Droplet Printing Research Articles

A new strategy for eliminating bottom hole defects during aluminum droplet printing within a broad temperature range

A new strategy for eliminating bottom hole defects during aluminum droplet printing within a broad temperature range

Droplet Printing Enabled by Cavity Collapse Ejection.

The behavior of cavity collapse in liquids is of fundamental importance in natural and industrial applications. It is still challenging to use the phenomenon of cavity collapse ejection in on-demand droplet printing technology. In this study, we investigate the cavity collapse ejection phenomenon in the submillimeter to millimeter scale and demonstrate that the cavity capillary energy is a critical factor affecting the state of the generated jet. Based on this phenomenon, we developed a droplet printing technology that can print nanoliter satellite-free droplets from a millimeter-sized nozzle, which reduces the risk of nozzle clogging. Using this printing technology, we demonstrated the printing of a nanoparticle suspension with 60% mass loading. Finally, we also showcased the printing of various inks for different applications using this technology, demonstrating the printability of cavity collapse-ejection printing technology in functional inks and showing potential to be applied in scenarios such as bioassays, the electronics industry, and additive manufacturing.

A high-precision automated liquid pipetting device with an interchangeable tip.

Liquid handling is a necessary act to deal with liquid samples from scientific labs to industry. However, existing pipetting devices suffer from inaccuracy and low precision when dealing with submicroliter liquids, which significantly affect their applications in low-volume quantitation. In this article, we present an automated liquid pipetting device that can aspirate liquid from microplates and dispense nanoliter droplets with high precision. Liquid aspiration is realized by using a micropump and a solenoid valve, and on-demand nanoliter droplet printing is realized by using a low-cost and interchangeable pipette tip combined with a piezoelectric actuator. Based on the microfluidic printing technology, the volumetric coefficient of variation of the dispensed liquid is less than 2% below 1 µl. A demonstration of concentration dilution for quantitative analysis has been successfully performed using the automated liquid pipetting device, demonstrating its potential in low-volume liquid handling for a wide range of biomedical applications.

All-aqueous printing of viscoelastic droplets in yield-stress fluids

All-aqueous printing of viscoelastic droplets (aaPVD) in yield-stress fluids is the core of an emerging voxelated bioprinting technology that enables the digital assembly of spherical bio-ink particles (DASP) to create functional tissue mimics. However, the mechanism of aaPVD is largely unknown. Here, by quantifying the dynamics of the whole printing process in real-time, we identify two parameters critical to aaPVD: (1) acceleration of print nozzle, and (2) droplet/nozzle diameter ratio. Moreover, we distinguish three stages associated with aaPVD: droplet generation, detachment, and relaxation. To generate a droplet of good roundness, the ink should be a highly viscous shear-thinning fluid. Using particle image velocimetry and scaling theory, we establish a universal description for the droplet displacements at various printing conditions. Along the direction of nozzle movement, the droplet displacement is determined by the detachment number, a dimensionless parameter defined as the ratio between the dragging force from the nozzle and the confinement force from the supporting matrix. Perpendicular to the direction of nozzle movement, the droplet displacement is determined by the Oldroyd number, a dimensionless parameter that describes the yielded area of the supporting matrix near the print nozzle. For a relaxed droplet, the droplet tail length is independent of droplet/nozzle diameter ratio but determined by the nozzle acceleration. We conclude that printing droplets of good fidelity requires a relatively large droplet/nozzle diameter ratio and intermediate nozzle accelerations. These ensure that the droplet is more solid-like to not flow with the nozzle to form a tadpole-like morphology and that the confinement force from the yield-stress fluid is large enough to prevent large droplet displacement. Our results provide the knowledge and tools for in situ generating and depositing highly viscoelastic droplets of good roundness at prescribed locations in 3D space, which help establish the foundational science for voxelated bioprinting. Analogues of pixels to two-dimensional (2D) pictures, voxels – in the form of small cubes or spheres – are the basic units of three-dimensional (3D) objects. All-aqueous printing of viscoelastic droplets (aaPVD) is the core of voxelated bioprinting, an emerging technology that uses spherical bio-ink voxels as building blocks to create 3D tissue mimics. Unlike existing technologies relying on the classic Rayleigh-Plateau instability to generate droplets, aaPVD exploits previously unexplored nonlinear fluid dynamics of complex fluids to precisely manipulate viscoelastic droplets in 3D space. The developed knowledge and tools not only help advance biomanufacturing but also stimulate new research directions in soft matter and complex fluids.

