## 3 Results and Discussion  
The process for fabricating the micro-porous Pd-Ni membrane filter consists of two steps: (1) the lithography process; and (2) the electroplating process, as shown in Figure 2. The lithography process was used to generate standard column-like patterns, an important process that determines the position of the filter pore and the size of the whole filter. This was followed by the electroplating process, which involved an electrochemical reaction to form a thick palladium-nickel alloy layer above the brass substrate. The required fine-scale specification was achieved using a similar technique that employs piezoelectric inkjet print heads. Figure 2: Schematic diagrams of micro-porous membrane filter fabrication process. Prior to measurements, images of each part of the Pd-Ni membrane filter were defined to confirm the direction in which aerosols were moving. The direction of entrance to the Pd-Ni membrane filter was the \"inlet\", and the direction of exiting to the outside was the \"outlet\", as shown in Figure 3a. The size of the spray droplets was mainly determined by the size of the filter pore. Therefore, the filter plate was an important part of the micro-nebulizer. The morphologies, structure, and size of the micro-porous Pd-Ni membrane filter were investigated using FESEM and optical microscopy. The diameter of the membrane filter was 15 mm and was circular in shape, and this membrane filter had 6517 nozzle pores, which were distributed in an area of 28.3 mm\({}^{2}\) (Figure 3a). High-magnification images (Figure 3c,e) indicate that the membrane filter surface exhibited a clean and clear morphology with electrodeposited particles being well separated and homogeneously distributed over the surface. Figure 3b,c shows images of the filter inlet, which had a curved, funnel-shaped structure. The average size of an inlet pore was 4.7 \(\upmu\)m with a distribution of \(\pm\)0.20 \(\upmu\)m. On the other hand, Figure 3d,e shows images of the filter outlet, which had a flat shape. In the outlet direction, a single filter had a 50 \(\upmu\)m diameter boundary and a 4.2 \(\pm\) 0.15 \(\upmu\)m diameter outlet pore. In addition, the pitch size between two hole centers is 70 \(\upmu\)m. Precise control of the electroplating process enabled the deviation in the pore diameters of both the inlet and outlet pores to be maintained at less than 0.5 \(\upmu\)m.  
The cross-sectional morphology of the electroformed membrane filter showed a thickness of approximately 41 \(\upmu\)m (Figure 4a). The filter could precisely control the electro-deposition thickness in the electroplating process, with a total thickness variation of less than 5%. Therefore, this electrochemical method could achieve a filter pore dimension deviation of less than 2 \(\upmu\)m.  
The composition of the Pd-Ni alloy was evaluated using EDS analyses to examine the composition of the fabricated Pd-Ni alloy filter. Figure 4b shows the location where the EDS measurement in the inlet pore direction of the Pd-Ni membrane filter was conducted. The EDS analysis of the Pd-Ni membrane filter revealed the existence of Pd and Ni elements, further confirming the formation of the Pd-Ni alloy (Figure 4c). The Pd and Ni element contents were 80.99 and 19.01 wt %, respectively. ICP-MS was used to quantitatively analyze the Pd-Ni membrane filter. The ICP-MS analysis of the surface of the Pd-Ni alloy filter to determine the elemental composition indicated that the palladium content was 7500.37 ppm, and the nickel content was 1704.80 ppm, respectively. Therefore, the weight ratio of Pd:Ni was 81.48:18.51. Figure 3: Optical and field-emission scanning electron microscopy (FESEM) images of the micro-porous membrane filter. (**a**) Schematic diagram of the pathway of an aerosol droplet. FESEM images in the (**b,c**) inlet and (**d,e**) outlet directions. The X-ray powder diffraction (XRD) pattern of the micro-porous Pd-Ni membrane filter provided more detailed structural information, as shown in Figure 5a.