## 3 Results and Discussion
### The Instabilities of Inkjet-Printed Lines
The formation process of the second bulge can be physically explained by the relationship of two kinds of a fluid flow existing along the line: capillary spreading and liquid addition by printing. When printing ink droplets under equilibrium status, the volume of the ink moving toward the line end by capillary force is smaller than one supplied by the printing. The unbalance of the two volume flows produces a large curvature on the line end, together with extra pressure. The extra pressure makes the ink flow short distance from the line end, resulting in the formation of the second bulge. Due to such mechanism, a new bulge appears abruptly between the first bulge and the line end at \(n=15\). Exceeding the number of ink droplets over 15, the injected ink begins to flow toward the second bulge and accordingly, the size of the second bulge becomes larger. After the second bulge grows large enough to induce pressure equilibrium in the liquid line, the initiation of the third bulge occurs in the same procedure explained above, i.e., at \(n=24\). When more than 30 ink droplets are deposited, there is observed an interesting phenomenon in terms of the size of bulges. Among the bulges developed along the printed line, the latest bulge is largest. The irregularity of the bulge sizes occurs because of the liquid accumulation resulted from the instantaneous breaking of the pressure balance in left and right side of the latest bulge, i.e., between the latest bulge and the previous bulges, and between the latest bulge and the line end. For example, initially the third bulge shows a similar formation and growth process with the first and second bulges, till reaching the pressure balance between the third bulge and the line end. Afterwards, the contact line of the third bulge collapses suddenly while printing ink droplets and accordingly, a pressure drop occurs at the location of the third bulge. It results in the simultaneous inflow of ink into the third bulge from both the previous bulges and the line end, making its size largest. The fourth and fifth bulges in Figure 8 also go through the same procedure of the size growth. Figure 8: Microscopic images of inkjet-printed lines with 40% droplet overlap on \(S\) 50 substrate for different numbers of droplets (\(n\)) at room temperature. In order to fabricate narrow inkjet lines, the surface energy of substrates should be low. However the hydrophobic \(S\) 50 substrate, which is prepared by single C\({}_{4}\)F\({}_{8}\) plasma, has a non-zero \(\theta_{\rm r}\) that makes difficult obtaining stable narrow liquid lines through the control of printing variables because of inducing bulging instability. In order to prevent the non-zero \(\theta_{\rm r}\) from hindering the fabrication of stable inkjet lines, the surface temperature of substrates needs to be increased. It is to facilitate the drying of ink liquid faster than the propagation of the bulge instability. This study investigated the effect of the substrate temperature on printed line morphology. It was performed by measuring the cross-sectional profiles of the dried single droplet produced on 5 50 substrate for various temperatures ranging from room temperature to 125 \({}^{\circ}\)C (Figure 9). The 3D images corresponding to each profile also are given in the figure, together with the viscosity variation of Ag colloidal suspension with temperature. At temperatures of 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C, stable pattern of a dried single droplet is obtained in an inverse parabolic shape. However for the temperatures exceeding 50 \({}^{\circ}\)C, a ring-like distribution of dried particles, so-called coffee ring effect, is observed from the profiles. It is created by the outward flow of the colloidal suspension replenishing the evaporation loss at a contact line. The coffee ring effect becomes stronger as \(T\) increases, because of the combined effect of enhanced evaporation rate at a droplet edge and contact line pining at high temperature.