Prediction model and compensation method for curing shrinkage of inkjet 3D printing parts

Inkjet 3D printing has huge potential for applications in electronics printing, ceramic printing, and other fields, primarily due to the excellent dimensional accuracy of printed products, which is particularly important for device performance. However, in inkjet 3D printing, the fusion and solidification of droplets can cause the samples to shrink in various directions, resulting in a deviation from the designed target size, and subsequently affecting the performance of the part. In this study, the curing shrinkage behavior of parts in inkjet 3D printing is examined. First, a shrinkage model of the droplet element is established using the finite element method in accordance with the material parameters, and the linear shrinkage rates of different droplet groups are obtained. A curing shrinkage prediction model of the printed parts is then established, and the size and shape of the cured parts are predicted accordingly. A height calculation model of the printed parts is established according to the concept of volume conservation, and the height prediction of the printed parts is realized according to the number of printed layers and their resolution. An approach for compensation of the print model graphic layers is developed using the established prediction model. Experiments using two printing materials with different degrees of shrinkage are conducted to verify the feasibility of this method. The results indicate that the proposed curing shrinkage prediction model can seamlessly capture the curing shrinkage behavior of printed parts for materials with different shrinkages. Furthermore, after shrinkage, the model-based print compensation method achieves the measured sizes that are closer to the target size than that without compensation. The dimensions of inkjet 3D printed parts can be accurately controlled using this method.

Research on the printing mechanism of electrohydrodynamic satellite-free droplets in pulsed voltage

Electrohydrodynamic (EHD) drop-on-demand (DOD) printing in pulsed voltage is a very promising technology for additive manufacturing at the micro- and nanoscale. However, jet/meniscus retraction during DOD printing creates additional satellite droplets, which can lead to blurred print patterns and thus reduced print resolution. In this paper, the mechanism of satellite droplet formation in EHD DOD printing in pulsed voltage is investigated in detail for the first time. At first, the numerical model for EHD printing is established and the correctness and credibility of the numerical results are verified experimentally. After that, the three key operating parameters peak voltage, voltage duration, and bias voltage are analyzed in detail on the formation mechanism of satellite droplets. The operating phase diagram without satellite droplets is proposed. Finally, the electric Bond number BoE and Ohnesorge number Oh are proposed for the detailed analysis of the physical parameters of the liquid on the generation of satellite droplets, and the prediction model of satellite droplet size is presented. The mechanism of satellite droplet formation in EHD printing is revealed, which provides a theoretical basis for the stable printing of satellite-free droplets and advances its application in high-resolution additive manufacturing.

Chelate complexed multi-elemental printing performance of a small and cost efficient picoliter droplet printing device for micro preparation

