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 Δp. 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 Qmin of the flow rate Q below which flow focusing is globally unstable independent of the pressure drop Δp applied to the gas stream. The dependence of the stability threshold Qmin 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 Hopt of the capillary-to-orifice distance H for which the stability threshold Qmin is minimum. We also determine the dependence of Hopt and the corresponding minimum flow rate Qopt with respect to the capillary diameter. In addition, we find that Qmin 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.