Electrohydrodynamic Jet Printing: Introductory Concepts and Considerations

Abstract

Electrohydrodynamic (EHD) jet printing is an emerging technique in the field of additive manufacturing. Due to its versatility in the inks it can print, and most importantly, the printing resolution it can achieve, it is rapidly gaining favor for application in the manufacture of electronic devices, sensors, and displays among others. Although it is an affordable and accessible manufacturing process, it does require excellent operational understanding to achieve high resolution printing of up to 50 nm as reported in literature. In this review, three main aspects are considered, namely, the ink properties, the printer system itself (including design, nozzle dimensions, applied potential, and others), and the substrate onto which printing is being carried out. Knowing how all these factors can be manipulated and brought together allows the users of EHD printing to achieve extraordinary resolution and consistent results. The review is concluded with a brief discussion on where one can see the potential for development in this field of research.

Keywords: electrohydrodynamic jet printing; ink properties; inkjet printing nozzles; process parameters for jet printing; substrate properties for inkjet printing.

Figure 1

Physics of electrohydrodynamic jet printing. (a) Summary of forces acting at capillary tip during EHD printing using the cone‐jet mode. Reproduced with permission.^{[ 8 ]} Copyright 2010, IOP Publishing. (b) The model shows the spatial distribution of the electric potential during EHD operation around the tip, with the scale bar indicating field strength. Adapted with permission.^{[ 9 ]} Copyright 2016, Royal Society of Chemistry. (c) Simple schematic of the components required to build an EHD printer.

Figure 2

a) Gold nanopillar of diameter ~50 nm and aspect ratio of ~17 (scale bar, 200 mm). b) Top and c) side views of nanopillars printed subsequently at 200 nm center‐to‐center distance (scale bar, 200 nm). d) Dots of 80 nm wide printed into a 1 μm lattice constant array (1 μm scale bar). e) Printed tracks with pitch sizes of 250, 200, 150, 100, and 75 nm (scale bar, 2 μm). The inset shows atomic force microscopy (AFM) (full black lines) and scanning electron microscopy (SEM) (red dashed lines) profiles of 150 nm pitch size. The height of AFM profiles is given in nanometers. The SEM profiles are in arbitrary units. Tracks have reproducible heights of ~40 nm and are well separated. Reproduced with permission.^{[ 20 ]} Copyright 2012, Springer Nature.

Figure 3

Effect of surface tension on applied voltage. Reproduced with permission.^{[ 47 ]} Copyright 1986, IEEE.

Figure 4

Surface tension responding to colloidal concentration. Plot of the surface tension versus weight percent of titania dispersions at pH 10 and pH 11. The plot shows that the surface tension of the dispersion decreases significantly from ≈0% to 5% and then increases above the surface tension of pure water at 10% and higher weight percentages. Reproduced with permission.^{[ 63 ]} Copyright 2003, American Chemical Society.

Figure 5

Influence of relaxation times. Representative shape of a) cone jet (τ_q/τ_h < 1) and b) ball cone (τ_q/τ_H > 1). c) Jetting window with respect to α for each fluid; the white region is for a classical EHD system, and the gray region is for a forced jet system (E is ethanol and T is terpineol). Reproduced with permission.^{[ 28 ]} Copyright 2013, American Chemical Society.

Figure 6

Evolution of cone jet of polyethylene oxide (PEO) (M_w = 1.0 × 10⁶ g mol⁻¹, 0.38 wt%) in water/glycerol mixture: a) thin jet from the insufficient flow rate and b) thick jet from the excessive flow rate. Reproduced with permission.^{[ 46 ]} Copyright 2016, Elsevier.

Figure 7

The nozzle wettability influences. (a) The ink droplet image at the moment of breakup. The contact angles of the nozzle inner wall and the breakup times are (a₁) θ = 30°, 37.14 μs; (a₂) θ = 60°, 38.78 μs; (a₃) θ = 90°, 51.89 μs; (a₄) θ = 120°, 58.45 μs; (a₅) θ = 150°, 61.73 μs. (b) The breakup time and droplet velocity changing with the wettability of the nozzle inner wall. a,b) Reproduced under the terms of the CC‐BY 4.0 license.^{[ 44 ]} Copyright 2017, The Authors, published by Springer Nature. The liquid rises the wettable nozzle outer wall in the dripping mode (c). After installation of the extender cap, the recess was small enough for rising droplet to reach the extender cap and adhere to its bottom surface (e). When the electric field is intensified the cone is formed with its base connected to the extender cap (f), instead of the nozzle tip itself (d). (g) The shaded regions show stability islands corresponding to the two nozzles. The inset shows a typical stability island with the corresponding zones. c–g) Reproduced under the terms of the CC‐BY 4.0 license.^{[ 43 ]} Copyright 2016, The Authors, published by Springer Nature.

Figure 8

Details of three‐needle coaxial device: (a) needle assembly, (b) needle dimensions and (c) relative placement of needles in device. ID and OD represent internal and outer diameters, respectively. a–c) Reproduced with permission.^{[ 116 ]} Copyright 2008, The Royal Society. (d) Fabricated multi‐nozzle and (e) fabrication flow chart of the multi‐nozzle. d,e) Reproduced with permission.^{[ 120 ]} Copyright 2008, AIP Publishing.

Figure 9

EHD printing of multilayer curved PEDOT:PSS‐PEO features. a,b) Optical photos of circular features with a layer number of 200. c,d) Optical photos of 300‐layer curved features. a–d) Reproduced with permission.^{[ 127 ]} Copyright 2018, American Chemical Society. e) Drop‐on‐demand printed “S” on glass slides with printing speed and droplet dimension controlled by the parameter of the AC‐pulse voltage. Reproduced with permission.^{[ 130 ]} Copyright 2014, IOP Publishing.

Figure 10

Optical images of the P3HT lines on differently modified surfaces. Reproduced with permission.^{[ 136 ]} Copyright 2014, American Chemical Society.

Figure 11

Optical images of printed Ag lines. Printed Ag nanoink on (a) PET film and (b) PEN film. Printed Ag nanopaste on (c) PET film and (d) PEN film. Scale bars are 100 μm. Quality of printing appears better on PEN than on the PET. Reproduced under the terms of the CC‐BY 4.0 license.^{[ 137 ]} Copyright 2015, The Authors, published by ECS.

Figure 12

Surface features for wetting control. (a) Schematic showing terminology for pillar width a, separation b and height h. (b–d) Scanning electron microscopy images of representative micro‐structured surfaces for (b) b/a = 1/3, (c) b/a = 1, and (d) b/a = 4. Reproduced under the terms of the CC‐BY 4.0 license.^{[ 142 ]} Copyright 2015, The Authors, published by Springer Nature.

Figure 13

The formation process of the jet over the different substrates. The nozzle‐to‐substrate distance H is fixed at 2 mm, the flow rate is Q = 50 nL/min, and the substrate is still. a) PET. b) Silicon. c) Steel. Reproduced with permission.^{[ 147 ]} Copyright 2013, Trans Tech Publications Ltd.

Figure 14

EHD‐printed PMMA. a) Regular patterns with relatively large line width due to sufficient discharge occurring before jet lands on substrate. b) Nonlinear lines due to whipping instabilities introduced by excessive applied bias.