Inkjet Droplets Research Articles

Predicting inkjet dot spreading and print through from liquid penetration- and picoliter contact angle measurement

AbstractIn this study we have evaluated the suitability of laboratory testing methods to predict inkjet printing results. We have developed and used testing liquids that are spanning the operational window of industrial High Speed Inkjet (HSI) printers while still covering the maximum possible range of viscosity and surface tension. First we correlated liquid penetration measured with ultrasound (ULP) and direct absorption (ASA) to print through from HSI prints. The best correlation (R2≈0.7{R^{2}}\approx 0.7) was found for the sized paper. For papers with increasing liquid penetration speed we found a decreasing ability of both testing methods to predict print through, for the strong absorbing paper the correlation drops toR2≈0.2{R^{2}}\approx 0.2. Second we correlated contact angle and drop diameter to the dot area from HSI prints. Contact angle turned out to be a better predictor for printed dot area than drop diameter. Evaluating the change in contact angle over time we found the highest correlation to the dot area in the print when measuring the contact angle as soon as possible, in our case 1 ms after deposition of the drop on the paper. We also compared contact angle with microliter drops to picoliter drops, which are in the size scale of the actual inkjet droplet. To our great surprise correlations for microliter drops were equal or better than for picoliter drops, particularly for highly absorbing papers. Thus in order to predict dot spreading on paper our results suggest to measure the contact angle with microliter drops. Overall we found that, using laboratory testing methods, print through and dot spreading for HSI printing can be quite well predicted for slow absorbing papers but not very well for fast absorbing papers.

Direct-Write Bioprinting Approach to Construct Multilayer Cellular Tissues.

As a cellular-assembly technique, bioprinting has been extensively used in tissue engineering and regenerative medicine to construct hydrogel-based three-dimensional (3D) tissue-like models with prescribed geometry. Here, we introduced a unique direct-write bioprinting strategy to fabricate a bilayer flat tissue in a hydrogel-free approach. A printed retina pigmented epithelium layer (RPE) was applied as living biopaper for positioning a fibroblast layer without using any hydrogel in bioink. We adjusted the number of cells in the inkjet droplets in order to obtain a uniform printed cell layer and demonstrated the formation of a bilayer construct through confocal imaging. Since our printing system introduced low levels of shear stress to the cells, it did not have a negative effect on cell survival, although cell viability was generally lower than that of control group over 1 week post-printing. In conclusion, our novel direct-write bioprinting approach to spatiotemporally position different cellular layers may represent an efficient tool to develop living constructs especially for regeneration of complex flat tissues.

Numerical Simulation of Inkjet Droplet Spreading with the Consideration of the Relationship between Dynamic Contact Angle and Contact Line Velocity

Numerical Simulation of Inkjet Droplet Spreading with the Consideration of the Relationship between Dynamic Contact Angle and Contact Line Velocity

Real time observation of binder jetting printing process using high-speed X-ray imaging

A high-speed synchrotron X-ray imaging technique was used to investigate the binder jetting additive manufacturing (AM) process. A commercial binder jetting printer with droplet-on-demand ink-jet print-head was used to print single lines on powder beds. The printing process was recorded in real time using high-speed X-ray imaging. The ink-jet droplets showed distinct elongated shape with spherical head, long tail, and three to five trailing satellite droplets. Significant drift was observed between the impact points of main droplet and satellite droplets. The impact of the droplet on the powder bed caused movement and ejection of the powder particles. The depth of disturbance in the powder bed from movement and ejection was defined as interaction depth, which is found to be dependent on the size, shape, and material of the powder particles. For smaller powder particles (diameter less than 10 μm), three consecutive binder droplets were observed to coalesce to form large agglomerates. The observations reported here will facilitate the understanding of underlying physics that govern the binder jetting processes, which will then help in improving the quality of parts manufactured using this AM process.

A Low-Cost Flash Photographic System for Visualization of Droplets in Drop-on-Demand Inkjet

Flash photography has been widely used to study the droplet dynamics in drop-on-demand (DoD) inkjet due to its distinct advantages in cost and image quality. However, the whole setup, typically comprising the mounting platform, flash light source, inkjet system, CCD camera, magnification lens and pulse generator, still costs tens of thousands of dollars. To reduce the cost of visualization for DoD inkjet droplets, we proposed to replace the expensive professional pulse generator with a low-cost microcontroller board in the flash photographic system. The temporal accuracy of the microcontroller was measured by an oscilloscope. The microcontroller’s temporal stability was compared with a professional pulse generator by tracking a large number of droplet positions. To validate the effectiveness of the whole setup, the droplet ejection and the droplet impact on a silicon wafer were quantitatively analyzed and compared with theoretical predictions. Finally, sample images of droplet ejected from a commercial inkjet cartridge were presented to show the flexibility of the system.

