Atomization process in electrospray deposition
In electrospray deposition process, a single Taylor cone-jet shaped meniscus forms by stabilizing electrostatic repulsion and surface tension31. If the applied voltage at the nozzle is high enough to produce electrostatic repulsive force within charged droplets, the liquid breaks up into finer charge droplets by overcoming the surface tension32. The atomization process can go through several steps. The primary charge droplets produced by a Taylor cone, shrink due to the evaporation of the solvent. The shrinkage process continues until the droplet radius approaches the Rayleigh limit33. Rayleigh limit describes the stability limit of a charged droplet, which is affected by the opposing forces of the surface tension and electrostatic repulsive force. Droplets in conditions close to the Rayleigh limit are unstable and can undergo Coulomb fission. The charge in the droplets at this limit can be expressed by the critical radius of the droplets as:
![](data:image/png;base64,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)
where, r, \({q}_{R}\), \({\gamma }\) and \({\text{ϵ}}_{\text{o}}\) are the radius of the droplets, the total charge on the droplet, surface tension of the liquid and electrical permittivity of the liquid, respectively. The diameter of the atomized droplet could be less than 10 µm having normal distribution behavior34. The energy required for droplet evaporation comes from the thermal energy of the ambient air. Due to the evaporation, droplet size decreases while the same amount of charges remains. As a result, the charge density of the droplet increases until the electrostatic repulsive force within the droplet overcomes the surface tension. At this point, the droplets are no longer stable, which is called Rayleigh instability. Then the droplets (parent droplets) experience Coulomb fission and release new droplets (child droplets) with a small amount of mass (around 2% of the parent droplet) and a greater charge density (about 15% of the parent droplet)35. The process of evaporation and fission is repeated by both parent and child droplets until reaching the substrate.
The spray deposition process could be affected by the accumulated charges of deposited materials on highly insulating substrate36,37. Recently, this issue was solved by applying AC voltage on substrate27. The AC voltage is effective for all substrates regardless of the conductivity27. In this research of multi-nozzle electrospray application, we applied AC voltage at the substrate to enhance the deposition efficiency.
Crosstalk and end effects of multi-nozzle electrospray
The electric field interference among neighboring nozzles, i.e., crosstalk effect, depends mainly on the distance between nozzles, when flow rate/pressure, voltage and nozzle inner diameter are constant10. In this section, we investigated the effect of the crosstalk using two nozzles with a distance adjustor, as shown in Figure 1. The nozzle-to-nozzle distance varied from 1 cm to 5 cm and the inner diameter of the nozzle was 330 µm. A detailed discussion on crosstalk effects is provided in the supplementary information (Section S1).
Figure 2(a-d) shows the electrospray behaviors of two nozzles with different nozzle spacing. Here, no significant crosstalk effects could be observed in the deposited shape when two nozzles were used. However, when we used a five-nozzle spray-head with a nozzle-to-nozzle distance of 1.2 cm, we could observe the crosstalk effects from the deposition shape as shown in Figure 2(e). In the case of a multi-nozzle spray head, having more than three nozzles, the cone jets could be deflected in different ways due to the end effects12. The end effect arises from asymmetric electric field in the boundary nozzles. The spray behavior of nozzles in the middle shows that the spray process was more stable and could result in more uniform coated results than that of the nozzles on both ends. Figure 3 helps to explain this phenomenon, which illustrates electrospray behaviors caused by the asymmetric electric field at the tip of the nozzles. The electric field at various points (A, B & C) in a nozzle, as shown in Figure 3(a), could be different according to nozzle configuration10,12. As illustrated in Figures 3(b) and (c), the spray emits from the point where the electric field is the strongest. In case of a linear array with five nozzles, the electric field is the highest at both ends10 (Point A in case of nozzle 1 and point C in case of nozzle 5). As a result, the spray jet from the edge nozzles (nozzles 1 and 5) deflected from the capillary axis. Also, the charged spray droplets in flight from a specific nozzle produce repulsive forces to sprayed droplets from neighboring nozzles. As a result, when multiple nozzles are used for electrospray deposition, the deposition layer may not be uniform over the entire area of the substrate.
Uniform deposition method based on motion control
As discussed in the previous section, non-uniform deposition areas from each nozzle have resulted from crosstalk and the end effect. Additionally, the sprayed substrate contains a non-sprayed region between two nozzles if the scanning movement is not intended to overlap. To increase the uniformity of deposition layers, we proposed ‘zig-zag’ relative motion of spray head with respect to the substrate (Refer to the experimental section). The zig-zag motion could increase the thickness uniformity of coated layers by averaging out non-uniform spray jetting from each nozzle. The scanning speed of motion movement can affect the deposition amount on substrates. For example, there can be acceleration and deceleration when motion direction changes. So, the deposition area should be sufficiently large compared to the substrate, such that scanning speed passing through the substrate should be constant.
