Periodic emission of droplets from an oscillating electrified meniscus of a low-viscosity, highly conductive liquid

Abstract

The generation of identical droplets of controllable size in the micrometer range is a problem of much interest owing to the numerous technological applications of such droplets. This work reports an investigation of the regime of periodic emission of droplets from an electrified oscillating meniscus of a liquid of low viscosity and high electrical conductivity attached to the end of a capillary tube, which may be used to produce droplets more than ten times smaller than the diameter of the tube. To attain this periodic microdripping regime, termed axial spray mode II by Juraschek and Röllgen [R. Juraschek and F. W. Röllgen, Int. J. Mass Spectrom. 177, 1 (1998)], liquid is continuously supplied through the tube at a given constant flow rate, while a dc voltage is applied between the tube and a nearby counter electrode. The resulting electric field induces a stress at the surface of the liquid that stretches the meniscus until, in certain ranges of voltage and flow rate, it develops a ligament that eventually detaches, forming a single droplet, in a process that repeats itself periodically. While it is being stretched, the ligament develops a conical tip that emits ultrafine droplets, but the total mass emitted is practically contained in the main droplet. In the parametrical domain studied, we find that the process depends on two main dimensionless parameters, the flow rate nondimensionalized with the diameter of the tube and the capillary time, q, and the electric Bond number B E , which is a nondimensional measure of the square of the applied voltage. The meniscus oscillation frequency made nondimensional with the capillary time, f , is of order unity for very small flow rates and tends to decrease as the inverse of the square root of q for larger values of this parameter. The product of the meniscus mean volume times the oscillation frequency is nearly constant. The characteristic length and width of the liquid ligament immediately before its detachment approximately scale as powers of the flow rate and depend only weakly on the applied voltage. The diameter of the main droplets nondimensionalized with the diameter of the tube satisfies d d ≈ (6/π ) 1/3 (q/f ) 1/3 , from mass conservation, while the electric charge of these droplets is about 1/4 of the Rayleigh charge. At the minimum flow rate compatible with the periodic regimen, the dimensionless diameter of the droplets is smaller than one-tenth, which presents a way to use electrohydrodynamic atomization to generate droplets of highly conducting liquids in the micron-size range, in marked contrast with the cone-jet electrospray whose typical droplet size is in the nanometric regime for these liquids. In contrast with other microdripping regimes where the mass is emitted upon the periodic formation of a narrow capillary jet, the present regime gives one single droplet per oscillation, except for the almost massless fine aerosol emitted in the form of an electrospray.

Capillary‐Based Microfluidics—Coflow, Flow‐Focusing, Electro‐Coflow, Drops, Jets, and Instabilities

“…When B E ≈ 1, the surface charge density approaches the Rayleigh limit, and the meniscus becomes electrically unstable, leading to the formation of conical tips, similar to those observed in microdripping in air. The emission is aperiodic at low q i and periodic at higher q i , similarly to what is observed in air …”

“…The emission is aperiodic at low q i and periodic at higher q i , similarly to what is observed in air. [127,133,134]…”

Bubble formation regimes in forced co-axial air-water jets

“…The images and the pressure signal were synchronized through a data logger system. Each registered image of the forming bubble was digitally analyzed using an in-house twostep interface detection routine, similar to that described by Hijano et al (2015). Firstly, a rough detection of the edges of the needle tip and of the forming bubble is performed at the pixel level.…”

Non-dimensional groups for electrospray modes of highly conductive and viscous nanoparticle suspensions

“…These suspensions can have high viscosity and high density due to high particle concentration 21,22 . Hijano et al 23 found that in an electrospray of low viscosity fluid, the process depends on two non-dimensional parameters: normalized flow rate and electric Bond number which is a measure of the square of the voltage. The current paper deals with the inception of a viscous droplet in the dripping regime as well as the microdripping regime.…”