Article pubs.acs.org/Langmuir
Systematic Design of Jettable Nanoparticle-Based Inkjet Inks: Rheology, Acoustics, and Jettability Himamshu C. Nallan, Jacob A. Sadie, Rungrot Kitsomboonloha, Steven K. Volkman, and Vivek Subramanian* Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720-1770, United States S Supporting Information *
ABSTRACT: Drop-on-demand inkjet printing of functional inks has received a great deal of attention for realizing printed electronics, rapidly prototyped structures, and large-area systems. Although this method of printing promises high processing speeds and minimal substrate contamination, the performance of this process is often limited by the rheological parameters of the ink itself. Effective ink design must address a myriad of issues, including suppression of the coffee-ring effect, proper drop pinning on the substrate, long-term ink reliability, and, most importantly, stable droplet formation, or jettability. In this work, by simultaneously considering optimal jetting conditions and ink rheology, we develop and experimentally validate a jettability window within the capillary number−Weber number space. Furthermore, we demonstrate the exploitation of this window to adjust nanoparticle-based ink rheology predictively to realize a jettable ink. Finally, we investigate the influence of mass loading on jettability to establish additional practical limitations on nanoparticle ink design.
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INTRODUCTION Drop-on-demand (DoD) inkjet printing has become an increasingly attractive method of fabrication in recent years because of its simplicity, cost effectiveness, and versatility. More recently, inkjet printing has demonstrated enormous applicability in areas such as the fabrication of nanomechanical cantilevers for utilization in sensing,1 displays using conjugated polymers,2 thin-film transistors (TFT),3 ceramics,4,5 deposition sol−gel materials,6,7 and biomaterials for tissue engineering.8 These applications have largely been facilitated through the development of families of new printable functional materials such as nanoparticles, polymer semiconductors, sol−gel precursors, and so forth. Unfortunately, whereas knowledge regarding materials synthesis has progressed dramatically in this regard, the associated knowledge regarding ink formulation has not kept up; ink formulation is still largely treated as an art, and the literature regarding this topic is sparse. What is needed, then, is a scientifically driven methodology for the design of inks using these novel printable materials. This is in fact the focus of this work. Two primary types of DoD inkjet technologies exist: thermal inkjets, in which ink is superheated within the nozzle chamber until a bubble forms, collapses, and ejects a drop, and piezoelectric inkjets, in which a voltage waveform excites a piezoelectric material within the nozzle, creating a deformation wave that expels a drop.9 Both methods are highly sensitive to the rheological properties of the ink.10 For example, improperly designed inks can cause the formation of satellite drops (small subsidiary drops formed during the ejection of the main drop) © XXXX American Chemical Society
that deposit in undesired locations, degrading pattern integrity. Similarly, improperly designed inks can cause nozzle clogging and or jetting changes because of the evaporation of volatile solvents at the nozzle orifice. Consequently, factors such as the suppression of satellite drop formation and solvent evaporation at the nozzle become integral to ink design.9 Additionally, because the material is deposited as a liquid, the solid patterning constituents are often suspended or dissolved within a liquid precursor, introducing additional concerns such as nozzle clogging (often described as the “first drop problem”) and complex rheology, which have been explored in the literature.5,11−14 Moreover, because pattern solidification occurs postdeposition, the interaction between the substrate and impinging drops also poses several challenges to producing high-quality patterns. The drop’s contact angle and interaction with other drops on the substrate greatly influence the pattern morphology,9,15 but, as Soltman demonstrated, the temperature, drop frequency, and drop spacing can be manipulated to control the deposition quality. Finally, drying droplets often exhibit the coffee-ring effect, in which solute is deposited primarily at the drop edge, compromising pattern uniformity; this phenomenon has been studied extensively by Deegan et al.16,17 Given these diverse dependencies and requirements, the starting point in inkjet technology is still the generation of stable, satellite-free droplets to ensure high-quality printing. Received: July 24, 2014 Revised: October 10, 2014
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do not suffer from satellites. Finally, we demonstrate predictive and reliable control over the jettability of an ink loaded with metallic nanoparticles, thus attesting to the viability of this approach to develop jettable inks functionalized with nanoparticle materials.
