Scaling up of extractor-free electrosprays in linear arrays

Abstract

Electrospray offers unique atomization of liquids, whereby micro- or nano-droplets with very narrow size distributions are generated from electrified Taylor cones. To scale up this process, many electrospray emitters must be operated simultaneously, while the flow rate per emitter must remain low enough to preserve stability. To cope with the electrostatic repulsion between the various elements in the system, it is common to position the emitters very near a counter-electrode. Instead, we have studied the conditions leading to robust scalable spraying by using linear arrays of electrosprays in which the counter-electrode is far compared to the inter-emitter separation. In our design, a row of emitter tubes protrudes out of a backplate, and the counter-electrode is a flat collector plate (droplet collection plate set at a high negative electric potential). In addition, electrodes at both ends of the array enable uniform electrical field conditions, while preventing electrical gaseous discharges. Strong electrostatic interactions are expected between the spray plumes and the Taylor cones. Nonetheless, we show that this geometry is scalable without bound, both by electric field computations as by experiments performed under different geometrical configurations, liquid flow rates per emitter, and electrical conductivities of the liquid (mainly, NaCl/MEG solutions). The onset voltage necessary to stabilize the spraying at all emitter positions approaches a plateau as the number of operated emitters increases. Eventually, at high enough potential difference, the cones and sprays misalign, pointing in directions in consonance with minute zig-zag misalignments of the emitters, revealing the importance of small imbalances in the electrostatic forces at the Taylor cones.