# Fabrication of Stretchable Transparent Electrode by Utilizing Self-Induced Vacuum Force
## 3 Results and Discussion
### Effect of Evaporation Rate on the Fabrication Process  
As discussed in the previous sections, a volatile solvent adsorbed onto the carbon particles is critical to inducing a vacuum force, and results in the penetration of the highly viscous elastomer into the intaglio pattern. However, the entire solvent should be evaporated during the thermosetting step before the elastomer is cured. If the vaporized solvent does not fully escape until the elastomer is cured, gases will be trapped in the cured elastomer, causing the bubbling defect. Furthermore, the vaporized solvent would be trapped in the elastomer if the viscosity of the elastomer is too high, even before the elastomer is cured. Because the viscosity of a thermoset elastomer increases upon heating, it is crucial that the solvent should be fully evaporated before the viscosity reaches the critical value. Hence, from Figure 8, it can be proposed that a stretchable electrode without the bubbling defect will be obtained under the condition of Case-A, while the same defect will be observed for the condition of Case-B. Figure 8: Schematic viscosity–temperature curve of the elastomer during thermal curing. Full evaporation of the solvent in the porous conductive material is desired before reaching the critical viscosity of the elastomer (Case A) to avoid the trapping of the vaporized solvent into the elastomer (Case B). Figure 7: Cross-section SEM image for electric conductor pattern prepared in (**a**) Ketjen Black and (**b**) Denka Black showing the characteristic of elastomer penetration into porous conductive materials on varying the porosity. Compared with Denka Black, the Ketjen Black has higher adsorption ability, thereby leading to superior penetration of the elastomer in the case of using the latter. For demonstration, several solvents with different evaporation rates were tested by performing the proposed fabrication method. Three solvents, namely, acetone, THF, and ethanol, were used to prepare carbon slurry. Among the solvents, acetone exhibited the highest evaporation rate of 7.7, where the evaporation rate of butyl acetate is defined as 1.0. Meanwhile, ethanol exhibited the lowest evaporation rate of 1.7. Furthermore, the vapor pressure of acetone was the highest at 24 kPa, while the vapor pressure of ethanol was lower than those of acetone and THF. The detailed properties related to evaporation are presented in Table 2. Here, it was anticipated that the carbon slurry, based on ethanol, would cause the bubble defect in the cured elastomer due to the inferior characteristic of evaporation for ethanol. Figure 9 depicts the stretchable electrodes fabricated using carbon slurry based on the use of the three different solvents separately. As expected, the bubble defect was observed in the electrode that used the carbon slurry based on the ethanol solvent; this defect resulted in poor electric conduction. According to the hypothesis, this fabrication condition is considered Case-B (note Figure 8). However, referring to the results, acetone and THF are feasible solvents for the proposed electrode-fabrication method (Case-A). Considering the characteristic of evaporation, acetone is a better solvent than THF. However, both the solvents show feasibility of the fabrication. Hence, we measured the electrical resistance of the electrodes, which also represents their electrical conductivities. To ensure reliability of the experiment, the average values of the resistances of 15 electrode samples were compared. In addition, the volume of the solvents in the synthesis of carbon slurry was varied from 12 to 17 mL in increments of 1 mL. Figure 10 depicts the averaged values of the resistances of the 15 samples with varying solvent volumes. The resistances were measured by simple two-probe DC technique (Fluke 15b Digital Multimeter). Both ends of the conductor were selected as measuring points. Here, the measurement were carried out under room temperature (25 \({}^{\circ}\)C). As expected from the table for the rates of evaporation, a superior