Experimental investigations on characteristics of stable water electrospray in air without discharge

An experimental study was conducted to resolve previous conflicting results on water electrospray in air at atmospheric pressure. Using a small flow rate relative to that used in previous studies and a small nonmetallic nozzle, we observed stable electrospray of water in air without discharge and distinguished three distinct operating regimes for applied voltage and flow rate. The well-known cone-jet mode was observed and the general scaling law of the generated droplet size in the cone-jet mode was confirmed by direct visualization of the meniscus, jet, and generated droplets. We also observed and analyzed whipping motion in the electrified water jet.

Figure 1

Schematics of the electrospray and optical imaging system.

Figure 2

Operation domain of water electrospray: straight-jet regime (I), transition regime (II), and whipping-jet regime (III). Dotted line represents the transition between modes. Square symbol presents the tilted-jet mode.

Figure 3

Morphology of water electrospray. (a) Straight-jet regime: microdripping (i), cone jet (ii), and tilted jet (iii). (b) Transition regime: microsimple jet (i), sinusoidal jet (ii), and tilted jet (iii). (c) Whipping regime: simple jet (i), whipping jet (ii), and tilted jet (iii). (d) Simple dripping.

Figure 4

Change in mean diameter of the generated droplets. (a) Mean diameter of the generated droplets as a function of the electric Bond number for various flow rates. (b) Transition from cone-jet mode to tilted-jet mode.

Figure 5

Relation between charge relaxation length and droplet diameter.

Figure 6

Variation of two dimensionless droplet diameters, $D_{\mathrm{d}}/r^{*}$ (black square) and $D_{\mathrm{d}}/R^{*}$ (red circle) as a function of ${\alpha }^{1/2}$.

Figure 7

(a) Photographic images of jet breakup in cone-jet mode and (b) the functional relationship between jet breakup length and flow: experimental measurements (symbol) and linear fit (red line).

Figure 8

Representative image of a whipping jet.

Figure 9

Normalized diameter of electrified jet as a function of dimensionless axial distance for (a) various Weber number values and (b) increasing electric Bond number values.

Figure 10

Liquid outline along the axial direction. Comparison of measured outline to (a) previously suggested functional form and (b) Eq. (8) in the present study.