A team of researchers from the Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science (IISc) has developed a low-cost, drop-on-demand printing technique capable of generating a wide range of droplet sizes using a variety of inks.
This new printing technique can be used not only for traditional printing but also for 3D printing of living cells, ceramic materials, electronic circuits, and machine components.
Coming to the problem this new technique solves, the printers we use have a nozzle with a small opening to eject droplets.
However, particles in the ink or a cell suspension can clog the opening, which limits the number of particles or cells that can be loaded initially. Consequently, the thickness of the layer that can be printed is also limited.
To solve this problem, in the new technique, researchers replaced the nozzle with a mesh that is covered with chemically treated nanowires that repel water.
How does it work?
When a large droplet impacts on the above mentioned mesh, it bounces back. However, a small part of the liquid is ejected through the mesh pore as a jet that breaks to create a micro-scale droplet, which is then printed onto a surface.
The team says that the short contact time does not give a chance for the ink to clog. As it doesnt clog, they were able to load the ink with larger quantities of nanoparticles, enabling printing of very thick lines in a single cycle. Moreover, the mesh can also be easily cleaned and reused which add to its benefits.
“The mesh costs only a small fraction of the nozzles that it replaces. This significantly reduces the operational cost when compared to conventional printing techniques,” says Prosenjit Sen, Associate Professor in CeNSE and senior author of the study.
The team has been working on developing nanostructured surfaces that can repel water. They knew that when large droplets hit such nanostructured meshes at high speeds, jets are ejected. The researchers while studying this phenomenon, found that the velocity of the ejected jet was surprisingly higher than the velocity of the impacting droplet.
“This was the first hint that some mechanism was playing a role in focusing the kinetic energy,” says Chandantaru Dey Modak, first author and Ph.D. student at CeNSE. “At this point, we started asking the following questions: What is this focusing mechanism? Can this mechanism be exploited to reliably generate single microscale droplets?”
The team captured high-speed videos of these impacting droplets (Linked Below) and found that an air cavity was being formed at the center of the droplet.
During the recoil phase of the impact, this cavity collapsed, focusing all the kinetic energy into a single point, resulting in the generation of individual droplets. No “satellite” droplets (secondary droplets that result in unwanted scatter) were generated. The size of the droplets ejected could also be tweaked by adjusting the pore size of the mesh.
The researchers were able to demonstrate the use of this technique for various applications. “Using drop impact printing, we could print 3D pillars of different sizes, an electronic circuit for semiconductor device applications, and bio-based droplet arrays for cell culture,” says Modak. “The capability to print a wide range of droplet sizes while using different kinds of inks for different applications makes this technique unique.”
Modak, C.D., Kumar, A., Tripathy, A. et al. Drop impact printing. Nature Communications, 11, 4327 (2020) DOI: doi.org/10.1038/s41467-020-18103-6