The mechanics of drop formation using radially polarized piezoelectric nozzles have been investigated by many researchers through experiments as well as simulations. Bogy and Talke showed that there exists an optimal dwell time for an applied trapezoidal pulse waveform at which the velocity of the ejected drop is maximized because of effective pressure wave superposition within the nozzle cavity.18 Tsai and Hwang explored drop formation dynamics using a bipolar pulse waveform.19 Fromm defined a range of stable drop formation using numerical calculations; from the Navier−Stokes equations, Fromm identified the relevant dimensionless parameter to be the ratio of the Reynolds (Re) number to the square root of the Weber (We) number (a quantity later called the Z number), concluding that fluids whose Z numbers are less than 2 cannot form stable drops.20 Since then, several authors have attempted to define a jettability range using the Z number, resulting in the following: 1 < Z < 10,21 4 < Z < 14,22 and 0.67 < Z < 50.23 The inconsistencies in reported Z-value ranges suggest that the Z number alone cannot define jettability. Although the other dimensionless parameters relevant to droplet formationRe, We, and capillary (Ca) numbereach balance only two of the force terms pertinent to drop formation (inertial, viscous, and surface tension), the Z value encompasses all three. Thus, it is possible that fluids having very different combinations of densities, viscosities, and surface tensions, not all of which are jettable, could have the same Z value. Alternatively, Derby suggested using a map of the We−Re space within which a printability region can be traced.4 Derby considered two additional constraints in his analysisa minimum value of We to ensure drop ejection occurs and a maximum value at which the drop splashes upon meeting the surface. Kim et al. simulated the drop-formation dynamics of a Newtonian fluid and generated a qualitative printability window in a Ca−We space (which approximately spanned 0 < Ca < 4 and 1.5 < We < 15), considering single-drop formation as the sole metric of jettability.24 All of these prior works therefore represent excellent foundational bases for the system study and definition of jettability but do not provide comprehensive frameworks for experimental ink design. In this work, we build upon existing knowledge to demonstrate a systematic methodology for designing jettable inks, particularly focusing on nanoparticle-containing inks. In recent years, nanoparticle-containing inks have become extremely important and are widely used for the printing of antennae,25 passive components,26 transistors,27 solar cells,28 and so forth. To our knowledge, a systematic method of engineering the jettability of a nanoparticle suspension has not yet been published; this is a major shortcoming in the available body of knowledge. This has largely left ink formulation as an art, preventing effective transition from materials synthesis to applications in printed electronics, rapid prototyping, and so forth. For the first time, in our work, we investigate the jettability window within the Ca−We space experimentally and show it to be consistent with prior observations and simulations. We use these results to develop a systematic methodology for designing nanoparticle-loaded jettable inks; these are shown to follow the aforementioned jettability window guidelines. Maximum allowable jetting frequency and positional accuracy are not explicitly considered in our design strategy because we employ a bipolar pulse waveform that eliminates the effect of frequency on droplet formation29 and because satellite drops are the primary cause of printing inaccuracy;30 inks developed using the design strategy herein
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EXPERIMENTAL SETUP
To explore drop-formation dynamics, we used a custom-built DOD inkjet printer, based on the setup used in Soltman,15 consisting of a MicroFab MJ-AT nozzle, a charge-coupled device (CCD) camera, a strobe and strobe driver, a jetting driver, and a system computer, which controlled the pulse waveform. A reservoir filled with ink (1.5 mL) was attached to the nozzle as well as a valve-controlled nitrogen gas source. The nozzle consisted of a glass capillary tube and a radially polarized piezoelectric actuator sleeve. Nozzle coatings, primarily used to suppress wicking outside the nozzle after drop ejection31 or enhance wicking within the orifice,32 are commonly used in many commercial inkjet heads. Such coatings were not employed within this work for two reasons. First, because a wide variety of inks were tested, the nozzle’s surface condition was held constant for simplicity. Also, wicking of the glass nozzle tip was not a problem because most of the inks employed within this work are hydrophobic. The nozzle’s orifice diameter was held constant at 60 μm, and all inks were jetted at a frequency of 1000 Hz. A schematic of a typical bipolar waveform is shown in Figure 1. Drop ejection occurs during the positive
Figure 1. Diagram of a typical bipolar pulse waveform. trapezoidal pulse. The negative pulse that follows serves to eliminate residual oscillations after each drop ejection, removing the influence of drop frequency on jetting performance. The voltage ramp times of the waveform were kept as short as allowable because longer ramp times can result in unnecessarily large drop volumes.18 The echo time was always set to twice the dwell time because this ratio results in the optimal suppression of unwanted residual deformation waves after each drop ejection.29 Solvents used for ink formulation were all purchased from SigmaAldrich. Binary solvent mixtures of varying compositions were also explored, and all composition values are given on a molar basis. Loaded inks employed gold nanoparticles synthesized according to the Brust method,33 following the methodology of Huang.26 The particles have a mean diameter of 2 to 3 nm and are encapsulated by hexanethiol ligands. The density of each multicomponent system was approximated as a weighted average of constituents on a volume basis. A Brookfield LVDV III rheometer (Middleboro, MA) was used to determine the viscosity of each system. All pure solvent systems were considered to be Newtonian because of their simple molecular structures and indeed showed Newtonian behavior over the experimental ranges of importance in this work. Regarding loaded inks, it is known that the addition of nanoparticles generally increases an ink’s low shear rate B
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viscosity and at sufficiently high particle loading (∼60% mass loading) can lead to non-Newtonian behavior, such as shear thinning at moderate shear rates and shear thickening at very high shear rates if significant hydrodynamic coupling arises between particles.34 As will be discussed, non-Newtonian behavior was not observed during jetting (at shear rates of ∼105 1/s for the mass loadings employed herein), and thus viscosity values measured at low shear rates were used to calculate the dimensionless parameters. The surface tension of all systems was measured using a custom-built apparatus based on the pendant drop method. The stroboscopic vision system was used to calculate the drop velocities.
