We present a robust and computationally efficient numerical scheme for simulating steady electrohydrodynamic atomization processes (electrospray). The main simplification assumed in this scheme is that all the free electrical charges are distributed over the interface. A comparison of the results with those calculated with a volume-of-fluid method showed that the numerical scheme presented here accurately describes the flow pattern within the entire liquid domain. Experiments were performed to partially validate the numerical predictions. The simulations reproduced accurately the experimental shape of the liquid cone jet, providing correct values of the emitted electric current even for configurations very close to the cone-jet stability limit.

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 $Q_{\text{min}}$ 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 $Q_{\text{min}}$ 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 $H_{\text{opt}}$ of the capillary-to-orifice distance $H$ for which the stability threshold $Q_{\text{min}}$ is minimum. We also determine the dependence of $H_{\text{opt}}$ and the corresponding minimum flow rate $Q_{\text{opt}}$ with respect to the capillary diameter. In addition, we find that $Q_{\text{min}}$ 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.

We examine both theoretically and experimentally the breakup of a pendant drop loaded with an insoluble surfactant. The experiments show that a significant amount of surfactant is trapped in the resulting satellite droplet. This result contradicts previous theoretical predictions, where the effects of surface tension variation were limited to solutocapillarity and Marangoni stresses. We solve numerically the hydrodynamic equations, including not only those effects but also those of surface shear and dilatational viscosities. We show that surface viscosities play a critical role to explain the accumulation of surfactant in the satellite droplet.

The characterization of the extensional rheology of polymeric solutions is important in several applications and industrial processes. Filament stretching and capillary breakup rheometers have been developed to characterize the extensional properties of polymeric solutions, mostly for high-viscosity fluids. However, for low concentration polymer solutions, the measurements are difficult using available devices, in terms of the minimum viscosity and relaxation times that can be measured accurately. In addition, when the slow retraction method is used, solvent evaporation can affect the measurements for volatile solvents. In this work, a new setup was tested for filament breakup experiments using the slow retraction method, high-speed imaging techniques, and an immiscible oil bath to reduce solvent evaporation and facilitate particle tracking in the thinning filament. Extensional relaxation times above around 100 μs were measured with the device for dilute and semi-dilute polymer solutions. Particle tracking velocimetry was also used to measure the velocity in the filament and the corresponding elongation rate, and to compare with the values obtained from the measured exponential decay of the filament diameter.