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REVIEW small science Open Access www.small-science-journal.com 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 Small Sci. 2022, 2, 2100073 2100073 (1 of 21) © 2021 The Authors. Small Science published by Wiley-VCH GmbH ADVANCED SCIENCE NEWS www.advancedsciencenews.com small science Open Access www.small-science-journal.com 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 26884046, 2022, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smsc.202100073 by Test, Wiley Online Library on [29/03/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License [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 Small Sci. 2022, 2, 2100073 2100073 (2 of 21) 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 © 2021 The Authors. Small Science published by Wiley-VCH GmbH ADVANCED SCIENCE NEWS www.advancedsciencenews.com (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 small science Open Access www.small-science-journal.com 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) Small Sci. 2022, 2, 2100073 2100073 (3 of 21) © 2021 The Authors. Small Science published by Wiley-VCH GmbH 26884046, 2022, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smsc.202100073 by Test, Wiley Online Library on [29/03/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License ADVANCED SCIENCE NEWS www.advancedsciencenews.com (d) (b) (e) 40 small science Open Access www.small-science-journal.com 20 EMMAMA 0 0 100 200 300 400 500 600 700 800 900 Profile width (nm) 26884046, 2022, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smsc.202100073 by Test, Wiley Online Library on [29/03/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 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 Small Sci. 2022, 2, 2100073 2100073 (4 of 21) 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 © 2021 The Authors. Small Science published by Wiley-VCH GmbH ADVANCED SCIENCE NEWS www.advancedsciencenews.com small science Open Access www.small-science-journal.com 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] 26884046, 2022, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smsc.202100073 by Test, Wiley Online Library on [29/03/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 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 Small Sci. 2022, 2, 2100073 2100073 (5 of 21) 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

Fig: 1