We introduce an inexpensive modular picoliter pipetting device using modified commercially available thermal inkjet cartridges. The system is capable of reliable deposition of 65 elements. The printing solutions were modified to prevent the dissolution of Ni from the nozzle head, keeping those at a pH of >9. Loss of cations to the glass surfaces inside the nozzle head is prevented by forming anionic complexes with ethylenediaminetetraacetic acid in basic conditions. For some elements, e.g., Si, additives like tetramethylammonium hydroxide are necessary.The volumes of the droplets were obtained, weighing 30,000–360,000 droplets ejected by the printer using a laboratory scale. The droplet volume was within a range of 142–184 pL, depending on the formulation. The delivered elemental masses and the reproducibility were determined using total reflection X-ray fluorescence analysis and inductively coupled plasma optical emission spectrometry. Delivered masses were in the range of 0.98 pg – 11.9 pg per element and droplet.The shape of deposits of about 31 pg per element was studied using atomic force microscopy, optical microscopy, and confocal laser scanning microscopy. They were mostly spherical and ca. 14 μm in diameter and ca. 2 μm in height. The broad range of printable elements allows for the micro preparation of solid compounds for establishing a grazing incidence X-ray diffraction analysis library. As an example, the MgAl2O4 spinel was synthesized by printing of the formulation and subsequent calcination. Here we show its performance for the preparation of minerals in the context of recycling of critical elements from pyrometallurgical slags.

The role of resin in optimizing the performance and printing properties of water-based inkjet inks for food packaging

Purpose This study aims to investigate the influence of commercially available resins in water-based magenta pigment inkjet ink formulations on the properties of ink printability and the characteristics of ink application in food packaging. The impact of the resin on the jettability of the existing printability phase diagrams was also assessed. Design/methodology/approach Inks with different resin loadings were tested for selected properties, such as viscosity, particle size and surface tension. Stability was determined using a Turbiscan AGS turbidimeter and LumiFuge photocentrifuge analyzer. The ink layer fastness against abrasion and foodstuffs was evaluated using an Ugra device and according to PN-EN 646, respectively. JetXpert was used to assess Ricoh printhead jetting performance. Findings Printability diagrams successfully characterized the jettability of polyurethane inkjet inks on a multi-nozzle printhead and the binder improved droplet formation and printing precision. Originality/value Magenta water-based inkjet inks with commercial resins have been developed for printing on paper substrates. To the best of the authors’ knowledge, for the first time, inkjet ink stability was evaluated using the Turbiscan AGS and LumiFuge analyzers, and jettability models were verified using an industrial multi-nozzle printhead.

Programmable pulsed aerodynamic printing for multi-interface composite manufacturing

•Wide Z number soft material printing •Active programmable droplet preparation •Multi-interface composite manufacturing Droplet-based printing is increasingly popular for flexible sensors, soft robotics, bioprinting, and bioinspired structures. However, programmable multi-interface droplet printing with a wide Z number (reciprocal of the Ohnesorge number) remains challenging. We present a method, programmable pulsed aerodynamic printing (PPAP), for patterning soft materials with extensive material compatibility and multi-scale and multi-interface properties. PPAP utilizes programmable pulsed airflow to shear pendant droplets at the nozzle orifice, covering a broad range of parameter domains. Owing to its co-flow configuration, PPAP can accurately print droplets with shell-core, Janus, and combined morphologies. The multi-interface droplet size is independent of its physicochemical parameters and can be predicted by the scaling law. Demonstrations include multi-scale, multi-interface composites: cell-loaded annular particles, liquid metal circuits, laser stimuli-responsive microcapsules, calcium alginate elastomers, and 3D multi-interface clusters. PPAP may promote the development of micro-nano functional structures and devices. Droplet-based printing is increasingly popular for flexible sensors, soft robotics, bioprinting, and bioinspired structures. However, programmable multi-interface droplet printing with a wide Z number (reciprocal of the Ohnesorge number) remains challenging. We present a method, programmable pulsed aerodynamic printing (PPAP), for patterning soft materials with extensive material compatibility and multi-scale and multi-interface properties. PPAP utilizes programmable pulsed airflow to shear pendant droplets at the nozzle orifice, covering a broad range of parameter domains. Owing to its co-flow configuration, PPAP can accurately print droplets with shell-core, Janus, and combined morphologies. The multi-interface droplet size is independent of its physicochemical parameters and can be predicted by the scaling law. Demonstrations include multi-scale, multi-interface composites: cell-loaded annular particles, liquid metal circuits, laser stimuli-responsive microcapsules, calcium alginate elastomers, and 3D multi-interface clusters. PPAP may promote the development of micro-nano functional structures and devices.

