Multiplexed electrospraying of water in cone-jet mode using a UV-embossed pyramidal micronozzle film

Abstract

The electrospraying of water in the cone-jet mode is difficult in practical applications owing to its low throughput and the electrical discharge caused by the high surface tension of water. A film with multiple dielectric micronozzles is essential for multiplexed electrospraying of water in cone-jet mode without electrical discharge. Thus, a pyramidal micronozzle film with five nozzles was fabricated using the UV-embossing process. The pyramidal micronozzle film consisted of pyramidal micronozzles, a micropillar array, and an in-plane extractor, which were proposed to minimize wetting and concentrate the electric field to the water meniscus at the tip of the pyramidal micronozzle. The electrospraying of water using a single pyramidal micronozzle was visualized by a high-speed camera at a flow rate of 0.15-0.50 ml/h with voltages of 0.0-2.3 kV, -1.6 kV, and -4.0 kV at the water, guide ring, and collector, respectively. Three distinct modes, the dripping, spindle, and cone-jet modes, were observed and distinguished according to the motion of the water meniscus at the nozzle tip. The steady Taylor cone and jet were observed in a voltage range of 1.3-2.0 kV in water, particularly in cone-jet mode. Multiplexed electrospraying of water in cone-jet mode at a flow rate of 1.5 ml/h was performed using a pyramidal micronozzle film, demonstrating the potential for a high-throughput electrospraying system.

Keywords: NEMS; Nanofluidics.

Fig. 1. Electrospraying using a capillary nozzle-collector configuration.

Electrospraying is typically conducted by using a capillary nozzle-collector configuration. A thin jet is emitted from the apex of the Taylor cone and disintegrated into charged microdroplets. The droplets have identical polarities and accumulate on the grounded collector

Fig. 2. Schematic diagram of the Taylor cone formation based on the nozzle tip design.

a Hydrophilic tip. The flow between the outer and inner rims of the nozzle leads to the unstable form of the Taylor cone. b Hydrophobic tip. The flow between the outer and inner rims of the nozzle is not created due to the hydrophobic surface. c Pyramidal tip. The gap between the outer and inner rims is geometrically reduced. The effects of flow on the formation of the Taylor cone can be minimized even if wetting occurs at the tip

Fig. 3. Fabrication process and results of the pyramidal micronozzle film.

a Fabrication of the nickel mold. Bulk micromachining and photolithography on a single crystalline silicon substrate were conducted to create an opposite pattern to that of the pyramidal micronozzle and the micropillar array (a-i to a-viii). The patterned substrate was replicated by electroforming nickel (a-ix to a-xi). b Fabrication process of the pyramidal micronozzle film. The pyramidal micronozzle and micropillar array was fabricated on a polycarbonate film via the UV-embossing process. Multiple layers of chromium and copper were deposited to pattern the in-plane extractor. Laser drilling was conducted to form a hole at the nozzle. The pyramidal micronozzle film was dip-coated in HDFS solution to create a hydrophobic surface. c SEM image of the UV-embossed pyramidal micronozzle and the micropillar array. d The pyramidal micronozzle film after removal from the nickel mold. The fabricated film was flexible. e Deposition results of the in-plane extractor. f Laser drilling results of the polycarbonate film

Fig. 4. Experimental setup for electrospray visualization.

The main high voltage was applied to the water. The in-plane extractor was connected to switch 1 to identify the effect of electric concentration at the nozzle tip. A guide ring was connected to switches 2 and 3. Switches 2 and 3 were not connected simultaneously. Switch 2 was connected when switch 1 was disconnected. Switch 3 was connected when switch 1 was connected (the grounded in-plane extractor). The flow rate was controlled using a syringe pump. A DSLR camera or a high-speed camera was used for the visualization of the electrospray

Fig. 5. High-speed images of the electrospraying of water by the pyramidal micronozzle at a flow rate of 0.15 ml/h.

a Dripping mode (0.8 kV). b Spindle mode (1.1 kV). c Cone-jet mode (1.3 kV)

Fig. 6. Experimental results of the cone-jet mode.

a DSLR camera images in cone-jet mode (voltage at water: 1.6 kV, flow rate: 0.50 ml/h). b Onset and end voltages of the cone-jet mode. The data plotted in black lines with diamond symbols are the results when switch 2 depicted in Fig. 4 was connected. The data plotted in green lines with triangle symbols are the results when switches 1 and 3 depicted in Fig. 4 were connected. c Morphology of the Taylor cone and jet at voltages of 1.3 kV (c-i), 1.5 kV (c-ii), and 1.7 kV (c-iii) at a flow rate of 0.15 ml/h. The meniscus of the Taylor cone was compressed toward the nozzle as the applied voltage increased. d Operation domain of the steady cone-jet mode of water. The domains of the other research studies were replotted from the data provided in the references. The domain obtained using UV-embossed pyramidal micronozzles showed good agreement with the other domains obtained by the other types of micronozzles

Fig. 7. Relationship between the charge-relaxation length and jet diameter.

The data plotted by the measurement of the jet diameter showed good agreement with the scaling law for steady cone-jet mode, which was suggested in the classical theory

Fig. 8. Multiplexed electrospraying of water in cone-jet mode using a pyramidal micronozzle film.

a Linear array of the five pyramidal micronozzles. The film was also fabricated based on the UV-embossing process, which is demonstrated in Fig. 3b. b Schematic setup for the multiplexed electrospraying of water. c Visualization of the multiplexed electrospraying of water without the glass beads. Nonuniform multiplexed electrospraying was visualized. d Effect of flow homogenization by the glass beads. Flow redistribution by the glass beads reduced the inequality of the flow rates. e Visualization of the multiplexed electrospraying of water with the glass beads. All of the nozzles were operated in cone-jet mode