One Dimensional Model for Droplet Ejection Process in Inkjet Devices

In recent years, physics-based computer models have been increasingly applied to design the drop-on-demand (DOD) inkjet devices. The initial design stage for these devices often requires a fast turnaround time of computer models, because it usually involves a massive screening of a large number of design parameters. Thus, in the present study, a 1D model is developed to achieve the fast prediction of droplet ejection process from DOD devices, including the droplet breakup and coalescence. A popular 1D slender-jet method (Egger, 1994) is adopted in this study. The fluid dynamics in the nozzle region is described by a 2D axisymmetric unsteady Poiseuille flow model. Droplet formation and nozzle fluid dynamics are coupled, and hence solved together, to simulate the inkjet droplet ejection. The arbitrary Lagrangian–Eulerian method is employed to solve the governing equations. Numerical methods have been proposed to handle the breakup and coalescence of droplets. The proposed methods are implemented in an in-house developed MATLAB code. A series of validation examples have been carried out to evaluate the accuracy and the robustness of the proposed 1D model. Finally, a case study of the inkjet droplet ejection with different Ohnesorge number (Oh) is presented to demonstrate the capability of the proposed 1D model for DOD inkjet process. Our study has shown that 1D model can significantly reduce the computational time (usually less than one minute) yet with acceptable accuracy, which makes it very useful to explore the large parameter space of inkjet devices in a short amount of time.

Inkjet droplet deposition dynamics into square microcavities for OLEDs manufacturing

The dynamics of inkjet deposition in square microcavities are investigated utilizing a three-dimensional multi-relaxation-time pseudopotential lattice Boltzmann (LB) model with large density ratios. A geometric scheme is considered within the pseudopotential LBM framework to obtain the desired contact angles. The effects of wettability, density ratios, droplet viscosity and impact velocity are explored to reveal the droplet–microcavity interactions. With the contact angles of microcavity increasing, the physical outcomes including the crown-like shape with a small round dot, circular hollow core, uniform film and convex film are identified and analyzed. At a lower density ratio ρr = 11.6, the surrounding denser gas resists the droplet recoiling flow resulting in an increasing hollow core. The appropriate higher droplet viscosity and decreasing impact velocity are preferred which could eliminate the hollow core in the recoiling phase and accelerate the inkjet deposition process straightforward. The revelation of droplet-microcavity dynamics is beneficial for optimizing inkjet deposition process and fabricating uniform OLEDs panels.

Interfacial Self‐Assembly of Colloidal Nanoparticles in Dual‐Droplet Inkjet Printing

AbstractThe well‐known coffee‐ring effect causes colloidal particles to convectively transport toward the contact line of an inkjet droplet leading to a nonuniform deposition of the colloidal particles. In this work, the self‐assembly of colloidal particles in a dual‐droplet inkjet printing configuration to produce a nearly monolayer closely packed deposition of colloidal particles that exhibits a colorful reflection are demonstrated. By controlling the ink surface tensions and jetting parameters, the wetting droplets (the second droplet) containing colloidal polystyrene (PS) nanoparticles quickly spread over the supporting droplets (the first droplet) upon impact. The well‐ordered deposition is achieved by tuning the solvent composition of the wetting droplets and functionalization of the PS nanoparticles to encourage a network formation among the colloidal particles at the air–droplet interface. The underlying self‐assembly mechanism is insensitive to substrates and can be applied to many other material/substrate combinations. Finally, the origin of the color generation is highlighted, and the potential of the dual‐droplet inkjet printing process is discussed for fabrication of optical devices.

Conical Refraction Bottle Beams for Entrapment of Absorbing Droplets

Conical refraction (CR) optical bottle beams for photophoretic trapping of airborne absorbing droplets are introduced and experimentally demonstrated. CR describes the circular split-up of unpolarised light propagating along an optical axis in a biaxial crystal. The diverging and converging cones lend themselves to the construction of optical bottle beams with flexible entry points. The interaction of single inkjet droplets with an open or partly open bottle beam is shown implementing high-speed video microscopy in a dual-view configuration. Perpendicular image planes are visualized on a single camera chip to characterize the integral three-dimensional movement dynamics of droplets. We demonstrate how a partly opened optical bottle transversely confines liquid objects. Furthermore we observe and analyse transverse oscillations of absorbing droplets as they hit the inner walls and simultaneously measure both transverse and axial velocity components.

Thermal bubble inkjet printing of water-based graphene oxide and graphene inks on heated substrate

Stable-jetting water-based graphene oxide (GO) and graphene (GR) inks without any surfactant or stabilizer are prepared from an unstable-jetting water-based starting solvent, with many thermal bubble inkjet satellite drops, by simply increasing the material concentration. The concentration-dependent thermal bubble inkjet droplet generation process is studied in detail. To overcome the low concentration properties of water-based thermal bubble inkjet inks, the substrate temperature is tuned below 60 °C to achieve high-quality print lines. Due to the difference in hydrophilicity and hydrophobicity of the 2D materials, the printed GO lines show a different forming mechanism from that of the GR lines. The printed GO lines are reduced by thermal annealing and by ascorbic acid, respectively. The reduced GO lines exhibit electrical conductivity of the same order of magnitude as that of the GR lines.

