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The determination of solid–liquid interfacial tension plays an important role in science and technology. Here, we propose a simple method to directly measure the contact angle between metal droplets and a graphite substrate for the determination of metal–graphite interfacial tension. The proposed method involves the synthesis of micro- and nanosized metal droplets on graphite by arc melting. Owing to its small volume, the rapid cooling of the prepared metal droplets on the graphite substrate leads to the freezing of equilibrium contact configuration after solidification. We observe that the measured contact angle between micro- and nanosized Au (or Ag) particles and the graphite substrate is almost size independent, even though the size of the particles synthesized herein is 1–3 orders of magnitude smaller than that studied in previous works. In addition, the interfacial tensions of Au and Ag on the step edges (edge plane) of graphite are found to be larger than that on the (0001) plane (basal plane). The proposed method provides a simple approach to determine the solid–liquid interfacial tension and may be effective in the study of interface related science and technology.Highlights: A simple method to directly measure the contact angle between metal droplets and graphite by the fast cooling of metal droplets on graphite. The contact angle between micro- and nanosized Au (Ag) particles and graphite is almost size independent. The contact angle and interfacial tension of Au (Ag) on the step edges of graphite are much larger than that on the basal plane of graphite. [ABSTRACT FROM AUTHOR]

We investigated the composition uniformity of InGaN epilayers in presence of metal droplets on the surface. We used Plasma Assisted MBE to grow an InGaN sample partially covered by metal droplets and performed structural and compositional analysis. The results showed a marked difference in indium incorporation between the region under the droplets and between them. Based on this observation we proposed a theoretical model able to explain the results by taking into account the vapour liquid solid growth that takes place under the droplet by direct impingement of nitrogen adatoms. [ABSTRACT FROM AUTHOR]

[Display omitted] • The occurrence state of Bi-Te-(S) phase minerals in the Dulanggou gold deposit can be divided into metal droplets and large patches. • Bi-rich metal droplets could be formed by dissociation from Bi-Te phase patches. • The gold in metal droplets has a higher purity than the gold in Bi-Te phase patches. There is a widely recognized association of bismuth (Bi)-tellurium (Te) minerals with gold in many hydrothermal gold deposits. However, the specific process for gold enrichment related to Bi-Te minerals remains enigmatic. The Dulanggou gold deposit, located on the southeastern margin of the Songpan-Garzê orogenic belt, China, provides an opportunity to research this process. Gold in the deposit is mainly associated with coarse-grained Bi-Te minerals and bismuth-rich polyphase metal droplets. Three mineralization stages are identified in the deposit. In Stage 1, a small amount of gold was deposited within dominate pyrrhotite. In Stage 2, a large amount of visible gold grains were precipitated with coarse-grained Bi-Te minerals. The various Bi-Te phases are mainly tsumoite, pilsenite, unnamed minerals (Bi 3 Te 2 , Bi 2 Te), joséite-B and trace hedleyite, which have a common characteristics of 2 ≥ Bi/Te > 1. In stage 3, around the Stage 2 large Bi-Te phase patches, bismuth-rich polyphase metal droplets pervasively occurred with Bi/Te ≥ 2. Other Bi-Te minerals in Stage 3 include hedleyite, unnamed minerals (Bi 3 Te, Bi 8 Te 3 , Bi 5 Te), joséite-B, and joséite-A, all intergrowth with gold and maldonite in the metal droplets. Mineral chemistry and phase diagrams of the Bi-Te mineral assemblages indicate that the mineralization temperatures at Stages 2 and 3 are > 325 ℃ and around 235 ℃ respectively, and the f Te 2 and f S 2 gradually decrease from Stage 2 to Stage 3. Importantly, a large amount of native gold was deposited with native Bi during Stage 3 and Bi contents in Bi-Te minerals increase with gold fineness from 799 ∼ 876 at Stage 2 to 833 ∼ 939 at Stage 3. These indicate that the metal droplets could be Bi-rich melts which were separated from Bi-Te phase patches and scavenged gold from earlier mineral assemblages to enhance gold enrichment. [ABSTRACT FROM AUTHOR]

