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Droplet emission from an electrified meniscus in the microdripping regime has been investigated experimentally and numerically. In this regime, the meniscus of a low viscosity, high electrical conductivity liquid hanging from the tip of a metallic tube, oscillates when subjected to a high electric field. When the liquid is fed at a constant flow rate, and for a certain range of voltages, the oscillations are periodic and, in each oscillation, the meniscus develops a ligament that eventually detaches forming a single drop. Thus, this technique is a simple way to produce monodisperse droplets at relatively high frequencies, whose diameters may be one-tenth of that of the tube. The process, governed by a dimensionless flow rate , the computed meniscus mean volume and oscillation frequency agree with the experimental results, although the volume and number of emitted droplets do not. An ultrafine electrospray emitted from the ligament's conical tip during part of the oscillation cycle is probably the cause of the disagreements. To tackle this, we combine the one-dimensional approximation of an electrified meniscus with experimental meniscus profiles to compute the electric field on the meniscus. Around the tip, this electric field turns out to be substantially smaller than the field computed by the model in the absence of the space charge, and agrees with the field induced by an electrospray emitted from a Taylor cone in air.

We apply an electric field to a moderately conducting liquid surrounded by another co-flowing liquid, all inside a glass-based microfluidic device, and study and classify the resulting emission modes. The viscosity and flow rate of the co-flowing liquid affect the number of modes observed in classical electrospray as well as their geometrical characteristics. In particular, we observe a two-dimensional whipping mode not described before. We also provide a qualitative description of some of the modes, including dripping, electrodripping, microdripping, the cone-jet mode, and both the two- and three-dimensional whipping modes.

A numerical description is given for the pulsating emission of droplets from an electrified meniscus of an inviscid liquid of infinite electrical conductivity which is injected at a constant flow rate into a region of uniform, continuous or time periodic, electric field. Under a continuous field, the meniscus attains a periodic regime in which bursts of tiny droplets are emitted from its tip. At low electric fields this regime consists of sequences of emission bursts interspersed with sequences of meniscus oscillations without droplet emission, while at higher fields the bursts occur periodically. These results are in qualitative agreement with experimental results in the literature. Under a time periodic electric field with square waveform, the electric stress that acts on the surface of the liquid while the field is on may generate a tip that emits tiny droplets or may accelerate part of the meniscus and lead to a second emission mode in which a few large droplets are emitted after the electric field is turned off. Conditions under which each emission mode or a combination of the two are realized are discussed for low frequency oscillatory fields. A simplified model is proposed for high electric field frequencies, of the order of the capillary frequency of the meniscus. This model allows computing the average emission rate as a function of the amplitude, duration and bias of the electric field square wave, and shows that droplet emission fails to follow the applied field above a certain frequency.