## 3 Results and Discussion
### The Instabilities of Inkjet-Printed Lines
The change of the ink viscosity with temperature also would affect the ring-like pattern. In the Figure 9, it is observed that the dried droplet diameter increases weakly along with temperature. Meanwhile, the volume of the dried ink droplet decreases with the temperature. It can be explained as follows: The ink droplet dried at room temperature contains both tetradecane solvent not evaporated yet and air void inside it. As the substrate temperature increases, the evaporation of the remaining solvent occurs actively and also the number of the voids decreases significantly. Finally the ink droplet becomes to have a concave peak, together with the reduced volume. In addition to the morphology transition of an ink droplet with temperature, the change of the morphology of inkjet-printed lines on \(S\) 50 substrate along with printing modes was investigated for various droplet overlaps and temperatures. The results by continuous printing procedure are shown in Figure 10. When temperature is increased to 75 \({}^{\circ}\)C, continuous lines are produced without break-up at low overlap ratio, i.e., 20% and the bulging instability becomes diminished at higher overlap ratios. The reason that the bulges still exist at \(T=75\)\({}^{\circ}\)C is because the inkjet lines retain a liquid state at the given \(T\). The individual droplets become solidified at a rate much slower than the printing speed. However at \(T=125\)\({}^{\circ}\)C, stable inkjet lines are produced on \(S\) 50 substrate for all the overlap ratios. On the contrary, at \(T=100\)\({}^{\circ}\)C the inkjet lines not containing bulges are obtained only at the low overlap ratio of 20%. The formation of bulges suppressed at higher \(T\) attributes to the reduction of ink flowing  
Figure 9: (**a**) Morphology transition of single droplets on \(S\) 50 substrate and (**b**) viscosity variation as a function of substrate temperature. into each bulge through ridges, which is a direct consequence of the rapid solidification of individual droplets. For the three temperatures, coffee ring effect also is confirmed from all the inkjet lines. Figure 11 shows the patterns of inkjet lines printed by interlacing printing procedure. Each inkjet line was produced by at least two printing sequences. The morphology of the lines was investigated under the similar conditions with the continuous printing procedure except the temperature range limited below 75 \({}^{\circ}\)C. The reason of using the downward-adjusted temperature range was to stably produce inkjet-printed lines with narrower width in the printing procedure. With 20% and 40% overlap ratios at room temperature, stable continuous lines with a non-zero \(\theta_{\mathrm{r}}\) were printed on the 5 50 substrate. This is obtainable because of the discontinuous nature of the interlacing printing procedure. Under the mode, the ink flow is not enough to introduce hydrodynamic instability in the printed lines. The inkjet lines become unstable with the irregular sizes of bulges at 60% overlap ratio. During the first pass of interlacing printing procedure, the incomplete but partially continuous lines are formed due to the high overlap ratio. They are subject to the pressure difference causing the formation and growth of a bulge during the second pass. In order to examine the effect of temperature on the interlacing mode-printed lines, the surface of 5 50 substrate was gradually heated from room temperature to 75 \({}^{\circ}\)C. As temperature increases, the solidification of individual ink droplets proceeds faster than the printing period between the first and second pass, leaving the distinct round edge on the substrate. The appearance frequency of bulges also decreases along with the temperature increase, due to the rapid evaporation of solvent. Figure 3:0 Microscopic images of inkjet-printed lines with 200 Hz printing frequency on \(S\) 50 substrate for various droplet overlaps and substrate temperatures when using interlacing printing procedure.