Drying optimization for uniform layer thickness
For better deposition of materials, the surface energy of the PCB substrates was improved by (Ar and O2 mixed, radio frequency (RF) driven atmospheric pressure plasma) plasma treatment prior to electrospray (Supplementary information, Movie S1). By combining Ar and O2, RF-generated plasma can increase the substrate wettability and adhesion qualities38,39. The detailed plasma pre-treatment effects are discussed in the supplementary information (Section S2).
Even if the plasma treatment could improve surface conditions by improving adhesion properties of the substrate, the surface as well as the thickness of the dried coating layer, is not uniform. The uniformity is affected by the evaporation process. The non-uniformity becomes more severe in the case of multi-nozzle spray deposition since a significant amount of materials could be deposited with non-evaporated solvents in it. The evaporation rate depends on the spray parameters such as droplet sizes and stand of distances, environmental conditions such as temperature and relative humidity and ink formulation parameters such as solute concentration and solvent volatility40. The solvent volatility is one of the most important properties which influence the evaporation rate and the ultimate particle morphology41. The severe agglomeration of particles on the substrate can avoid by immediate evaporation of the solvents40. In the case of a single nozzle electrospray system, evaporation issue rarely causes a problem since the solvent atmosphere is not so dense and the deposition rate is relatively low. However, when the deposition rate is increased by employing multi-nozzle electrospray, the solvent atmosphere could be dense, resulting in a slower evaporation rate. In this case, the ambient atmosphere is unable to supply adequate energy for the evaporation of solvents in a large number of deposited droplets. As a result, the deposited materials with unevaporated solvents could coalesce on the surface, as shown in Figure 4(a). This effect creates severe non-uniformity of the coated layer, as shown in Figure 4(b). The coalescence of the droplets in-flight along their trajectory towards the substrate is extremely rare because charged droplets repel one another. Rather, it occurs right after deposition on substrates. In conventional inkjet application, a substrate (platen) heater is often considered to expedite the solvent evaporation of deposited materials42. In our present case of using the multi-nozzle spray system, the platen heater has limitations of improving the uniformity of layer, as shown in Figure 4(c). Furthermore, the cured coated layer did not show sufficient hydrophobic characteristics having a contact angle of only 131˚.
To improve the uniformity of the coating by accelerating solvent evaporation, a hot blower (STEL670, STANLEY, USA) was integrated into the spray system. An airflow duct was designed to effectively supply hot air to the substrate, as shown in Figures 5(a-d). Photos of the hot blower and the flow duct are presented in the supplementary information (Section S3). The use of the hot blower has the further benefit of in-situ curing during the deposition without requiring an additional curing process.
Two different configurations were investigated to optimize the drying process using the hot blower. The first configuration was based on two independent operations for spraying and drying. Figures 5(a) and (b) shows hardware configuration based on separate drying unit. We investigated optimum temperature and wind speed using this configuration. In this case, right after printing the spray deposition pattern (zig-zag motion) once, the substrate travels to the location where the hot blower is positioned for the solvent evaporation and curing of deposited materials. The process of spraying and drying is repeated multiple times to increase the thickness of the coated layer. However, it could have limitation since there was a time delay in the drying process.
The second configuration uses a combined unit of spraying and drying, as shown in Figures 5(c) and (d). This configuration allows simultaneous deposition and evaporation process with a minimal time delay. In this case, the optimum conditions of the hot blower found in the first configuration are used for drying the deposited materials. The distance (D) between the nozzle head and the hot blower is important in this case, as the hot blower close to the nozzle head can blow away the spray droplets in flight. The effect of the distance on deposition quality is discussed later.
To understand the effect of substrate temperature on coating performance using the first configuration, a forward-looking infrared camera (E6390, FLIR, Sweden) was used to measure the temperature of the substrates according to different power (Level 1 to Level 4) of the hot blower. The wind speed at the outlet of the airflow duct was measured by using a typical anemometer (ST-111, Sincon, South Korea) based on the wind levels (Level 1 and Level 2) of the hot blower. Table 1 shows the temperature of the PCB substrate according to the heating levels and wind speeds of the hot blower when the hot blower was located 5 mm above substrates.
Table 1
Temperature of the PCB substrate at various heating levels (temperature at the outlet of the hot blower) and wind speeds, measured using FLIR camera.
|
Temperature of the substrate (°C)
|
Wind speed (m/s)
|
Leve 1 (100˚C)
|
Level 2 (150˚C)
|
Level 3 (200˚C)
|
Level 4 (250˚C)
|
1.5 (Level 1)
|
39.3
|
48.0
|
76.2
|
102.0
|
3.6 (Level 2)
|
47.7
|
62.0
|
98.8
|
150.0
|
The proper drying process is important in order to ensure coated layer uniformity. Figure 6(a) shows surface conditions of PCB substrate right after spray deposition using a zig-zag pattern five times. Figure 6(b) shows the contact angles of coated layers on the substrates. Here, all the conditions for spray deposition were the same except hot blower parameters such as power and wind speed. Note that the temperatures of the hot blower and substrate can be increased according to the power of the hot blower, as shown in Table 1. When the blower power was minimum (Level 1), the temperature of the substrates was measured to be around 50℃. On the other hand, the substrate temperature can be increased to 150℃ by maximizing the power (Level 4), which is close to the boiling point (170℃) of the solvent, 2-butoxy ethanol. In case of the substrate temperature could increase more than 150℃, additional post-processing of curing is unnecessary.