In this paper, we report on a detailed experimental study carried out with the StarJet technology to investigate the mechanical adhesion properties of directly printed solder bumps on electroless nickel immersion gold (ENIG) plated PCB boards. The aim of this study is to determine the maximum bond strength achievable by this method and to find suitable printing parameters that allow for the production of reliable and consistent solder bumps by non-contact printing of molten solder (type SAC305). Molten solder droplets of about 250 μm diameter were printed at melt temperatures between 250 and 400 °C onto ENIG surfaces kept at temperatures in the range of 100 to 200 °C. Using shear force tests, the adhesion of the printed bumps was investigated as a function of the main process parameters: 1. printhead temperature, 2. substrate temperature, and 3. substrate preheating time. The formation of an intermetallic compound (IMC) between the solder and the ENIG was confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) measurements. As a result of the comprehensive experimental parameter study, suitable printing parameters for establishing bond strengths corresponding to maximum shear force values of 3000 to 4000 mN could be found, i.e. high printhead temperature of 400 °C, short preheating and time of

This article presents a design and development of a drop-on-demand (DOD) droplets generator. This generator uses molten metal as a liquid and can be used in fabrication, prototyping and any kind of printing with solder droplets. This setup consists of a vibrator solenoid with tunable frequency to produce a semi-spherical shape of molten metal, close to the surface of fabrication. This design also has a nozzle with micro-size orifice, a rod for transmitting force and a heater to melt the metal and keep it in superheat temperature. This DOD can produce droplets in different sizes (less than 550 µm) by controlling the vibration frequency of solenoid. This ability together with the accuracy of the droplets in positioning (the error is less than ±20 µm for 1.5 mm amplitude) can be used in different applications. Moreover, in this paper, the impact of initial position of the head and temperature on the average diameter of droplets and the impact of the frequency on the shape of the droplets have been tested and discussed

Formation of uniform traces is fundamental, but challenging, in droplet-based 3D printing. Due to the scalloped surface topography of metal droplets and complex thermal deformation between them, sequentially printed traces tend to form low-quality shapes. Here, an alternate droplet printing method is proposed to print uniform metal traces. Instead of printing droplets successively, discrete droplet array with uniform intervals was first printed. Then, the gaps between droplets were fully filled up in a later printing iteration. Herein, the proper combination of the step interval and the thermal state in the droplet printing process were first investigated to ensure the success of the proposed printing method. Four typical shapes of segments were identified using multiple three-droplet deposition experiments. Moreover, metal trace printing experiments further revealed five distinct trace topographies. In addition, the relationship between the topographies of printed traces and the printing parameters (i.e., temperature of the substrate and printing step size) was studied to establish a trace-shape map. Finally, experiments show that metal traces with the dimensionless roughness ( Ra/D d ) of 1.8% were successfully printed. The results demonstrate a significant reduction of the roughness comparing to the literature results printed in consecutive sequence. [ABSTRACT FROM AUTHOR]

This article presents a design and development of a drop-on-demand (DOD) droplets generator. This generator uses molten metal as a liquid and can be used in fabrication, prototyping and any kind of printing with solder droplets. This setup consists of a vibrator solenoid with tunable frequency to produce a semi-spherical shape of molten metal, close to the surface of fabrication. This design also has a nozzle with micro-size orifice, a rod for transmitting force and a heater to melt the metal and keep it in superheat temperature. This DOD can produce droplets in different sizes (less than 550 μm) by controlling the vibration frequency of solenoid. This ability together with the accuracy of the droplets in positioning (the error is less than ±20 μm for 1.5 mm amplitude) can be used in different applications. Moreover, in this paper, the impact of initial position of the head and temperature on the average diameter of droplets and the impact of the frequency on the shape of the droplets have been tested and discussed. [ABSTRACT FROM AUTHOR]

We present a novel pneumatic actuation system for generation of liquid metal droplets according to the so-called StarJet method. In contrast to our previous work, the performance of the device has been significantly improved: the maximum droplet generation frequency in continuous mode has been increased to fmax = 11 kHz (formerly fmax = 4 kHz). In addition, the droplet diameter has been reduced to 60 μm. Therefore, a new fabrication process for the silicon nozzle chips has been developed enabling the production of smaller nozzle chips with higher surface quality. The size of the metal reservoir has been increased to hold up to 22 mL liquid metal and the performance and durability of the actuator has been improved by using stainless steel and a second pneumatic connection to control the sheath flow. Experimental results are presented regarding the characterization of the droplet generation, as well as printed metal structures.