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RESULTS AND DISCUSSION To realize a systematic methodology for formulating jettable inks, it is first necessary to define jettability, including the identification of failure modes that define the bounds of the jettable rheological space. Next, on the basis of the defined jettability criteria, simple unloaded pure-solvent inks can be used to explore the rheological relationships underlying jettability. These experiments then provide a clear definition of the rheological characteristics of a jettable ink. The resulting rheological space can then be validated using “real” nanoparticle-loaded inks to validate the universality of the selected criteria. At this point, a clear ink design strategy emerges, which can be experimentally tested. Within this work, therefore, we follow this methodology. We begin by defining a jettability criterion. Jettability Criterion. When investigating jettability, it is first necessary to optimize the waveform used; this normalizes for any acoustic variation from one solvent system to the next. Pressure wave propagation within the nozzle chamber is strongly dependent on the medium’s bulk modulus, which can vary significantly across ink systems. Therefore, for a given nozzle chamber length and ink, the dwell time has to be matched for an efficient superposition of pressure waves within the chamber that lead to droplet ejection.18 For a constant waveform amplitude and droplet volume, operating at the optimal dwell time maximizes the droplet velocity whereas the harmonics of the dwell time produce smaller, local maxima because energy is lost through viscous dissipation.18 However, within this work, the droplet volume was not constant from system to system; in fact, the fluid properties, pulse waveform shape, and, most importantly, pressure on the nozzle-reservoir system all affect the droplet volume. Also, for a given waveform setting, it is possible to adjust the nozzle pressure to a trade off between the droplet velocity and volume, effectively tuning the drop stability. Consequently, as shown in Figure 2a,b, neither the velocity nor drop mass reach a discernible maximum if the dwell time is varied when holding the waveform amplitude constant. Interestingly, however, plotting the momentum against the dwell time, as shown in Figure 2c, reveals the expected periodic trend. Thus, whereas the literature suggests defining the optimal dwell time as the value that maximizes the droplet velocity, we have defined it to maximize the drop momentum in order to normalize the effects of nozzle pressure. Having chosen the optimal dwell time, it is now possible to define a jettability criterion that can then be tested across multiple inks, all operating at their individual optimal dwell time values. To be considered “jettable”, an ink must be able to form a single drop that can be stably jetted for long periods of time without becoming unstable. Although avoiding satellite drop formation is often desirable, experimentally we find that satellite drops that are reabsorbed by the main drop while in flight do not appear to impact the jettability. Therefore, for a
Figure 2. Droplet mass (a), velocity (b), and momentum (c) plotted vs dwell time at a constant voltage for o-xylene at a 30% mass loading of gold nanoparticles. The nozzle pressure was adjusted to produce stable droplets.
jettable ink, any satellite drops or fluid tails must be reabsorbed to form in a single droplet within 1.3 mm of the nozzle tip, a typical working distance for inkjet printing. Under optimized dwell time conditions, the voltage was varied to cover the range over which stable jetting occurred to identify the velocity bounds for stable jetting (because the voltage directly affects the ejected drop velocity). Jettablility Window. The common dimensionless numbers relevant to drop formation dynamics are as follows: Reynolds number, Re =
ρ v 2d 2 ρvd inertial force = = viscous force ηvd η (1)
Weber number, We =
ρ v 2d 2 ρ v 2d inertial force = = σd σ surface tension (2)
C
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Figure 3. (a) Jettable window within the capillary number−Weber number parameter space. The solvent systems used to create the window are oxylene, tetralin, anisole, decanol, hexanol, octanol, and binary solvent mixtures of hexanol/octanol, octanol/decanol, decanol/cyclohexanol, and oxylene/tetrahydrofuran. (b) Drop stability breakdown mechanisms corresponding to the first four regions of the jettability window. (c) Multiple droplet breakups resulting from wavelike instability corresponding to the stability breakdown in region IV.