To ensure the curing of the deposited layers, the coated substrates were placed on a hot plate at 170℃ for 30 minutes. Figure 6(b) shows the water contact angles of the silica-coated substrates according to the temperatures of the substrate and wind speed at the outlet of the flow duct. The contact angles were different according to drying conditions. The contact angles increased according to the temperature as well as the wind speed. The coated layer using the maximum power of the hot blower and with a wind speed of 3.6 m/s showed superhydrophobic characteristics with a contact angle of 151°. However, heating alone without proper airflow could not make superhydrophobic layers when multi-nozzle was used.
The second configuration, in which the hot blower attached to the electrospray head, is used for drying the deposited materials instantly. However, if the distance between the electrospray head and the hot blower becomes too close, the hot blower can create adverse airflow that can affect the deposition by flying away droplets in flight. Thus, the distance between the multi-nozzle head and the hot blower can affect deposition quality, which can measure from the contact angle of the coated layer. To investigate the effect, we developed an experimental setup to adjust the distance (D) between the nozzle head and the hot blower, as shown in Figures 5(c) and (d). Refer to the video in the supporting material (Movie S2) for a better understanding. Figure 7 shows the water contact angle variation with the distance between the nozzles and the hot blower. As shown in Figure 7, the hot blower should locate more than 90 mm from the spray nozzle in order to ensure supper-hydrophobicity of the coated layer. The videos in the supporting material (Movie S3, Movie S4 & Movie S5) refer to observing the uniform superhydrophobic characteristics.
Characterization of deposited layer
The deposited silica layer could have micro/nano hierarchical roughness, which leads to hydrophobicity by reducing the contact area between water droplets and the solid surface43,44 For better understanding of the microscopic structure, the characterization of surface roughness was carried out by using atomic force microscopy (AFM). Refer to the details in the supplementary information (Section S4). The root mean square surface roughness (RRMS) of the coated layers was in the range of 80 nm to 100 nm. Note that the original (uncoated) PCB substrate had a surface roughness of 11.03 nm.
Sufficient thickness of the coated layer is important in order to ensure the reliability of the layer in presence of mechanical harshness. However, in case of using plastic substrates including PCB substrates, it was very difficult to measure the thickness of the coated layers. To roughly understand layer thickness, we used glass substrate for fabricating superhydrophobic layers with the same conditions for multi-spray deposition discussed earlier. The contact angle of the coated glass substrate was 154\(^\circ\). The thickness of the coated layers on glass could easily be measured by using a 3-D profiler and FIB-SEM, as shown in Figure 8(a) and Figure 8(b). For thickness measurement using the 3D profiler, a part of the coated layer was removed by a knife, as shown in Figure 10(e). By using the conventional electrospray method, the thickness of the coated layer on glass substrate was only 0.87 µm27. When an alternating current voltage is applied to the substrate in a single nozzle deposition system, the thickness was measured to be 1.91 µm27. By implementing a multi-nozzle system, the coating layer thickness was further increased to 2.85 µm in the present method. It took 60 s to achieve this thickness of 2.85 µm over 5\(cm\times 5 cm\) region of the PCB, while the previous method using the single nozzle takes the same amount of time to achieve a thickness of 1.91 µm on a single spot with a diameter of 4 cm27. The improvement in the deposition efficiency is due to the combined effect of AC voltage on the substrate holder and multi-nozzle deposition with a hot blower.
To investigate the reliability of coated layers, we performed the mechanical and chemical tests by using sandpaper abrasion and chemical attack (HCl), respectively. Even after the sandpaper abrasion (refer to Materials and Methods), the water contact angle was measured to be 147\(^\circ\), which indicates that the hydrophobicity of the surface remained almost the same as the original contact angle of 152\(^\circ\). For chemical attack, the superhydrophobic PCB substrate was submerged in 0.1 M HCL for 5 minutes (Supplementary information, Movie S7). There was no rapid chemical reaction when the coated substrate was immersed in HCL acid. The contact angle after the chemical test was measured 149\(^\circ\). The FE-SEM image of the surface before and after abrasion and chemical test is shown in Figure 9. There is no significant change in the surface morphology of the coating after the sandpaper abrasion and HCL submersion tests.