Laser-induced forward transfer is a direct-write process suitable for high precision 3D printing of several materials. However, the driving forces related to the ejection mechanism of the donor material are still under debate. To gain further insights into the ejection dynamics, this article presents results of a series of imaging experiments of the release process of nanosecond LIFT of a 200 nm thick gold donor layer. Images were obtained using a setup which consists of two dual-shutter cameras. Both cameras were combined with a 50× long-distance microscope and used to capture coaxial and side-view images of the ejection process. Bright field illumination of the scene was accomplished by a 6 ns dual-cavity laser source. For laser fluence just above the transfer threshold of 140 mJ/cm2, the formation of a jet and the subsequent release of a single droplet is observed. The droplet diameter is estimated to be about 2 µm. For laser fluences above 400 mJ/cm2 the formation and rupture of a blistering bubble is observed, which ultimately leads to an undesirable ejection of multiple droplets [ABSTRACT FROM AUTHOR]

We present a novel pneumatic actuation system for generation of liquid metal droplets according to the so-called StarJet method. In contrast to our previous work, the performance of the device has been significantly improved: the maximum droplet generation frequency in continuous mode has been increased to fmax = 11 kHz (formerly fmax = 4 kHz). In addition, the droplet diameter has been reduced to 60 µm. Therefore, a new fabrication process for the silicon nozzle chips has been developed enabling the production of smaller nozzle chips with higher surface quality. The size of the metal reservoir has been increased to hold up to 22 mL liquid metal and the performance and durability of the actuator has been improved by using stainless steel and a second pneumatic connection to control the sheath flow. Experimental results are presented regarding the characterization of the droplet generation, as well as printed metal structures. [ABSTRACT FROM AUTHOR]

Abstract: Within the scope of additive manufacturing of metal parts, a droplet generator for metal melts is proposed. Its magnetohydrodynamic (MHD) working principle is shown and the actuation force available for droplet generation is calculated. The design of such a printhead is shown to consist only of very few, non-moving parts and therefore it is of great simplicity. First results gained with a prototype printhead show that successful droplet generation is possible, but that pollution of the melt with oxides and sparks developing during the droplet generation process have to be overcome to fully exploit the potential of the proposed print head. [Copyright &y& Elsevier]

Abstract: In laser droplet generation from a metal wire a droplet is generated by laser heating of the wire-end. In the paper a problem of laser droplet sequence generation, which results in unsuccessful droplet detachment is considered. To solve this problem, a control system and strategy for laser droplet sequence generation are proposed. The control bases on in-process monitoring of the initial position and temperature of the wire-end are presented. The results show that by ensuring proper initial wire-end position and stationary wire-end temperature which can be influenced by laser pulse parameters, a reliable generation of a droplet sequence can be achieved. [Copyright &y& Elsevier]

• Molten metal droplets print recyclable and unencapsulated circuits. • The droplets partially embed plastic substrates to obtain good adhesion. • The droplets solidify timely to maintain the shape of printed patterns. • The pattern directly removes from the substrate and remelts for recycling. Recyclable electronics are important in green manufacturing and sustainable development, but in infancy, printing strategies still need more exploration and improvement. Metal drop-on-demand (DOD) printing is an ideal candidate for fabricating recyclable circuits, owing to its advantage of inherent direct recyclable feedstocks. Liquid metals with melting points at room temperature have been utilized to print circuits. However, developing liquid metal-based composites or encapsulation is still needed to maintain shapes of printed patterns and improve the adhesion between liquid metals and substrates, restricting the directly recycling of printed circuits. Herein, tin solder is utilized to directly print recyclable circuits via metal drop-on-demand printing method. The molten tin solder droplets directly melt plastic substrates and partially embed the substrate to obtain good adhesion. Molten droplets solidify timely to maintain the shapes of printed 2D conductive patterns and 3D microstructures without encapsulation. Moreover, the electrical conductivity of printed lines is close to the homogeneous bulk metal due to the direct metallurgical bonding between droplets. The conductive traces can be directly recycled by mechanically peeling from the substrate and remelting in the crucible. To validate this concept, a 555 circuit is successfully printed and recycled to fabricate a new 3D cross circuit. This work opens a new direction to directly fabricate recyclable circuits by using metal drop-on-demand printing. [ABSTRACT FROM AUTHOR]