Encapsulated silver nanoparticles in water/oil emulsion for conductive inks

Abstract Silver nanoparticle (AgNP) inks are the most commonly used conductive ink for printed conductive tracks. However, the ink stability against aggregation and sedimentation is poor and results in short shelf life, severe nozzle clogging, and bad printing qualities. Herein, a new method is developed to improve the stability of AgNP inks. In this work, a water-in-oil type emulsion is used as a carrier for water-based silver nanoparticles. Different water/surfactant ratio are examined for the complete AgNP uptake in water nano droplets. The morphology of AgNP after emulsification is carefully examined with electro-microscopy to elucidate the particle engulfment in water. The encapsulated AgNP show a much lower sedimentation rate due to the greatly reduced overall density of encapsulated AgNPs/water droplets. Moreover, the water/oil interfaces around AgNPs prevent aggregations, and thus leads to better long-term ink stability. The physical properties of AgNP emulsion meet the requirement of the printer and stable inkjet droplet can be produced. Conductive tracks with electrical resistance 4.92 μΩ-cm and line width 170 µm can be printed on photo papers, which demonstrates the potential of the emulsion for inkjet printing application.

On International Standards for In-flight Measurements of Inkjet Drops

The first International Electrotechnical Commission (IEC) standards for vision-based measurements of inkjet droplet speed and volume from inkjet print-heads were recently prepared for publication by the Technical Committee working group WG3 for Printed Electronics equipment. These IEC/ISO standards could be more widely adopted for other inkjet printing applications and establishing the relative performance of drop-on-demand (DoD) inkjet print-heads and inks. The underlying behavior of inkjet drops and effects influencing these standards for in-flight drop measurement methods are discussed.

Quantification of Marangoni flows and film morphology during solid film formation by inkjet printing

We visualized experimentally the internal flow inside inkjet droplets of polystyrene–anisole solution during solid film formation on substrates at room temperature. The effects of contact angle and evaporation rate on the internal flow and film morphology were quantitatively investigated. The transport process during film formation was examined by measuring the relationship between internal flow and film morphology, which provided three remarkable findings. First, self-pinning and the strength of outward flow on the free surface under 2.3 Pa s determined film morphology. The solute distribution, corresponding to rim areas in ring-like films and a convex trough in dot-like films, had already developed at self-pinning. Second, the mass fraction at self-pinning close to the contact line converged to one, regardless of the film morphology. This implies that self-pinning is independent of parameters such as the contact angle and evaporation rate. Third, at room temperature, the solutal Marangoni numbers were 20–30 times larger than the thermal ones. Thus, the outward flow on the free surface caused by the solutal Marangoni effect dominates in droplets before self-pinning. The solutal Marangoni number at self-pinning and thickness variation at the center of the film displayed a good relationship for droplets with different contact angles and evaporation rates. This suggests that film morphology can be technically controlled by solutal Marangoni number at room temperature.

A disposable flexible humidity sensor directly printed on paper for medical applications

The present study demonstrates an inkjet – printed interdigitated electrode array on paper substrate and its evaluation as humidity sensor. Inkjet droplet formation analysis has been performed in order to achieve repeatable results regarding generated droplets, based on the driving pulses applied on the inkjet piezoelectric element. Droplet formation has been monitored using stroboscopic effect. Three different paper substrates, namely high glossy inkjet photo paper, glossy inkjet photo and matte inkjet photo paper have been evaluated to investigate compatibility with the ink. Relative humidity measurements have been carried out in a controlled environment. Material degradation, long term response and memory effect are some of the aspects which were studied within the frame of the present work. The proposed sensor provides the opportunity for novel biomedical applications given the flexible substrate nature and the low – cost, single – step fabrication approach.

Dynamic behavior with rapid evaporation of an inkjet water droplet upon collision with a high-temperature solid above the limit of liquid superheat

The dynamic behavior of the rapid evaporation of an inkjet droplet of water upon collision with a high-temperature silicon substrate has been investigated experimentally. The colliding droplet spreads into a thin disk-shaped liquid film and then splashes away. The splashing behavior depends on the solid temperature, which varies significantly due to nucleation and evaporation while spreading along the solid surface. Even when the temperature of the contact interface of the substrate, as estimated by transient heat conduction, significantly exceeds the kinetic limit of liquid superheat, numerous fine bubbles are simultaneously generated, presumably by spontaneous nucleation, beneath the liquid film and break up the film, which is dispersed into fine droplets. The surface temperature at the collision point is measured using a small platinum film sensor and exhibits a significant decrease compared to the heat conduction to the liquid for this peculiar nucleation.

