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Electrohydrodynamic Jet Printing: Introductory Concepts
and Considerations
Nhlakanipho Mkhize and Harish Bhaskaran*
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.
1. Introduction
1.1. Definitions
[1-3]
Electrohydrodynamic (EHD) jet printing is a noncontact printing
technique, which has gained much attention in recent years.
It works by applying an electric field to induce ink ejection from a
conductive nozzle onto a substrate. EHD jet printing is a specific
application of the well-studied electrohydrodynamic atomization
(EHDA). Table 1 shows the different phenomena which fall
under this umbrella term. [5-7] For the purposes of this work,
we will refer to near-field (less than 1 mm) cone-jet printing
as EHD.
For these EHDA methods, the electrostatic force resulting
from the normal component of the electric field is sufficient
to overcome the surface tension of the ink, as shown in
N. Mkhize, H. Bhaskaran
Department of Materials
University of Oxford
Parks Road, Oxford OX1 3PH, UK
E-mail: harish.bhaskaran@materials.ox.ac.uk
ID
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smsc.202100073.
© 2021 The Authors. Small Science published by Wiley-VCH GmbH. This
is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction
in any medium, provided the original work is properly cited.
DOI: 10.1002/smsc.202100073
Figure 1a.[8] Figure 1b shows how the mag-
nitude of the electric potential around a
nozzle looks like in space. [9] Figure 1c
shows a simple schematic of the setup
required to carry out EHD printing. A con-
ductive capillary is hooked up either to a
syringe pump or air supply to supply the
ink. The high voltage source provides the
electric force required to induce the print-
ing. The translation stage holds the sub-
strate being printed on and controls how
the deposition pattern is formed, usually
a predetermined design loaded into control
software. This stage can also translate in
the z direction, and this modulates the
strength of the electric field (for constant
potential). The microscope camera allows
for visualization of the printing process.
An ink in a capillary, as shown in
Figure 1a, will not flow until a force is
applied which will overcome the surface tension and capillary
forces by which it is held stationary. The only forces pushing
down on it are a weak gravitational force and any pressure which
exists within the system (due to the syringe pump or applied air
pressure). When an electric potential is applied to the system,
charge will migrate to the meniscus surface of the ink, depend-
ing on whether it has mobile charge carriers or not. The conduc-
tivity of the ink depends on the number of charge carriers per
unit volume and their drift velocity. [10] When sufficient charge
has built up on the ink interface, an electrostatic potential is cre-
ated between the meniscus and the grounded substrate. The elec-
trical stress now present has a normal and tangential component,
as the inks are generally not perfect conductors. [11] The normal
stress destabilizes the meniscus and the tangential component
supports the formation of the meniscus into a cone (termed a
Taylor cone [12]) which results in jetting. This was modeled
numerically by Rahmat et al. and found to be consistent. The
electric forces pull the ink toward the deposition substrate.[13]
This works best for liquids which are sufficiently dielectric,
meaning that they can sustain the electric field. [14] Where the
dielectric constant is too low, errant satellite droplets are possi-
ble. [15]
Early work by Hayati et al. on the mechanism of stable jet
formation in EHDA demonstrates visually the effect of the elec-
trical stresses on a fluid by showing the circulation patterns
which can arise.[16]
The advantage of EHD printing comes from the formation of a
characteristic Taylor cone (first described by Sir Geoffrey Taylor).
The jet will be of smaller spatial dimensions than the nozzle used
if the correct conditions are applied, unlike what is observed with
conventional inkjet printing."
[17] Taylor found that for an ideal
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Table 1. EHD atomization modes as summarized by Jaworek and Krupa..[6]
EHDA category
Dripping mode
Subcategory
Dripping
Microdripping
Spindle
Jet mode
Cone jet
Multijet
Precession jet
Oscillating jet
Description
Large drops (nozzle diameter) pinch off when the electric field is switched on.
Characteristic of flow rates faster than jet formation
Small droplets pinch off from meniscus
Characteristic of flow rates slower than jet formation
A large spindle shaped fragment is pinched off before a jet can occur
Characteristic of an extremely high electric field
Flow rate is equivalent to jetting rate
A stable cone and jet form
Less space charge results in less interference allowing jet to remain along the central axis
A stable cone forms, but multiple jets arise
Likely due to fast flow rate, or excessive applied potential
A steady cone and jet are formed, but precess about the nozzle circumference
Caused by electrostatic force build up due to sprayed droplets (space charge)
Flow rate sufficient to create a steady jet, however off the central axis
Caused by excessive charge build-up
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[1,19]
conducting liquid, the angle at the cone apex is 20₁ = 98.6°. [18]
This size reduction in jet diameter is especially important in pur-
suits such as nanofabrication, where the size limits of devices
and structures are currently restricted with existing extrusion
technologies, such as inkjet printing. With EHD, high aspect
ratio printing of up to 50 nm lateral resolution is shown
(Figure 2a-e). [20] The versatility of the process is further enhanced
by the wide variety of materials which can be deposited, in any
configuration, without the need for set templates or masks. [1,21] It
is a true additive manufacturing technique as only the material
required is deposited, thus eliminating waste. [22] This is one of
the reasons why the technique is touted as being one of the most
economical manufacturing techniques available. The mathemat-
ics of the stable jet mode is beyond the scope of this work, but can
be found in seminal work done by Hohman et al. [23] Other jet
modes have been reported by Jaworek and Krupa.[6] The advan-
tages and disadvantages of EHD compared with other direct writ-
ing techniques are shown in Table 2.