capillary number, Ca =
ηvd ηv viscous force = = surface tension σd σ
normalize the minimal effect of the surface tension force, highlight the impact of the viscous and inertial forces, and compare the jettability across ink systems. An experimentally elucidated jettability window in Ca−We space is shown in Figure 3a; each data point represents a jettable ink. Outside the jettable bounds, the absence of any data points indicates that no jettable inks were found. The solvent systems used to generate the space include pure solvents (o-xylene, tetralin, anisole, decanol, hexanol, and octanol) and binary solvent mixtures (hexanol/octanol, octanol/decanol, decanol/cyclohexanol, and o-xylene/tetrahydrofuran). As previously discussed, ρ and σ do not change drastically between solvent systems; thus, Ca and We are primarily determined by v and η. Each system, when plotted on a log−log scale, results in data that lies along a line with slope m = 1/2, in part because Ca is proportional to v and We is proportional to v2 and also because η in Newtonian systems is constant across all shear rates, i.e., jetting velocities. Because the data is normalized by surface tension, visually each parallel line represents an individual ink system operating at different velocities along the line, with more viscous systems
(3)
Z number, Z =
inertial force × surface tension viscous force
= =
ρv 2d3σ ηvd σρd η
(4)
where η, ρ, and σ are the viscosity, density, and surface tension of the ink and v and d are the droplet velocity and the nozzle diameter, respectively. The surface tension force does not vary significantly from one ink to the next for the inks considered in this work. Additionally, d is held constant, and ρ varies negligibly. As such, the inertial force is effectively dictated by v, which the pulse waveform controls. It has been suggested in the literature that viscosity is a key standalone measure of jettability.4 Thus, the jettability of an ink system should be governed primarily by the viscous and inertial forces. Therefore, we employ the Ca−We parameter space to investigate the impact of ink rheology on jettability. This space allows us to D
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occupying lines of higher Ca values. In addition, the length of the lines is dictated by the velocity range for a given system’s jettability. Thus, the sets of unloaded solvent data that together create the jettability window lie along parallel lines of Z number contours ordered from most viscous (highest Ca number) to least viscous (lowest Ca number) systems. Furthermore, the sloped nature of a given ink system also implies that if the system lies either above or below the jettability window it will continue to lie outside of the window, remaining unjettable for all velocities, because it will lie on a line parallel to the sloped bounds. In other words, we expect this jettability window to be universal and applicable as a basis for the definition of an ink design strategy. Literature data from Tai et al,.23 Tsai and Hwang,19 and Bogy and Talke18 (also plotted in Figure 3a) agree well with our findings. In mapping the jettability window, we observed several characteristic jetting stability breakdown mechanisms in the areas enumerated in Figure 3b,c. In the low-We regions, I and II, the amplitude of the pulse waveform is too low to eject a drop, so only a small perturbation in the meniscus occurs. In the high Ca and We region, III, the ink pillar that emerges from the nozzle upon ejection becomes relatively long prior to pinch off. One or more satellite drops then form because of Rayleigh instability along the length of the droplet tail. At low Ca and high We, as in region IV, the relatively inviscid fluid is subjected to a large inertial force, resulting in wavelike instability in the liquid thread at pinchoff, leading to multiple breakups and spraying behavior.35 Between regions III and IV, a local maxima is reached in the Weber number, when the fluid is viscous enough to withstand higher velocity without instabilities within the liquid thread but not viscous enough for the ink pillar to become excessively long and result in satellites droplets. These experimentally observed breakdown mechanisms, shown in Figure 3b,c, agree well with the results of Kim and Baek’s simulation-based analysis.24 Furthermore, it is interesting that the window spans Z values from approximately 1 to 60, encompassing the ranges suggested in the literature. Ink Design Considerations. At this point, having defined a rheological window over which inks are expected to be jettable, it is now possible to validate the defined window using loaded inks. We particularly focus on nanoparticle-loaded inks in this work given their tremendous technological importance. One of the most important aspects of an effective nanoparticle ink is the ability to achieve high mass loading (ratio of nanoparticle mass to solvent mass), which is a measure of solubility. High mass loading is desirable because it maximizes material deposition per drop, improving process throughput. A solvent capable of delivering high mass loading should be prioritized as the “base” solvent to ensure ink functionality. Afterward, several methods can be utilized to tune the jettability of the system (for example, to bring an ink that is not jettable into the jettable window). In the case of our gold nanoparticles, high mass loading has been demonstrated with hexane and α-terpineol. We test the viability of the observed jettability window by investigating moving nanoparticle-loaded inks in and out of the window. Specifically, we investigate altering the ink jettability by adding cosolvents and nanoparticles to the ink system. Both of these approaches change the rheological properties of the ink and can be used to test the universality of the jettability window bounds identified above. Because the volume fraction of nanoparticles in all inks employed herein is very low (
