The combination of electrospray and flow focusing

Abstract

An ultra-fine liquid atomization procedure combining the advantages of electrospray and flow focusing is presented. Both techniques are known to produce strikingly small and steady liquid micro-jets issuing from menisci held by capillary forces. Such menisci take the form of a cusp-like drop attached to the feeding tube (flow focusing: FF) or a Taylor cone (electrospray: ES). The issuing micro-jets are forced or ‘sucked’ from the parent meniscus either by pressure or electrohydrodynamic forces. Subsequent capillary breakup of the jet leads to fine sprays of remarkable quality. Here we describe the joint effect of pressurization and electrification in a flow focusing device, and the subsequent coupling of both ES and FF phenomena. For any given liquid and flow rate, the combined procedure gives rise to significantly smaller droplet sizes than observed in any of the source techniques. The co-flowing gas stream removes space charges; in addition, the perforated plate facing the feed tube provides an electric barrier, shielding the jet-meniscus or ‘production’ area from the spray or ‘product’ area. As a result, space charges and electrified droplets are removed from the production area, thus avoiding the ambient electric saturation which becomes a limiting factor in ES-spraying: a significantly enhanced spraying stability ensues, with a much wider operation range than FF or ES. Other unexpected outcomes from the combination are also shown. A theoretical model is developed to predict the emitted droplet size: a first integral of the momentum equation yielding a generalized Bernoulli equation, and an explicit approximation for the jet diameter and droplet size, accurate within a broad parametrical band.

Absolute to convective instability transition in charged liquid jets

“…It seems that the results are almost not influenced by the nature of the source of momentum for the liquid, either set in motion under the action of a coflow or propelled by an electric field. 44 In this regard, the physical analogy found in experiments between electrosprayed and flow-focused capillary jets 13 is fully consistent with our results.…”

Electro-Flow Focusing: The High-Conductivity Low-Viscosity Limit

“…In addition, flow focusing [10,11] has earned attention in the microfluidic community owing to its simplicity, robustness, and reliability as a means to generate steady microjets and nearly monodisperse microdroplets. Numerous applications exist, e.g., the production of high quality aerosols and microspheres [12].…”

Droplet group production in an AC electro-flow-focusing microdevice

“…The active techniques encompass the use of electric, magnetic, or centrifugal fields, just to name a few. External forces are not only used to generate droplets (Gu et al, 2008;Gañán-Calvo et al, 2006), but also to manipulate them downstream as, for example, droplet coalescence, splitting and mixing (Ray et al, 2017;Budden et al, 2013;Chokkalingam et al, 2014). Droplet cluster formation has been achieved using passive methods with a potential to serve as building blocks for new materials (Shen et al, 2016).…”