## 1 Introduction  
Inkjet printing of solution-processable materials has recently received much attention as a promising means to realize printed electronics. The additive, non-contact process has advantages such as low fabrication cost, reduced waste output, and less risk of breakage in producing electronic components on a substrate, compared to traditional photolithography. In addition, electronics with various sizes and geometries can be manufactured in fast speed by the data-driven approach the process employs. These benefits have been steadily utilized for the mass production of large-area electronics including passive circuit elements, organic transistors, and organic light emitting diodes, giving high manufacturing flexibility and efficiency [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. However the applications have also revealed the limitations of development of inkjet printing technology. The reliability of inkjet printing process, and the limited miniaturization and poor electrical performance of printed components or devices are the examples [1; 3; 4; 7; 8; 10]. In inkjet printing, continuous lines can be obtained by arranging the ink droplets ejected from a nozzle at smaller interval than their diameter on a substrate. The deposited droplets spread to finally form inkjet-printed lines with various patterns and widths, depending on the surface energy of the substrate. The surface energy is one of the key factors to control the spreading of ink droplets. Therefore, hydrophobic substrates which have generally a low surface energy are used for well-defined and high-resolution inkjet printing. They induce the formation of smaller droplets and with higher contact angle than hydrophilic substrates. However the hydrophobic surfaces may introduce the hydrodynamic instability such as merging and bulging phenomena into inkjet-printed lines during the droplet deposition. Hence, it is important to understand the fluidic behaviors of ink droplets on a substrate, which result from the interaction between an ink and a substrate, and the printing variables including the frequency of droplet generation, the volume and size of droplets and so on [6; 9; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Ultimately, the understanding will make a direct contribution to the improvement of working performance of printed electronics. There have been many studies about the fluidic behaviors of ink droplets on a substrate. [PERSON] investigated extensively the instability of inkjet-printed lines on a homogeneous flat surface, observed during bulging process [25]. The study found that the lines become unstable by forming a series of liquid bulges when the contact angle of the liquid with the substrate is larger than the advancing contact angle while the receding contact of the liquid line is zero. [PERSON] and [PERSON] explained the growth of the bulging instability in terms of three parameters, i.e., droplet spacing, time delay between droplet landings, and surface temperature of a substrate, assuming zero receding contact angle [23; 26]. According to their study, the instability increases as the drop spacing and the time delay decreases. However it, together with coffee ring effect in the inkjet-printed lines, can be improved by maintaining the surface temperature within 17 to 30 \({}^{\circ}\)C. [PERSON] and [PERSON] demonstrated experimentally that the bulging instability depends on droplet spacing and print-head traverse velocity [24]. In addition, they elaborated the phenomena physically in terms of the spreading flow due to capillarity and the pressure-driven flow co-existing in printed lines, without considering the hysteresis of contact angles of an ink droplet on a substrate. The above researches introduce the conditions of various printing parameters to produce inkjet-printed lines without fluid instability at zero receding contact angle. However, printing the lines stably at non-zero receding contact angle has been little studied.