High-Resolution, Multiplex Antibody Patterning using Micropillar-Focused Droplet Printing, and Microcontact Printing.

Antibody arrays have great implications in many biomedical settings. However, commonly used patterning methods have difficulties in generating antibody arrays with both high resolution and multiplexity, limiting their applications. Here, a convenient and versatile technique for the patterning of multiple antibodies with resolution down to 20µm is reported using micropillar-focused droplet printing and microcontact printing. Droplets of antibody solutions are first printed and stably confined on the micropillars of a stamp, and then the antibodies absorbed on the micropillars are contact-printed to the target substrate, generating antibody patterns faithfully replicating the micropillar array. The effect of different parameters on the patterning results is investigated, including hydrophobicity of the stamps, override time of the droplet printing, incubation time, and the diameters of the capillary tips and micropillars. To demonstrate the utility of the method, multiplex arrays of anti-EpCAM and anti-CD68 antibodies is generated to capture breast cancer cells and macrophages, respectively, on the same substrate, and successful capturing of individual cell types and enrichment among the cells are achieved. It is envision that this method would serve as a versatile and useful protein patterning tool for biomedical applications.

Prediction of Both E-Jet Printing Ejection Cycle Time and Droplet Diameter Based on Random Forest Regression.

Electrohydrodynamic jet (E-jet) printing has broad application prospects in the preparation of flexible electronics and optical devices. Ejection cycle time and droplet size are two key factors affecting E-jet-printing quality, but due to the complex process of E-jet printing, it remains a challenge to establish accurate relationships among ejection cycle time and droplet diameter and printing parameters. This paper develops a model based on random forest regression (RFR) for E-jet-printing prediction. Trained with 72 groups of experimental data obtained under four printing parameters (voltage, nozzle-to-substrate distance, liquid viscosity, and liquid conductivity), the RFR model achieved a MAPE (mean absolute percent error) of 4.35% and an RMSE (root mean square error) of 0.04 ms for eject cycle prediction, as well as a MAPE of 2.89% and an RMSE of 0.96 μm for droplet diameter prediction. With limited training data, the RFR model achieved the best prediction accuracy among several machine-learning models (RFR, CART, SVR, and ANN). The proposed prediction model provides an efficient and effective way to simultaneously predict the ejection cycle time and droplet diameter, advancing E-jet printing toward the goal of accurate, drop-on-demand printing.

Study on flow and heat transfer characteristics of 3D molten aluminum droplet printing process

The molten metal droplet printing technology can effectively realize a precise preparation of complex electronic devices at the micro scale by relying on the metallurgical bonding among the tiny molten metal droplets, which can improve the metallographic structure and mechanical properties of the fabricated parts. In this paper, the interaction process between metal droplets and movable substrates during the preparation of electronic devices is studied based on the coupled level set volume of fluid method (CLSVOF) and equivalent heat capacity method. The influence of the micro-bubbles and wall infiltration characteristics on the impingement spreading flow of metal droplets and their heat transfer cooling process is also investigated. The evolution mechanisms of droplet geometry and the distribution characteristics of droplet internal temperature and heat flow density during the impingement spreading process are discussed. The mechanism of high-temperature region formation under superhydrophobic wall conditions is analyzed. Results show that the microbubbles inhibit the spread of molten metal droplet. Under the same conditions, the spreading radius of hollow metal droplet is smaller than that of solid droplet, and their spreading height is higher than that of solid droplet. Different wall infiltration characteristics have similar degrees of influence on the spread of solid and hollow droplets. A larger wall contact angle for the same impingement mode at the same moment corresponds to a smaller droplet spreading radius and the higher the spreading height. With the participation of microbubbles, the heat transfer cooling process inside metal droplet becomes slow and demonstrates a less uniform temperature and heat flow density distribution. A high-temperature region is easily formed inside droplet under superlyophobic wall conditions, which triggers an uneven heat transfer cooling process at the bottom of the droplet. The results benefit to reveal the law of metal microdroplet impingement molding on different working surfaces and provide a theoretical reference for the preparation of electronics-related devices.