1.2. Scope of Review
To fully make the best of the benefits of EHD printing technol-
ogy, the interplay between properties of materials used and sys-
tem parameters must be understood. (7,24,25] The interaction of
the electric field with the chosen ink relies heavily on the sys-
tem's parameters including applied potential, stand-off height,
nozzle diameter, and operating environment (e.g., humidity,
temperature, etc.). [2
[26] In addition, there are properties of the
ink that require due consideration, namely surface tension, vis-
cosity, conductivity, and fluid nature. In the following sections,
we look at each of these named variables in turn and relate them
to the overall EHD process. [27,28] Several key studies looking at
the interplay of only some of these variables have been
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reported. [28-30] Lee et al. and Choi et al. have done significant
work in describing the majority of these parameters. [28,31]
The aim of this work is to enhance understanding of the EHD
process and highlight the intricacies involved to achieve the best
possible result and resolution, based on existing methodologies,
as well as those which we describe here. For applications of EHD
printing, and current progress, several excellent reviews exist
within the larger framework of additive manufacturing, [1,32-35]
biological studies, [36-39] flexible sensors, [40] and electronics.[41]
The references cited in this work are not exhaustive, and were
chosen due to their instructive or illustrative nature.
2. Ink Considerations for EHD Printing
Inks are at the core of EHD printing. Their properties, and
intended functionality, determine the ideal printer settings.
The inks available can be categorized as homogenous solutions
(pure solvents or solubilized material), suspensions (such as
colloids of quantum dots, nanoparticles, insoluble material), pol-
ymers, melts (such as molten metal, wax, etc.), and biomolecular
inks (DNA, proteins, and bacteria). Table 3 shows just some of
these many inks and their applications. As an essential compo-
nent, understanding how the ink properties affect EHD printing
is of the utmost importance. In this section, we examine a few of
the properties and outline their implications.
2.1. Surface Tension
A free droplet, unencumbered by electrostatic, aerodynamic,
or gravitational forces will always assume a spherical shape.
This is because of a sphere costing the least energy to form.
This minimization of energy (and areal coverage) is because
of the surface tension (y) of the molecules. This property is a con-
sequence of the fact that molecules at the free surface (liquid–air
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(a)
Normal
electric stress
Capillary nozzle
Surface
tension
liquid
Gravity + Pressure
air
Tangential
Viscosity
Electric stress
Electric polarization
stress
(c)
Substrate
Syringe pump
Microscope
camera
(b)
tip
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V
180
140
electric
potential
100
Electric field
80
60
20
substrate
Computer
control
Capillary
High voltage
supply
Substrate
XYZ Translation
Stage
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 oper-
ation 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.
interface) have higher energy than within the bulk. [42] When we
consider the droplet on a solid surface, there exists a force which
is a result of the surface tension. This force acts in the plane of
the free surface, perpendicular to a free edge in that surface. The
force (F) is proportional to the length (L) of the edge.
F=yL
(1)
Surface tension is an important parameter to consider for the
inks used in EHD printing. The electrical force actuated needs to
be able to overcome the surface tensile force with which the
meniscus is held to the capillary. [43] It has been demonstrated
by He et al. that if the surface tension is too low, the ink will
form satellite droplets before a stable jet can be formed, due
to a longer pinch off time (the time it takes for a droplet to detach
from the jet). [44] Studies also show that if the surface tension is
too high, the electrostatic force applied will not be sufficient to
cause the jetting of the material, [45] and only meniscus pulsing
will be observed, resulting in no printing. Several articles make
reference to a specific range of surface tensions which are
considered useful or optimal for EHD printing. [28,46] "Islands
of stability" have been demonstrated, considering other proper-
ties such as conductivity and viscosity, which we discuss shortly.
To understand the influence of the surface tension on EHD,
let us first look at a very simplified mathematical treatment.
A basic condition for jetting to occur is that the electrostatic force
be sufficient to overcome the surface tension (Equation (2))
Fsurface tension
Felectrostatic
(2)
If we treat the force between the capillary and the ground
plane as being capacitive and the surface tensile force as being
the product of the surface tension and the distance over which
the force is applied, we obtain Equation (3)
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(d)
(b)
(e)
40
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20
EMMAMA
0
0 100 200 300 400 500 600 700 800 900
Profile width (nm)
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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.
