A pneumatically driven inkjet printing system for highly viscous microdroplet formation
© The Author(s) 2016
Received: 7 April 2016
Accepted: 1 June 2016
Published: 9 June 2016
This paper introduces a pneumatically driven inkjet printing system that forms highly viscous microdroplets in the nanoliter volume range. The printing system has a unique printing mechanism that uses a flexible membrane and an effective backflow stopper. While typical inkjet systems can handle liquids with a limited range of viscosity due to energy loss by viscous dissipation at the nozzle and ineffective backflow management within their systems, our printing system can print liquids with viscosity as high as 384.5 cP. In the viscosity range 1–384.5 cP, we investigated printing characteristics such as printed droplet volume, standoff distance, and maximum possible frequency. The droplet formation showed outstanding reliability, with the droplet volume exhibiting a coefficient of variation less than 1.07 %. Our printing system can be directly used in inkjet applications with functional liquids over a broad viscosity range.
Inkjet technology, which delivers a minute droplet onto a desired position, offers several advantages for fabricating micropatterns, including reduced material usage, minimized use of hazardous etching chemicals, compatibility with various substrates, and reduced number of process steps . Thus, inkjet technology has been regarded as an effective fabrication tool for the use of liquid functional materials and has been implemented into research areas such as fabrication of organic thin-film transistors , organic solar cells , 3D structures , and cellular structures . The properties (i.e., viscosity, surface tension, and density) of the printing liquids govern the operation of an inkjet printing system. Among these properties, viscosity is often a limiting factor because the ejection of the droplets depends on the amount of energy loss caused by viscous dissipation at the nozzle [6, 7]. In general, the viscosity must be less than 30 cP . Because of the viscosity limitation, many researchers have attempted to develop appropriate functional liquids for use in inkjet systems (e.g., lowering viscosity via dilution). The limited viscosity of the printing liquids restricts the content of functional pigments, leading to limited functionality (i.e., the printing liquids in an inkjet system can carry a lower amount of functional pigments than those in a contact printing system) . Thus, contact printing methods such as screen and offset printings are preferred for handling highly viscous functional liquids. If the allowable viscosity range for inkjet systems can be increased, the printed results will exhibit greater performance and better throughput. Therefore, to extend the range of applications of inkjet printing, it is essential to develop a printing mechanism that can eject viscous liquids as well as appropriate printing liquids.
In this study, we present an inkjet printing system that forms highly viscous microdroplets in the nanoliter volume range. To print highly viscous liquids, a unique pneumatically driven printing mechanism with a flexible membrane and an effective backflow stopper was used in the printing head. Since the effective backflow stopper can provide complete confinement of the liquid within the chamber along with the flexible membrane, energy loss during the ejection of the droplets can be dramatically reduced. Therefore, effective energy transfer can be realized which enables highly viscous microdroplet formation.
The performance of the printing system was assessed by application of liquids with a broad range of viscosity (1–384.5 cP). The system showed reliable performance, with the droplet volume exhibiting a coefficient of variation (CV) less than 1.07 %. For analyzing printing characteristics, the dependence of the printing volume, minimum standoff distance, and maximum printing frequency on the liquid viscosity was examined. While typical inkjet systems can use liquids with a limited range of viscosity, our printing system can print liquids of viscosity as high as 384.5 cP without additional energy (e.g., heating of printing liquids to reduce viscosity). Therefore, our printing system can be directly used in inkjet applications with functional liquids over a broad viscosity range, without a need for controlling viscosity of printing liquids.
Results and discussion
The conditions for each printing liquid, such as the magnitudes of positive and negative pressures, the push time for deflecting the membrane downward by positive pressure, and the pull time for deflecting the membrane upward by negative pressure, were optimized to operate the printing head. To generate a droplet, an appropriate kinetic energy must be transferred to the liquid. The kinetic energy is adjusted by positive pressure and push time. The higher the viscosity of the printing liquid, the higher the energy required to generate a droplet. For glycerol–water mixture concentrations of 0, 40, 60, 80, 84, 88, and 92 %, the required minimum positive pressures to eject a minimum volume of each viscous liquid were 2, 3, 10, 65, 150, 200, and 200 kPa, respectively, at a fixed push time of 3 ms. In the case of the 92 % glycerol–water mixture, a higher push time of 5 ms was set to transfer more energy because 200 kPa is the maximum pressure in our current configuration. Negative pressure is optimized to prevent air bubble entrapment in the chamber of the printing head and was fixed at −5 kPa for all printing liquids. For each printing liquid, sufficient pull times were provided within 600 ms to return the liquid meniscus at the end of the nozzle to its initial state. The height of the liquid column in the reservoir was maintained at 40 mm for each test.
Minimum standoff distance
Droplet formation frequency
We developed a pneumatically driven inkjet printing system that forms highly viscous microdroplets in the nanoliter volume range. The performance of the printing system at high viscosity was assessed via the application of liquids with a broad range of viscosity (1–384.5 cP). The operating conditions were optimized to determine the minimum droplet volumes for each liquid. The printed droplet volumes were in the range 12.2–63.5 nL and increased with the viscosity of the printing liquids. The printing system showed outstanding reliability in droplet formation, with a CV ≤ 1.07 % for the droplet volume. The minimum standoff distance was in the range 1.09–3.51 mm and depended on the viscosity of the printing liquids. The standoff distance was increased by high viscosity and excessive energy, leading to long tails during droplet formation. The printing frequency was estimated as the time to form a droplet and was found to be strongly affected by the high viscosity of the printing liquids. Liquids with high viscosity required a long retraction time (i.e., the time taken by the liquid to return to its initial state) for the residual liquid at the end of the nozzle after pinch-off. While typical inkjet systems can print liquids with a limited viscosity range, our printing system can print liquids of viscosity ranging from 1 to 384.5 cP without the need for additional energy (e.g., heating of printing liquids to reduce the viscosity). Therefore, without requiring viscosity control of the liquids, our printing system can be directly used in inkjet applications with functional liquids over a broad viscosity range.
IHC participated in design and fabrication, carried the experiments, contributed to the data analysis and drafted the manuscript. JK conceived of the study, participated in its design and coordination, reviewed all test results, and finalized the drafted manuscript. Both authors read and approved the final manuscript.
This research was supported by a Grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (Grant number: HI15C0001).
The authors declare that they have no competing interests.
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