Search

Electrohydrodynamic (EHD) nanodrip printing is a novel high resolution printing technology, which enables direct, non-contact, and conformal writing of a wide range of material systems. By using an electric field to pull the material solution out of a nozzle, EHD printing can generate droplets that are over an order of magnitude smaller than their respective nozzle opening. In this way, nanoscale entities can be printed from micron-sized nozzles, hence enabling a host of new devices and platforms composed of nanoscale functional components. As a bottom-up additive manufacturing process, it allows for the efficient usage of materials, as the often expensive material is placed only where required. Moreover, the printing is carried out at ambient conditions, removing the need for expensive vacuum systems and clean room environments. This promising technology paves the way for the inexpensive and flexible prototyping of new concepts in nanotechnology, with great potential in digital microfluidics, security printing, as well as the patterning of optical structures and electronics. The thesis is centered around exploiting the capabilities of EHD nanodrip printing by expanding its range of application to digital droplet microfluidics in a first part part, and showcasing its versatility as a security printing platform utilizing unique ink systems, in a second part. In a final part, the scalability of the EHD nanodrip printing process is demonstrated by the development of a chip-based multi-nozzle printhead, capable of high-resolution parallel printing. The controlled and rapid handling of ultrasmall liquid volumes holds great promise in many scientific and application areas. In this thesis, EHD nanodrip printing is exploited to demonstrate a novel open-atmosphere microfluidic platform, where femtoliter sessile droplets (approximately 2-5 μm in radius) can be formed sustainably despite high volatility, manipulated, and freely translated. The omnidirectional planar movement of the droplets is achieved by a precisely controlled, on-demand sequence of coalescence events, where the directed motion of an already deposited droplet is dictated by the positioning of the subsequent droplet printed adjacent to it. With the presented platform, the fundamentals of the coalescence kinetics of sessile microdroplets in a partially wetting regime are experimentally investigated. The study has revealed that even for the minute sizes of the droplets examined, the contact line motion still dominates the coalescence process. Knowing the physics of the droplets propulsion mechanism and its limits, the coalescing moving droplets are tasked to perform typical microfluidic operations so as collecting, transporting and merging solid materials on otherwise unpatterned substrates, exemplifying capabilities for digital microfluidic applications. In reference to security printing, the high-resolution capabilities of EHD nanodrip printing is combined with the use of novel ink systems, to demonstrate the microfabrication of next-generation security tags. Here, custom-made lead-halide perovskite nanocrystals (quantum dot) inks are employed. The quantum dot inks are especially engineered to have the same emission color, but vastly different fluorescence lifetimes. EHD nanodrip printing is used to precisely co-align and pattern the different quantum dot inks into a single security tag, which appears unicolor, and is multi-fluorescent-lifetime encoded. In doing so, the microprinted security code is protected from conventional readers. The versatility of the proposed security printing system is exhibited by showing that a portable, cost-effective time-of-flight prototype camera can decipher these codes in a single-shot rapid manner. While promising high resolution, current EHD printing research predominantly involves the use of a single nozzle to print a single ink, which limits the efficiency of the process significantly. To overcome the limitations of conventional EHD printing and bring the technology one step closer to a commercially viable microscale additive manufacturing platform, the final part of the thesis presents a chip-based batch microfabricated printhead that can incorporate a multitude of densely arranged nozzles. By using established semiconductor processing techniques, multiple arrays of thousands of nozzles are feasible on a single chip, increasing the printing speed proportionally. The printhead chip also comprises of several reservoirs, each capable of holding a different ink, hence allowing for the simultaneous printing and fast patterning of multi-material structures. The functionality of the printhead is evaluated by analyzing the printing behavior of the nozzles and assessing the quality of the printed structures. The scalability of the EHD printing process is demonstrated by printing with an array of 3 x 20 simultaneously actuated nozzles, wherein superb sub-micrometer registration from the nozzles is achieved. Using specific movement commands, the print output of the nozzle array is made to overlap into single entities, so as to write large-area high-resolution fine conductive lines., ISBN:978-3-907234-66-2