Stimuli-Triggered Multishape, Multimode, and Multistep Deformations Designed by Microfluidic 3D Droplet Printing.

Elastomers generally possess low Young's modulus and high failure strain, which are widely used in soft robots and intelligent actuators. However, elastomers generally lack diverse functionalities, such as stimulated shape morphing, and a general strategy to implement these functionalities into elastomers is still challenging. Here, a microfluidic 3D droplet printing platform is developed to design composite elastomers architected with arrays of functional droplets. Functional droplets with controlled size, composition, position, and pattern are designed and implemented in the composite elastomers, imparting functional performances to the systems. The composited elastomers are sensitive to stimuli, such as solvent, temperature, and light, and are able to demonstrate multishape (bow- and S-shaped), multimode (gradual and sudden), and multistep (one- and two-step) deformations. Based on the unique properties of droplet-embedded composite elastomers, a variety of stimuli-responsive systems are developed, including designable numbers, biomimetic flowers, and soft robots, and a series of functional performances are achieved, presenting a facile platform to impart diverse functionalities into composite elastomers by microfluidic 3D droplet printing.

Flexible droplet printing of prominently luminescent patterns of europium-doped yttrium oxide nanospheres

Flexible techniques for producing luminescent patterns with high-resolution and moderate efficiency at the microscale are potentially important for pattern display, customized anti-counterfeiting and data security. An innovative droplet-based aerosol jet printing procedure has been developed for the creation of prominently luminescent patterns of europium-doped yttrium oxide (Y2O3:Eu3+) inorganic phosphors in case of a rather low calcining temperature. Microscale luminescent patterns of pomegranate-shaped Y2O3:Eu3+ nanospheres were effectively created by synchronizing the droplet deposition behavior and the microreactions therein. The deposition temperature was crucial to the pattern development accompanying a morphological transformation from dense film to spherical nanostructures by providing an anisotropic evaporation regime. The red emission of the printed luminescent patterns was nearly identical to that of the equivalents calcined at 1000 °C, indicating the possibility for future applications in flat panel display systems.

Modification of Commercial 3D Fused Deposition Modeling Printer for Extrusion Printing of Hydrogels.

In this paper, we report a simple modification of a commercially available printer with fused deposition modeling (FDM) technology for the implementation of extrusion printing of hydrogels. The main difference between an FDM printer and a gel-extrusion printer is their material propulsion system, which has to deal with ether a solid rod or liquid. By application of plastic 3D printing on an FDM printer, specific details, namely, the plunger system and parts of the gel supply system, were produced and combined with a modified printer. Two types of printing of polymer hydrogels were optimized: droplet and filament modes. The rheological ranges suitable for printing for each method were indicated, and the resolution of the samples obtained and the algorithms for creating g-code via Python scripts were given. We have shown the possibility of droplet printing of microspheres with a diameter of 100 microns and a distance between spheres of 200 microns, as well as filament printing of lines with a thickness of 300-2000 microns, which is appropriate accuracy in comparison with commercial printers. This method, in addition to scientific groups, will be especially promising for educational tasks (as a practical work for engineering students or for the introduction of 3D printing into school classes) and industrial groups, as a way to implement 3D extrusion printing of composite polymer hydrogels in a time- and cost-effective way.

