Baekhoon Seong

Direct Alignment and Patterning of Silver Nanowires by Electrohydrodynamic Jet Printing

Highly aligned and patterned silver nanowires (Ag NWs) are investigated by using electrohydrodynamic (EHD) jet printing. Interaction between the flow field and the electric field as well as the mechanical stretching of the fiber jet can successfully align the Ag NWs inside the jet fiber. This technique can be applied in fabricating 1D nanostructures-based printed micro/nanoscale devices.

Directly printed stretchable strain sensor based on ring and diamond shaped silver nanowire electrodes

In this study, we fabricated a stretchable Silver nanowires (Ag NWs)/PDMS composite strain sensor with arbitrary micro-pattern electrodes using dispensing nozzle printing. In order to ensure a mechanically stable design, we proposed two types of electrodes: patterns of overlapped rings and diamonds. We also demonstrated that the electrical resistance could be modified according to the printing speed because the number of conductive fillers was proportional to the liquid ejection time. We also conducted static simulation for the two geometries to study the effect of the patterns when the strain sensor is stretched. We achieved highly stretchable strain sensor (up to 60% strain) with a suitable electrode design. Based on experimental results, it is expected that directly drawn electronic skin (E-skin) via the printing method can be fabricated with multifunctional sensing abilities in the near future.

3D printing of high-resolution PLA-based structures by hybrid electrohydrodynamic and fused deposition modeling techniques

Recently, the three-dimensional (3D) printing technique has received much attention for shape forming and manufacturing. The fused deposition modeling (FDM) printer is one of the various 3D printers available and has become widely used due to its simplicity, low-cost, and easy operation. However, the FDM technique has a limitation whereby its patterning resolution is too low at around 200 μm. In this paper, we first present a hybrid mechanism of electrohydrodynamic jet printing with the FDM technique, which we name E-FDM. We then develop a novel high-resolution 3D printer based on the E-FDM process. To determine the optimal condition for structuring, we also investigated the effect of several printing parameters, such as temperature, applied voltage, working height, printing speed, flow-rate, and acceleration on the patterning results. This method was capable of fabricating both high resolution 2D and 3D structures with the use of polylactic acid (PLA). PLA has been used to fabricate scaffold structures for tissue engineering, which has different hierarchical structure sizes. The fabrication speed was up to 40 mm/s and the pattern resolution could be improved to 10 μm.

Metal-mesh based transparent electrode on a 3-D curved surface by electrohydrodynamic jet printing

Invisible Ag mesh transparent electrodes (TEs), with a width of 7 μm, were prepared on a curved glass surface by electrohydrodynamic (EHD) jet printing. With a 100 μm pitch, the EHD jet printed the Ag mesh on the convex glass which had a sheet resistance of 1.49 Ω/. The printing speed was 30 cm s−1 using Ag ink, which had a 10 000 cPs viscosity and a 70 wt% Ag nanoparticle concentration. We further showed the performance of a 3-D transparent heater using the Ag mesh transparent electrode. The EHD jet printed an invisible Ag grid transparent electrode with good electrical and optical properties with promising applications on printed optoelectronic devices.

Deposition of Controllable Nanoparticles by Hybrid Aerodynamic and Electrostatic Spray

Generating uniform and size controllable nanoparticles are important for various analytical and industrial applications. Here, we propose the hybrid aerodynamic and electrostatic spray system with a large amount of size-controlled nanoparticles. The aerodynamic force easily produces a large amount of sprayed droplets and the electric field assists in secondary breakup of sprayed droplets. The charge of droplets was evaluated to understand the mechanism of controllable particle size because the size and uniformity of particles are determined by the distributions of droplets after evaporation. The distributions of particles are evaluated with a scanning mobility particle sizer (SMPS) technique and a field emission scanning electron microscope (FE-SEM) images. The size and aggregation of the particles are reduced and controlled by electric field strengths.

Self-Assembly of Silver Nanowire Ring Structures Driven by the Compressive Force of a Liquid Droplet

In a nanowire dispersed in liquid droplets, the interplay between the surface tension of the liquid and the elasticity of the nanowire determines the final morphology of the bent or buckled nanowire. Here, we investigate the fabrication of a silver nanowire ring generated as the nanowire encapsulated inside of fine droplets. We used a hybrid aerodynamic and electrostatic atomization method to ensure the generation of droplets with scalable size in the necessary regime for ring formation. We analytically calculate the compressive force of the droplet driven by surface tension as the key mechanism for the self-assembly of ring structures. Thus, for potential large-scale manufacturing, the droplet size provides a convenient parameter to control the realization of ring structures from nanowires.

