The demand for additive manufacturing is increasing along with interest in the field of 3D Printing technology. Laser-based sintering is a major fabrication method for complex three-dimensional parts and can make patterns with a width thinner than a human hair, without the need for masks. Laser-based processes, however, have some drawbacks, and a representative side effect is an indented shape owing to Marangoni flow—a phenomenon usually observed and studied in the field of inkjet printing technology, but which also occurs in laser processing technology. Marangoni flow can be calculated by droplet evaporation; Ristenpart et al. measured the relative thermal conductivities of the substrate and liquid to verify the effect on the direction of circulation in liquid evaporation.¹ Marangoni flow can be controlled by the selection of the liquid, enabling the production of precise patterns. The relationship between local heating and flow of sessile water was verified by Askounis et al.² who compared the resultant thermal patterns in terms of heating location, time, and heating power. Local heating produces four stages of thermal flow in the droplet; the flow finally converged to non-directional flow regardless of the conditions, which means that the fabricated pattern is affected more by materials than by the process. The Marangoni flow direction has been investigated in inkjet processes by Li et al., according to solvent and process parameters.³ Their research revealed that drop spacing and Z number of ink for inkjet printing determines the morphology of the dried pattern. Moreover, ethylene glycol was found to countervail the outward Marangoni flow. Hu et al. applied recirculating Marangoni flow to prevent the “Coffee Ring” effect, wherein particles are drawn from the inside to outside according to the drying time in solution.⁴ A uniform pattern was fabricated using a solvent with a high boiling point, which reinforces recirculation of Marangoni flow without binder material. Thokchom et al. researched how Marangoni flow in influenced by particle concentration, which affects the contact angle of the droplet.⁵ Specifically, when the solution has high particle concentration, the surface tension of the solution decreases because the low surface energy of the liquid–solid interface. They also investigated the effect of the substrate temperature, which determines the diameter of the evaporated droplet. Elashnikov et al.applied Marangoni flow in laser machining to make valleys that connected to each other, making a specific pattern on the substrate.⁶ On the other hand, Chung et al. found that in laser sintering, Marangoni flow occurs when the laser scan speed is faster than the evaporation of the solvent and the sintering of the nanoparticles, and that it has an adverse effect on the sintered pattern.⁷ Therefore, the effect should be controlled properly to prevent indents forming in the pattern of sintered nanoparticles, which obstruct the deposition of a dense pattern.

In the aforementioned studies, Marangoni flow was studied in various ways according to material and process, and it was found that Marangoni flow predominantly depends on the material properties and heating conditions. In spite of broad research on Marangoni flow, material dependency has not yet been revealed in laser sintering. Hence, this study focuses on nanoparticle laser sintering to analyze Marangoni flow with regard to nanoparticle concentration and heating conditions.

Laser nanoparticle sintering has been studied since 2004 using gold nanoparticles. Since then, silver, copper, and nickel nanoparticles have been applied to make electrode patterns with high electrical conductivity and stable structural properties. The process has evolved enough to fabricate high-quality patterns, but the nanoparticles show limits such as corrosion in storage and surface oxidation after sintering. Recently, copper and nickel oxides have been applied to satisfy the shortcomings in previous research. However, this still does not solve the side effects of fluidic motions such as Marangoni flow—which causes irregular pattern cross section and causes not only high electrical resistivity but also easy deterioration. For these reasons, Marangoni flow is analyzed herein with regard to its effect on the microstructure of the pattern according to the laser source and blending conditions of the metal oxide nanoparticle solution. Subsequently, the optimal conditions for producing highly conductive electrical patterns on a substrate are discussed.

Nanoparticles were synthesized by a hydrothermal process. Deionized (DI) water was heated to its boiling temperature, and dissolved copper ions were reacted with reduction agents. During this process, the copper ions reduce to Copper(II) oxide and then to Copper(I) oxide. The Copper(I) oxide nanoparticles were washed several times until clean and then diluted with an alcohol solvent and a surfactant. Samples were prepared by coating the solution onto a glass substrate using a spin coater, followed by laser sintering, which varied according to the laser source and concentration of the compound in solution, as shown in Fig. 1 The laser sources used were infrared (IR) Continuous Wave (CW) and pulsed lasers. The pulsed laser can be tuned in terms of pulse width, power, and repetition rate. After the laser sintering process, the fabricated copper electrode patterns were observed by Scanning Electron Microscopy (SEM). The SEM images show nanoparticles with different densities, shapes, and degrees of sintering. Oxidation and deterioration was not considered which was analyzed in previous study⁸ and the process was verified that organic layer protects the electrode pattern with reliable performance.

Fig. 1 
Schematic of copper(I) oxide nanoparticle sintering system

In this study, laser sintered copper electrode patterns were analyzed in terms of Marangoni flow and microstructure, according to nanoparticle density and laser irradiation. The surface tension gradient and temperature gradient are major factors in determining Marangoni flow—the flow dominates when the concentration of nanoparticles is low, and the laser power is relatively high. The surface tension of the low-weight-percent nanoparticle solution can change drastically according to temperature, and the high-power irradiation causes rapid temperature change. When the laser power is too low, the process leaves residual unsintered or weakly sintered nanoparticles in the pattern.

Marangoni flow is generated by both weak and strong laser irradiation when the nanoparticle density in solution is low. It is experienced by the nanoparticle solution as the solvent evaporates when the laser power is weak and by the nanoparticle during sintering when the laser power is strong. Hence, the nanoparticle solution should have a high concentration of nanoparticles to prevent Marangoni flow.

Laser power density is another major factor that determines the quality of the process. A short pulse duration causes non-uniform pattern width, but narrower patterns can be made than with a longer pulse duration. A long pulse duration enables high heat conduction, not only to the nanoparticle solution but also to the substrate. This helps to make wide, even patterns, but it also causes thermal damage to the substrate. Further research is needed to clarify the interaction between high-density nanoparticle solutions and ultra-short-pulse lasers.