Enhanced Stability of Electrohydrodynamic Jets through Gas Ionization

Theoretical predictions of the nonaxisymmetric instability growth rate of an electrohydrodynamic jet based on the measured total current overestimate experimental values. We show that this apparent discrepancy is the result of gas ionization in the surrounding gas and its effect on the surface charge density of the jet. As a result of gas ionization, a sudden drop in the instability growth rate occurs below a critical electrode separation, yielding highly stable jets that can be used for nano- to microscale printing.

Figure 1

(a) Comparison of centerline deflection of a jet at the deployment point on the bottom electrode produced with small electrode separations (solid symbols) vs the centerline deflection of a jet formed between 25.1 mm electrode separation (open symbols) at the equivalent position with respect to the nozzle (see inset). Data were determined from images of the PEO solution (conductivity: $431\mu \mathrm{S}/\mathrm{cm}$) jet under $1\mathrm{ml}/\mathrm{h}$ flow rate and $4119\mathrm{V}/\mathrm{cm}$ electric field. Maximum deflection of the jet refers to the largest horizontal length scanned by the jet within the captured images. (b) Variation of measured current and maximum deflection with electrode separation for PEO experiments done using different salts and nozzle materials. Solid circles: LiCl doped PEO solution ($430\mu \mathrm{S}/\mathrm{cm}$) with stainless steel needle; solid triangles: KCl doped PEO solution ($431\mu \mathrm{S}/\mathrm{cm}$) with stainless steel needle; open circles: KCl doped PEO solution ($436\mu \mathrm{S}/\mathrm{cm}$) with Teflon needle having copper wire connection; and open triangles: KCl doped PEO solution ($428\mu \mathrm{S}/\mathrm{cm}$) with copper coated needle.

Figure 2

Experimental determination of temporal instability growth rate for glycerol. 150 images of the jet are captured at 10 000 fps and $2\mu \mathrm{s}$ exposure time in each electrode separation. Using a Matlab program, the centerline of the jet is located and its deflection from vertical is found in every image. Each disturbance wave is traced in time and changes in its amplitude are determined by assuming that it is convected downstream with the average velocity of the stream. (a) Representative images from one of the experiments. (b) Variation of amplitude of disturbances as a function of time.

Figure 3

Representative images for glycerol solution filaments at: (a) electrode $\mathrm{\text{separation}}=8\mathrm{mm}$, relative $\mathrm{\text{humidity}}=25\pm 0.1\%$, $\mathrm{\text{current}}=5.46\mu \mathrm{A}$; (b) electrode $\mathrm{\text{separation}}=16\mathrm{mm}$, relative $\mathrm{\text{humidity}}=25\pm 0.1\%$, $\mathrm{\text{current}}=5.2\mu \mathrm{A}$; and (c) electrode $\mathrm{\text{separation}}=16\mathrm{mm}$, relative $\mathrm{\text{humidity}}=70\pm 0.1\%$, $\mathrm{\text{current}}=7.8\mu \mathrm{A}$.