii) 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 Delta 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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3450321]