Morphology of water electrosprays in the simple-jet mode

Experiments were conducted in order to study and characterize electrohydrodynamic atomization in the simple-jet mode for inviscid liquids. The operational window of this mode regarding the electric potential and liquid flow rate is presented. From the data it could be concluded that this mode can be divided by the characteristics of its breakup mechanism and that these characteristics are a function of the liquid Weber number and the electric Bond number for a given setup. Additionally we were also able to calculate the average charge per droplet and define the average size of primary and satellite droplets. The dispersion of the spray was also studied regarding its relation to the liquid Weber number and to the electric Bond number. We conclude that simple-jet mode electrosprays are a good option for applications which require monodisperse micrometer droplets with high throughput.

Figure 1

Different hydrodynamic and electrohydrodynamic droplet formation mechanisms: (a) dripping regime, uncharged jet; (b) transition regime, uncharged jet; (c) jetting regime, uncharged jet; (d) simple-jet mode with varicose breakup, charged jet; (e) simple-jet mode with whipping breakup, charged jet.

Figure 2

Effect of the electric field on the charged droplets in the simple-jet mode (spray envelope). For all the pictures the liquid used is de-ionized water pumped through the nozzle at 420 mL h${}^{-1}$. The indicated potentials were applied on the ring (not shown in the picture) with the nozzle grounded.

Figure 3

Electrospray and optical system scheme.

Figure 4

Representation of the nozzle-ring setup and images of the spray. (a) Close view of the jet breakup with water (L1) at −6 kV and 360 mL h${}^{-1}$ and representation of the nozzle-ring setup with the defined axis and some variables. (b) Snapshot of the jet with the dispersed droplets and a small part of the metallic nozzle. (c) Superimposed image showing the spray envelope, the breakup length ($h_B$), and the envelope angle (θ).

Figure 5

Diagram representing the operational window of the simple-jet mode in relation to the electric Bond number ($B$) and the liquid Weber number (We) for de-ionized water. The control parameters related to $B$ (applied potential) and We (flow) are represented on the right and upper axis, respectively.

Figure 6

Sequences according to the sets (a)–(c) defined in Fig. 5.

Figure 7

Whipping line and dispersion line from linear fits of experiments with water and sodium chloride aqueous solutions with different concentrations (17, 20, 35, and 70 g L${}^{-1}$) for different values of We and $B$. The inset shows the electric conductivity of the solutions for the different concentrations.

Figure 8

Normalized electric current against $B$ for three different flows (We $=$ 5.3, We $=$ 6.3, We $=$ 13.4) for NaCl solution (35 g L${}^{-1}$). The dotted lines connecting the symbols are to guide the eyes.

Figure 9

(a) and (b) Jet nondimensional radius [$r_j$(2$a^{-1}$)] against the electric Bond number at $z{a}^{-1}$ $=$ 9.4 for different values of the liquid Weber number and electric Bond number (a) and for a constant electric Bond number and different values of We (b) for liquid L2. Each data point represents the average of the measurements with the error bar representing the minimum and maximum measured values.

Figure 10

(a) and (b) Normalized diameter of the primary (a) and satellite (b) droplets generated for different values of the liquid Weber number and electric Bond number for de-ionized water and a solution of water and sodium chloride (35 g L${}^{-1}$). Error bars are data standard error.

Figure 11

Superposed images of the droplets for We $=$ 5.2 and $B$ $=$ 0 (a), $B$ $=$ 280 (b) and $B$ $=$ 500 (c), and for $B$ $=$ 500 with We $=$ 5.2 (d), We $=$ 7.5 (e), and We $=$ 10.3 (f). Both experiments were performed with de-ionized water (L1).

Figure 12

Influence of $B$ on θ for different values of We (a) and of $B$ on Lθ for different We (b). Each data point represents the average of the measurements with the error bar representing the minimum and maximum measured values.