• Solidification influences the dimensional accuracy of two overlapped droplets. • Solidification causes spread error and remelting causes retraction error. • Solidified layer thickness and remelting depth determine the length of droplets. • The length error of the two droplets is suppressed by the pinned contact line. Accurate deposition of metal droplets is crucial to limit geometrical defects in metal droplet-based 3D printing. However, unpredictable movements of the droplets displaced from the original landing location due to surface tension-driven flow before it is hindered by solidification, named landing error, lead to broken traces, irregular waved surfaces, or collapsed structures. In the present work, the effect of solidification at the early-time coalescence of two deposited metal droplets on the landing error was revealed. High-speed images were presented to observe coalescence dynamics. A volume of fluid (VOF) based numerical model was developed to analyze the generation mechanism of the landing error. An interesting phenomenon is observed that the second droplet absorbs more mass from the first droplet to form a bigger spherical crown with the increase of substrate temperature, resulting in the distinct final shapes. Inspired by this, a mass conservation model was proposed to predict the dependence of the landing error on substrate temperature. Metal droplets overlapping experiments were further conducted to verify the model and establish the relationship between the landing error and the substrate temperature. Predictions agreed well with experimental results. Finally, a strategy of increasing the thermal conductivity of the substrate to suppress the landing error at high substrate temperatures was proposed, which significantly expanded the processing window of the accurate deposition. This work can provide useful theoretical and experimental guidance for accurate printing of high-quality lines or patterns by utilizing metal droplets. [ABSTRACT FROM AUTHOR]

Although the superhydrophobic surfaces are attractive for various fields, the poor durability is still the main obstacle for their applications. In this study, a mechanically robust and multiscale structure NiCrZrN coating has been prepared by multi-arc ion plating. The as-deposited coating shows cauliflower-like hierarchical structure, related to the different size and shape of metal liquid droplets produced during deposition. Benefiting from the multiscale structure, the contact angle of coating surface is up to 135.6°. More importantly, the coating shows not only high mechanical properties but also superior abrasion resistance. The contact angle is still as high as 130° after 70 cycles using 1400 cm sandpaper abrasion test under the pressure of 10 kPa. By modifying with FAS, the contact angle and the rolling angle can reach 165.9° and ∼0°, respectively. It is expected that the hydrophobic NiCrZrN coating is a good candidate for industrial applications under harsh environment. Moreover, the flexible choice of deposition parameters during the novel multi-arc ion plating approach opens a new window to design and optimize the hydrophobic surface. • NiCrZrN coating deposited by Multi-arc ion plating with cauliflower-like structure. • The contact angle is still as high as 130° after 70 cycles sandpaper abrasion test. • The contact angle of NiCrZrN coating surface modified by FAS is up to 165.9°. [ABSTRACT FROM AUTHOR]

Droplet-based 3D printing is a promising technique for metal forming in space, owing to the advantages of dispensing without large-scale energy equipment or customized materials. However, the extreme environment in space, for example microgravity, would change the printing mechanisms significantly compared with those on the ground, which hinders the application of droplet-based 3D printing in space. To develop a droplet-based 3D printing technique suitable for space manufacturing, the droplet deposition behavior under microgravity should be physically simulated in normal gravity environment. Here, a novel experimental system comprising an anti-gravity electric field was developed to suppress the gravity effects on droplets deposition by manipulating the droplets to perpendicularly deposit on a vertical substrate. Parameter mappings of regulation voltage were established based on theoretical modeling and experimental results. The controllable region of flight trajectories locates in the top half of the electric field. The charging voltage shows better regulation accuracy than the deflection voltage. Under the manipulation of the electric field, droplets deposit on the substrate and solidify into regular morphologies. The solidified contour and adhesion force of the deposited droplets were measured. Finally, a freestanding pillar and a bending wall structure were fabricated. This work demonstrates the feasibility and effectiveness of the anti-gravity electric field in manipulating droplets to perpendicularly deposit on vertical surfaces against gravity, and forming 3D structures. Image 1 • A method to suppress the effects of gravity on droplet deposition is first proposed. • A parameter mapping for regulation voltage selection is established through theoretical modeling and experimental research. • The suppression of gravity on droplet deposition is realized via anti-gravity electric field. [ABSTRACT FROM AUTHOR]