Study of Layer Formation During Droplet-Based Three-Dimensional Printing of Gel Structures

Additive manufacturing, also known as three-dimensional (3D) printing, is an approach in which a structure may be fabricated layer by layer. For 3D inkjet printing, droplets are ejected from a nozzle, and each layer is formed droplet by droplet. Inkjet printing has been widely applied for the fabrication of 3D biological gel structures, but the knowledge of the microscale interactions between printed droplets is still largely elusive. This study aims to elucidate the layer formation mechanism in terms of the formation of single lines and layers comprised of adjacent lines during drop-on-demand inkjet printing of alginate using high speed imaging and particle image velocimetry. Inkjet droplets are found to impact, spread, and coalesce within a fluid region at the deposition site, forming coherent printed lines within a layer. The effects of printing conditions on the behavior of droplets during layer formation are discussed and modeled based on gelation dynamics, and recommendations are presented to enable controllable and reliable fabrication of gel structures. The effects of gelation on droplet impact dynamics are found to be negligible during alginate printing, and interfaces are found to form between printed lines within a layer depending on printing conditions, printing path orientation, and gelation dynamics.

Comparative Influences of Fluid and Shell on Modeled Ejection Performance of a Piezoelectric Micro-Jet

The piezoelectric micro-jet, which can achieve the drop-on-demand requirement, is based on ink-jet technology and small droplets can be ejected out by precise control. The droplets are driven out of the nozzle by the acoustic pressure waves which are generated by the piezoelectric vibrator. The propagation processes of the acoustic pressure waves are affected by the acoustic properties of the fluid and the shell material of the micro-jet, as well as the excitations and the structure sizes. The influences of the fluid density and acoustic velocity in the fluid on the nozzle pressure and support reaction force of the vibrator are analyzed in this paper. The effects of the shell material on the ejection performance are studied as well. In order to improve the ejection performance of the micro-jet, for ejecting a given fluid, the recommended methods of selecting the shell material and adjusting excitations are provided based on the results, and the influences of the factors on working frequencies are obtained as well.

Ultrafast imaging method to measure surface tension and viscosity of inkjet-printed droplets in flight

In modern drop-on-demand inkjet printing, the jetted droplets contain a mixture of solvents, pigments and surfactants. In order to accurately control the droplet formation process, its in-flight dynamics, and deposition characteristics upon impact at the underlying substrate, it is key to quantify the instantaneous liquid properties of the droplets during the entire inkjet-printing process. An analysis of shape oscillation dynamics is known to give direct information of the local liquid properties of millimeter-sized droplets and bubbles. Here, we apply this technique to measure the surface tension and viscosity of micrometer-sized inkjet droplets in flight by recording the droplet shape oscillations microseconds after pinch-off from the nozzle. From the damped oscillation amplitude and frequency we deduce the viscosity and surface tension, respectively. With this ultrafast imaging method, we study the role of surfactants in freshly made inkjet droplets in flight and compare to complementary techniques for dynamic surface tension measurements.

Evaporation of an inkjet droplet on a flat substrate

Understanding the evaporation behavior of inkjet droplets has become increasingly important as printed electronic technology develops. In this study, the evaporation phenomena of a 5–6-pL droplet were studied. Four types of non-polar liquid droplets were prepared via the inkjet method and placed on substrates with small contact-angle hystereses. The observed contact radius and contact angle during evaporation were in good agreement with the theoretical model. This model, that of diffusion-controlled evaporation, was obtained based on a microliter droplet or bulk liquid wherein the evaporation was considered to be a quasi-steady state. The square of contact radius R decreased linearly with the evaporation time t, and the gradient of the R2 vs t plot provided the diffusivity of vapor in the air. The experimentally obtained diffusivity values were helpful for estimating the evaporation speeds and times of droplets at any contact angle.

Fabrication of Bendable Circuits on a Polydimethylsiloxane (PDMS) Surface by Inkjet Printing Semi-Wrapped Structures.

In this work, an effective method was developed to fabricate bendable circuits on a polydimethylsiloxane (PDMS) surface by inkjet printing semi-wrapped structures. It is demonstrated that the precured PDMS liquid film could influence the depositing morphology of coalesced silver precursor inkjet droplets. Accordingly, continuous and uniform lines with a semi-wrapped structure were fabricated on the PDMS surface. When the printed silver precursor was reduced to Ag nanoparticles, the fabricated conductive film exhibited good transparency and high bendability. This work presented a facile way to fabricate flexible patterns on a PDMS surface without any complicated modification or special equipment. Meanwhile, an in situ hydrazine reduction of Ag has been reported using the vapor phase method in the fabricating process.