Experimental study of the piezoelectric drop-on-demand drop formation in a coaxial airflow
Graphical abstract
Introduction
The inkjet printing technology has created new prospects in the micromachining of various functional devices including solar cells [1], RFID tags [2], biosensors [3], printed circuit boards [4], OLED displays [5] and smart textiles [6], etc. It is a non-contact method for material deposition and has a number of advantages such as low cost, short process and high flexibility, etc. According to the methods of drop formation, inkjet printing can be mainly divided into two categories – continuous inkjet (CIJ) [7] and drop-on-demand (DOD) [8]. Since in the latter method, one droplet is ejected from the nozzle under a pressure pulse when it is required, DOD inkjet printing has become advantageous over the CIJ method. In the piezoelectric DOD technique which has occupied a predominant position in DOD printing, the piezoelectric actuator in the printhead is flexibly deformed when it is excited by a voltage pulse, and the pressure wave generated by the actuator drives the ink to enter the nozzle and emit through the nozzle to form a short jet which then condenses into a drop. After the drop hits the substrate, it changes from the spherical shape to a flat shape, and the ink material is deposited on the substrate after the diffusion and evaporation process [9].
The dynamics of drop formation is not only key to obtaining the desired drop characteristics and optimized quality of dots formed on the substrate, but is also an important indicator for evaluating the reliability of the printhead and printability of the ink. Therefore, a number of works have been performed on the drop formation process of DOD generators as well as its sensitivity to the waveform, nozzle geometry and ink properties [[10], [11], [12], [13], [14]]. For the recent works, Wu and Hwang investigated the effect of echo-time of a bipolar pulse waveform on molten metallic droplet formation by squeeze mode piezoelectric inkjet printing [15]. Harris et al. developed a piezoelectric droplet generator with an adjustable-height fluid reservoir and the drop formation can be controlled by changing the driving waveform parameters, outlet pressure and nozzle diameter [16]. He et al. studied the roles of wettability and surface tension in droplet formation during the DOD inkjet printing and found that higher surface tension values promote earlier droplet breakup and faster drop velocity [17]. Xu et al. investigated the pinch-off locations as a function of material properties and operating conditions during DOD inkjet printing of viscoelastic alginate solutions and four breakup types are identified on the basis of the first pinch-off position [18]. Wu and Xu used an imaging system to monitor the drop formation process of a cell-laden bioink and conducted a full factorial design of experiments (DOE) to investigate the effects of polymer concentration and excitation waveform on drop velocity and volume [19]. Liu and Derby presented an experimental study on the thresholds for drop generation and formation of satellite drops for DOD inkjet using inks covering a range of fluid properties. A parameter space bounded by Z number and the Weber number of the fluid jet prior to drop formation was provided for the behavior of drop generation [20]. Previous studies have elucidated the importance of understanding the drop formation in controlling and optimizing this process to obtain the required jetting performance and print quality.
Despite of the advantages of the piezoelectric DOD inkjet printing, the formation of satellite drops which can result in a degradation of print quality or failure of functional device is still one of its main issues. It has been known that the satellite behavior can be changed through the modification of the nozzle design and drive waveform for the piezoelectric actuator [9]. However, modification of the nozzle design cannot be achieved online and is also laborious and time consuming, while the effects of drive waveform for the piezoelectric actuator on the satellite formation are quite complex [21,22]. In addition, when the waveform parameters are changed to eliminate the satellite, behavior of other parameters such as the drop velocity and volume may be undesirably changed accordingly [20,23,24]. In this paper, the design of a coaxial airflow-assisted droplet generator for aqueous solutions is proposed as a new solution for reducing the satellite in piezoelectric inkjet printing. Formation of liquid drops or jets in a coaxial airflow has caught much attention of scientists from different fields and played important roles in a wide variety of engineering applications [25]. Although there have been a number of studies reporting on the formation process and flow characteristics of droplets or liquid jets in a coaxial airflow such as gas-liquid flow focusing [[26], [27], [28]], atomization of a liquid jet in an annular gas stream [[29], [30], [31]], and melt-blowing [32,33], etc., these studies are mainly focused on continuous flows which are different from drop-on-demand inkjet printing in terms of jet formation method, flow scale as well as applications. It has been reported that in some pneumatic drop-on-demand generators for producing molten metal droplets, inert gas is supplied surrounding the nozzle exit to form a coaxial flow around the hot metal droplet to prevent it from oxidization during its generation and flight [[34], [35], [36]]. With this system, Amirzadeh and colleagues found that increasing the pressure of the coaxial flow reduced the droplet diameter [34,35]. In addition, Yokoyama et al. reported a piezoelectric solder droplet generator with coaxial inert gas flowing in the vicinity of the nozzle exit for preventing the solder droplet from oxidizing [37]. However, in this research, little attention was paid to the effect of the co-flowing gas on the droplet formation process. Up till now, studies on the drop formation process in a coaxial airflow for piezoelectric DOD inkjet are still scarce in literature. In this paper, a vision monitoring system is constructed for experimentally studying the drop formation behavior of the coaxial airflow-assisted piezoelectric DOD droplet generator for aqueous solutions. The effects of some waveform parameters including driving voltage amplitude, pulse frequency and pulse width on the piezoelectric DOD drop formation at different coaxial airflow pressures are analyzed. This study is expected to provide a new idea for the design of the DOD droplet generator and a preliminary insight into the DOD droplet jetting behavior in a coaxial airflow.
Section snippets
Structure of the droplet generator
A coaxial airflow-assisted piezoelectric DOD droplet generator has been designed. Its structure is shown in Fig. 1. From top to bottom, the supporting structure of the droplet generator is generally composed of the top cover, the main body with fluid chamber, the nozzle holder and the air ejection plate. For the purpose of easy manufacturing, they are all made of brass. Gaskets are installed between adjacent parts to prevent the leakage of gas or fluid. The fluid chamber is in a conical shape.
Experimental details
A vision monitoring system is constructed to study the droplet jetting performance from the coaxial airflow-assisted piezoelectric droplet generator, as shown in Fig. 3. A waveform is generated by a two-channel function generator (AFG 3022C, Tektronix, Inc.) and is then amplified by a power amplifier (ATA-2032, Xi’an Aigtek Electronics Corporation) for driving the droplet generator. Fig. 4 shows the waveform for driving the piezoelectric actuator of the droplet generator. The waveform
General effect of the coaxial airflow on the drop formation process
Fig. 5(a) and (b) show the sequences of snapshots of the drop formation process without and with coaxial airflow, respectively. The ejection, stretching, pinch-off of the liquid and flight of the drop can be observed in both cases. As can be seen, the existence of coaxial airflow has no effect on the pinch-off time of the liquid thread from the nozzle exit (t = 1430 μs), while the shapes of the liquid thread before pinch-off are slightly changed. After pinch-off, breakup of the free liquid
Conclusions
In this paper, a coaxial airflow-assisted piezoelectric DOD droplet generator for aqueous solutions is designed and fabricated for investigating the effect of coaxial airflow on the drop formation process. A vision monitoring system is constructed for the experimental study of droplet ejection behavior based on the reproducibility of drop formation. Based on the observations, effects of some important waveform parameters for the actuation of the droplet generator - the voltage amplitude, pulse
Funding
This work was supported by the National Natural Science Foundation of China [grant number 11972116]; the Shanghai Natural Science Fund [grant number 18ZR1402200]; and the Fundamental Research Fund for the Central Universities [grant number 2232019G-05].
CRediT authorship contribution statement
Jian Zhou: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Zeguang Pei: Conceptualization, Methodology, Resources, Data curation, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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