Our world is dominated by exquisite design and complex machinery, but sometimes it is the most common natural phenomenon that have inspired us to revolutionize how we work. In our latest research venture, we bring together the micro-fluid and tip discharging, for a purpose that nudges the boundaries of technological innovation—developing a high-productivity and large-area fabrication technique for piezoelectric films, patterns, and nanoparticles.
A silent and nonluminous electrical discharge from a pointed conductor maintained at a potential that differs from that of the surrounding gas.
In 1917, Zeleny first observed that a strong electrostatic field could destabilize a microfluid interface separating drops from the surrounding air. The interface takes on a conical shape, now referred to as Taylor cone, and occurs when the microfluid is charged beyond the Rayleigh limit. In a sufficiently intense electric field, either a string of droplets or a thin liquid jet that subsequently disintegrated into a large amount of droplets will be propelled from the cone’s tip. Such electrostatically driven cone-jetting phenomena occur widely in nature and application. Two well-known examples are the ejection of streams of charged droplets from the tips of raindrops in a thunderstorm cloud and one immensely popular application for assaying large biomolecules: electrospray mass spectrometry. The Taylor cone can be formed from a microscopic pendent drop extruded from a round capillary, or macroscopic liquid films flowing to the tip. Printing strategies inspired by such phenomena include electrospraying, electrospinning, and droplet focus printing. For these nozzle-based printing strategies, ink flow defined by the inner diameters of nozzles, as well as the capillary phenomena put an upper limit on the printing speed. Additionally, nozzles with smaller apertures suffer from clogging and viscous losses, consequently, inks are limited to low-viscosity solutions free of large particles, which limits material versatility.
To unleash the potential of high-speed and versatile printing allowed by electrostatic printing, in our work, we develop an electrostatic disc microprinting (EDP) strategy to directly fabricate piezoelectric films, arbitrary micro-patterns and nanoparticles. The EDP process occurs via triggering the liquid-air interface instability of inks through an externally spiny thin disc (Fig. 1).
Fig. 1 Realization of the EDP. (a) Schematic of EDP printer. (b) High-speed movie capture of the EDP generated by applying ~5 kV between the disc and the substrate. (c) Simulation of the fluid volume fraction around the thin spiny disc. The thin disc is equipped with six tips to simplify the simulation model.
Benefiting from the large-volume printing process and in situ electrostatically induced modifications, the EDP process offers (i) on-demand printing strategies with a high depositing speed of ~ 109 μm3 s-1, which is the fastest in the existing techniques for piezoelectric micrometer-thick films (Fig. 2); (ii) lead zirconate titanate (PZT) films with excellent piezoelectric properties (d33 of ~560 pm V-1) outperforming the majority of previously reported PZT films (Fig. 3); (iii) free-standing PZT nanoparticles in the size regime of 100 nm (Fig. 3); (iv) PZT patterns with a fine feature size (minimum width is ~ 20 μm) (Fig. 3); and (v) printing capability for wide-ranging classes of materials such as dielectric ceramic, metal nanoparticles, insulating polymers, and biological molecules.
Fig. 2 Map of fabricating capabilities regarding manufacturing speed and the accessible film thickness (<50 μm).
However, we recognize the challenges ahead. Mask assisted EDP method may limit the accuracy and reproducibility of the deposited patterns, and miniaturizing the pattern to a nano-scale remains a significant hurdle. Yet, its benefits outweigh these concerns. The solution-based printing method allows easy processing of different inks and ink concentrations on the same substrate, enabling printing programmability. Moreover, it is compatible with complementary metal oxide semiconductor (CMOS) and MEMS techniques.
Fig. 3 Fabrication of piezoelectric films, micro-patterns and nanoparticles. (a) SEM image of surface topography of the annealed PZT film. (b) SEM images showing the surface and cross-sectional topography of the PZT columnar structures. (c) Schematic of the formation process for PZT columns. (d) SEM images of the printed PZT arrays of circular structures, squares and printed micro-letters (“City”) on Si substrates. (e) Schematic of the process for printing micro-patterns through a hole-containing dielectric mask. (f) Diameter variation of the PZT particles as a function of sol concentrations. The inset shows the SEM image of free-standing PZT particles with different diameters. (g) Schematic of the formation process for PZT particles.
By electrostatically triggering the liquid-air interface instability of inks through a conductive spiny disc, we present a versatile and fast microprinting technique. We’re not only excited about the potential of our work but also about its ability to inspire future innovations in electronics, biotechnology and beyond. We are ready to embrace the challenges and opportunities that it brings.