Printing Convertible Microstructures into Elastomer Matrix for Multiliquids Adaptive Information Encryption

AbstractAdaptive optical performance based on convertible microstructures is very useful for information encryption. However, the facile patterning of microstructures in reliable manner and the unclonable coding are still major challenges. The direct 3D printing of water droplets is reported as templates to polydimethylsiloxane (PDMS) precursor, followed by water evaporation in curing, which introduces convertible microcrease in the elastomeric matrix. The samples show optical response to liquids with the conversion of inner creases to ponds or cavities. The stochastic array of the resulted microstructures ensures the unclonability of the codes. Water‐responsive fluorescence emission is observed when a hydrosoluble dye is added to the microstructures, further enhancing the encryption safe level. Besides, the high stability of the PDMS matrix to ultraviolet irradiation and extreme temperature environment endows the material large application potentials in varied fields. This research provides a potential solution for facile, tough, and fundamentally safe information encryption.

Metal droplet printing of tube with high-quality inner surface via helical printing trajectory and soluble support

ABSTRACT Metal droplet-based 3D printing provides unique advantages for fabricating micro complex parts. Especially, assisted by soluble support, structures with high-quality inner surfaces can be directly printed without post-processing, which is very promising for the fabrication of waveguides and antenna horns. Here, a spatially distributed equidistant helical deposition strategy with only one-step positioning was proposed to improve the inner surface quality of the parts, and a five-axis motion stage was designed for matching its motion planning. The influence mechanisms of key process parameters on the forming quality were investigated. The droplet positioning errors, the aggregation behaviour, and the hole-defects formation on sloping surfaces were analyzed. As a proof-of-concept, a horn-structured tube was directly printed via the proposed printing method, which possessed both high-quality cavity surfaces and high density. This work paves the route towards the efficient additive manufacturing of metal tubes with high-quality inner surfaces.

Direct printing of surface-embedded stretchable graphene patterns with strong adhesion on viscous substrates

The graphene patterns with piezoresistance behaviors, originating from the structural deformation and band-structure shift under strain, represent an attractive characteristic for developing the small strain (<10%) sensors for wearable devices. However, the insufficient adhesion between the patterns and substrates has significantly limited their utility and reliability. Here, the surface-embedded graphene patterns were directly deposited on polydimethylsiloxane (PDMS) surfaces without hydrophilic treatment via the embedded droplet printing (EDP). The viscous PDMS films instead of the solid ones were used as substrates, and the graphene patterns were partly embedded onto the viscous PDMS surfaces. To assess the adhesion performance, a series of tests were performed. In the bending and tensile tests, the patterns strongly adhered to the PDMS films. Further, the patterns had a favorable increase in resistance in the tensile strain range of 0–3.5%. The resistance of the surface-embedded graphene patterns exhibited a negligible change for over 3 min in the ultrasonic bath. Finally, the relative resistance R/R0 remained constant after the first two adhesion tests using a 3M tape. The surface-embedded graphene patterns exhibited a strong adhesion to the flexible/stretchable substrates, indicating the application prospect in the field of flexible devices.

Strain-Engineered Adhesion and Reversible Transfer Printing of Water Droplets and Nanoparticles.

Transfer printing has been playing a crucial role in the fabrication of various functional devices. In spite of the extensive progress in technology, challenges are remaining, in the aspects of accuracy, efficiency, and adaptivity. Here, we propose a reversible transfer printing technique of tailoring adhesion by selectively stretching the surfaces. Through molecular dynamics simulations, we demonstrate the transfer of nanoscale substances such as water droplets, colloids, and nanoparticles between two graphene surfaces with strains switched on and off. We reveal the mechanism of the dynamic behaviors by analyzing the energies and driving forces of the substances during the process of transfer. The work not only advances the fundamental understanding of adhesion but also can inspire applications in the design of next-generation electronic and biomedical devices.

Flexible Droplet Printing of Prominently Luminescent Patterns of Europium-Doped Yttrium Oxide Nanospheres

Flexible Droplet Printing of Prominently Luminescent Patterns of Europium-Doped Yttrium Oxide Nanospheres