## 3 Results and Discussion
### The Instabilities of Inkjet-Printed Lines  
By continuous printing procedure, inkjet lines were printed on the five substrates with 200 Hz printing frequency at room temperature (Figure 5). The overlap ratio between neighboring droplets was adjusted to 20%, 40% and 60%. On the hydrophobic \(S\) 30 substrate, most of the printed droplets maintained their circular liquid patterns without forming any continuous inkjet lines for all the overlap ratios. The droplets returned into the separated circular configuration as soon as deposited on the substrate surface and accordingly, the merging of the droplets occurred immediately from  
Figure 4: **(a)** SEM images with top-view and oblique-view and **(b)** AFM images for the plasma-treated surface of \(S\) 30, \(S\) 70\({}^{1}\), and \(S\) 70\({}^{2}\) substrates. Figure 3: High resolution of C1s spectra and F/C ratio for the fluorinated surface on \(S\) 30, \(S\) 50, \(S\) 70\({}^{1}\) and \(S\) 70\({}^{2}\) substrates after plasma treatment. the beginning of inkjet printing. The size of the circular patterns increased due to the increase of ink amount merged, as the overlap ratio increased. On the \(S\) 50 to the \(S\) 70 substrate, a train of bead-like liquid patterns was observed at 20% overlap ratio. When the overlap ratio increased to 40%, the bead-like patterns transformed into the continuous lines where many bulges are regularly distributed along the line. The size of the individual bulges connected by liquid ridges got larger when increasing the overlap ratio from 40 to 60%. The formed bulges are observed to be more prevalent on the \(S\) 50 substrate than the \(S\) 70 substrates under the same overlap ratio. Besides, when comparing the \(S\) 70\({}^{1}\) and the \(S\) 70\({}^{2}\) each other, the bulges on the latter substrate are more irregular and less appeared than ones on the former substrate, in spite of their similar dried droplet diameters. This attributes to the difference in the wetting characteristics of the ink on the substrates, which are primarily governed by the chemical nature of the substrate surface such as F/C ratio and surface polarity. Besides the ink in the transition regime from spherical (i.e., droplet) to cylindrical shape (i.e., line) are more influenced by the former than the latter. On the hydrophilic \(S\) 90 substrate, stable straight lines were printed at 40% and 60% overlap ratios. This is related to the large spreading of ink droplets on the substrate. The outflow of ink in the droplets reaches farther for the conservation of line volume and accordingly, the possibility of bulge formation becomes very low. It helps the uniform and wide spreading of the ink droplets when they meet to produce straight lines. According to the previous researches regarding with bulging instability, the appearance frequency of bulges decreases by increasing either the droplet spacing or the time interval between droplet landings when printing liquid lines with zero receding contact on a homogeneous surface. However it is opposite to what was confirmed in Figure 5. Stable inkjet lines could not be obtained on \(S\) 70 substrates even at low droplet overlap. The recognition about the inconsistency with the previous reports led to investigating the effect of printing frequency on bulging instability additionally. The investigation was performed on \(S\) 50 substrate by changing the frequency from 50 to 100 Hz for various droplet overlaps at 125 \({}^{\circ}\)C (Figure 6). The lower printing frequency yields a larger time interval between successive droplet landings at a fixed droplet overlap. The result is given in Figure 6. It demonstrates that the printing frequency has little effect on the bulge formation in terms of the size and the appearance frequency of bulges. According to [PERSON]'s work, inkjet-printed lines with significant hysteresis between advancing and receding contact angle (CA), i.e., \(\theta_{\text{a}}\) and \(\theta_{\text{r}}\), become unstable, forming either a train of merges or regular bulges [25].