One-Step Sub-micrometer-Scale Electrohydrodynamic Inkjet Three-Dimensional Printing Technique with Spontaneous Nanoscale Joule Heating

A one-step sub-micrometer-scale electrohydrodynamic (EHD) inkjet three-dimensional (3D)-printing technique that is based on the drop-on-demand (DOD) operation for which an additional postsintering process is not required is proposed. Both the numerical simulation and the experimental observations proved that nanoscale Joule heating occurs at the interface between the charged silver nanoparticles (Ag-NPs) because of the high electrical contact resistance during the printing process; this is the reason why an additional postsintering process is not required. Sub-micrometer-scale 3D structures were printed with an above-35 aspect ratio via the use of the proposed printing technique; furthermore, it is evident that the designed 3D structures such as a bridge-like shape can be printed with the use of the proposed printing technique, allowing for the cost-effective fabrication of a 3D touch sensor and an ultrasensitive air flow-rate sensor. It is believed that the proposed one-step printing technique may replace the conventional 3D conductive-structure printing techniques for which a postsintering process is used because of its economic efficiency.

Shaping characteristics in alginate hydrogel encapsulation via a two-fluid spraying method

Recently, an alginate hydrogel containing amphiphilic liposomes was suggested for application to an antifouling coating. In this study, we investigate the shaping characteristics of the alginate hydrogel by a spraying method for application to the coating. Sodium alginate droplets from the spraying nozzle are known to be differently shaped by several external forces during the encapsulation process when such a droplet impacts onto the surfaces of liquid containing calcium ions. We adopted a two-fluid spraying method for fast alginate droplets with a scalable size suitable for mass production. Various shapes, such as mushroom, petal, sphere, and thin-sheet alginate hydrogel shapes containing liposomes, were obtained via this experimental approach. Supported by a theoretical analysis, we determined regimes for the various shapes depending on the droplet size and the concentration of the sodium alginate solution. Alginate hydrogel of different shapes based on the regimes will be used for the coating throughout a different post-processing.

Biomimetic, Flexible, and Self-Healable Printed Silver Electrode by Spontaneous Self-Layering Phenomenon of a Gelatin Scaffold

Organic-inorganic hybrid layer-by-layer (LBL) composite structures can not only increase the strength and ductility of materials but also well disperse nanomaterials for better-conducting pathways. Here, we discovered the self-assembly process of an organic and silver (Ag) LBL hybrid structure having excellent sustainability during the long-term bending cycle. During the assembly process, the organic and Ag hybrid structure can be self-assembled into a layered structure. Unlike other conventional LBL fabrication processes, we applied the hydrogel scaffold of a biological polymer, which can spontaneously phase separate into an LBL structure in a water/alcohol solvent system. This new hydrogel-based Ag LBL patterns can successfully be printed on a flexible polyimide film without nozzle-clogging problem. Although these Ag LBL patterns cracked during the bending cycle, carbonized organic compounds between the Ag layers help to self-heal within few minutes at a low temperature (<80 °C). On the basis of our new hydrogel-based Ag ink, we could fabricate a fully printed reliable microscale flexible heater. We expect that our self-layering phenomenon can expand to the broad research field of flexible electronics in the near future.

Atomization of Liquid Via a Combined System of Air Pressure and Electric Field

Abstract. Conventional electrospray and air spray methods have the vulnerabilities of limited flow rate (throughput) and droplet size, respectively. Since high throughput with uniform size of droplet is required for various applications, an improved technique should be adopted. Here, we report a combined system of an air pressure and an electric field and evaluate the atomization performance of it. The air flow allowed applying high flow rate range and the electric field reinforced the atomization process to generate fine droplets. A correlation between two forces was investigated by comparing the droplet produced by each method. The atomized droplets were measured and visualized by image processing and a particle image velocimetry (PIV). The quantitative results were achieved from the parametric space and the effect of both forces was analyzed. The motion of charged droplets followed the outer electric field rather than the complex vortex in the shear layer so that the droplets accelerated directly toward the grounded collector.