YL =
εAV2
2d²
(3)
where y is the surface tension of the ink, in N/m, L is the distance
over which surface tension (in m) applies (in this case circum-
ference of the pipette), A is the area over which the electric field is
experienced, V is the applied potential, and d is the stand-off
height. Rearranging for V, we obtain the following (Equation (4)).
V =
2yLd2
εA
α
surface tensions, which a single material could exhibit―
especially under variable temperature conditions.
Using two different inks, Choi et al. [31] derived more involved
scaling laws that enable further understanding of the relation-
ship between the electric field strength, the jet pulsing frequency,
and diameter as the jet leaves the capillary tip. They carried out
EHD experiments using several capillary sizes, and also com-
pared existing literature. As a starting point, they describe the
flow rate of the jet using the Poiseuille-type flow rate equation
(Equation (5)) first described by Chen et al. [48]
(4)
Q
xd (AP + ½ e E-
From this, we see that V x 10.5. The applied potential is directly
proportional to the square root of the surface tension. Smith plot-
ted this relationship for a series of common solvents and found a
linear trend (Figure 3). [47] The higher surface tension of some
solvents is a result of the strong intermolecular forces, such
has hydrogen bonds. This relationship is notable, as it means that
the applied voltage does not increase rapidly over a range of
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πα
128μL
1
2
4y
dN
where Q is the flow rate, d₁ is the diameter of the nozzle, μ is the
viscosity of the ink, L is the nozzle length, AP is the change in
pressure, & is the permittivity of free space, E is the electric field
strength, and y is the surface tension on the meniscus.
Approximating the electric field E is done using a model of
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Table 2. Additive manufacturing techniques. A comparison of the advantages and disadvantages of some commonly used additive manufacturing
processes on the microscale and nanoscale.
Technique
Inkjet printing
References
Advantages
Disadvantages
Noncontact patterning technique thus safe to use for multilayer
processes
Resolution limited by nozzle diameter
[159]
Variety of materials processable
Unable to process materials with high viscosity
Does not require vacuum
Microcontact
printing
Requires no mask or template
High speed
Useful for micro- and macroscale printing
Widely commercialized
Highly reproducible
Compatible with roll-to-roll processes
Remarkably high resolution
Rapid fabrication process
Does not require vacuum or high temperatures
Allows for many materials to be patterned
Wide applicability
Compatible with flexible substrates
Exceptional nanoscale resolution (15 nm)
Scalable with multitip arrays
Prone to nozzle clogging
Mainly used for planar patterning
Poor drying leads to inhomogeneous films via the "coffee ring
effect"
Contact mode patterning
Requires fabrication of template stamp using elastomer.
Limited to planar 2D structures
New stamps required for different patterns
[160-164]
Dip-pen
lithography
Large printing tip arrays require complex fabrication
Mechanically fragile tips
[32,165-
168]
No substrate modification required to achieve high resolution
Expensive equipment required
Limited material patterning
Able to deposit sensitive biological matter
Does not require vacuum conditions
Unlimited pattern design
Gravure printing
High throughput
High speed (1 ms¹)
EHD jet printing
Suitable only for planar printing
[169,170]
Rigid substrates difficult to process
High initial setup cost
Wide range of ink viscosity processable (10-200 CP)
Sub-10 μm resolution
Resolution not limited by nozzle diameter
Scalable (nano- to microscale printing)
3D structures can be created
Wide variety of materials can be processed
Rigid and flexible substrate compliant
Low setup cost
Highly tunable process
Process hybridization possible
Prone to nozzle clogging
Glass tips are easily broken
Only solutions/suspensions processable
Challenging to print on insulating substrates
Substrate damage possible if excess potential applied
Printing varies strongly with varying field strength
Crosstalk probable in multi-tip arrangements
Multiple tips can be used in tandem
[1,2,171-
173]
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a semi-infinite wire perpendicular to an infinite planar
electrode (Equation (6))
of important parameters mentioned earlier. The jet diameter
scales according to Equation (7)
E = 4V0/dN In
8H
dN
(6)
7dN
dx
E
(7)
where Vo is the applied potential difference and H is the distance
between the tip and substrate. We shall refer to this distance
from now on as the stand-off height. Through a series of deri-
vations, which are not part of the scope of this review, Choi
presents two scaling laws which can give the relative magnitudes
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which means for an increase in electric field, there is a linear
decrease in jetting diameter. The second scaling law describes
the relation between the jet pulsing frequency and the E field,
and is stated as Equation (8)
© 2021 The Authors. Small Science published by Wiley-VCH GmbH/nElectro hydrodynamic jet printing and dimensions
Some reading to identify the size
Look in the market and find metal nozzle that u can use
Evaluate and see what kind of prints you are going to do
Think about the materials
Then manufacture using 3d printer or other or Just manufacturing through workshop through
traditional techniques
Once you have the printing system do
Look out Size of the liquid by doing some jetting
Read some of the papers that do printing and see other data on what they are collecting