Writing the future of 3D printing

Stewart McCorkle Faculty Fellow Brett G. Compton coauthored a comprehensive review on the physics of direct-ink writing (DIW), a highly versatile type of 3D printing, which was published on January 5th in Volume 58 of the journal Annual Review of Fluid Mechanics.

Annual Review of Fluid Mechanics is the world's most impactful research journal covering fluid physics and mechanics. The journal releases just one volume each year and does not accept submissions; instead, authors must be invited to contribute articles on their areas of expertise.

"Summarizing the state of the art really gives current researchers a better springboard to attack the major problems a field is currently facing," said Compton, an associate professor in the Department of Mechanical and Aerospace Engineering at the University of Tennessee, Knoxville who specializes in additive manufacturing of composite and ceramic materials using the DIW process. "It's a huge honor to be able to participate in one of these reviews, and it was a joy to work with my co-authors in putting it together."

Last year, Annual Review of Fluid Mechanics invited University of Maryland Associate Professor Alban Sauret, an expert in theoretical fluid mechanics, to review decades of research on DIW and its underlying physics.

Sauret recruited Compton and University of Hawai'i at Mānoa Associate Professor Tyler R. Ray, an expert in small-scale fluid mechanics and in DIW-derived electronic devices, to the review. Together, the three researchers distilled decades of fluid mechanics research to highlight the unique and yet-unsolved physics questions of DIW.

"I hope that this review stimulates fundamental work on the central challenges of DIW," Compton said. "I would be very excited to see the field move forward in that direction. I think it's a technology that's worthy of more research activity."

Printing with Complex Fluids

DIW printing is a bit like using cookie icing to build a coil pot. The 'inks' used in DIW are complex fluids, paste-like materials composed of small particles in a liquid matrix, similar to the sugar crystals suspended in butter in a cookie icing.

Thanks to this composition, complex fluids can behave like liquids or solids depending on their environment, just like icing that easily flows through the outlet of an icing bag but then regains a rigid structure once out of the bag.

The machines can print with a dizzying array of complex fluids, from concrete and polymer-ceramic mixtures to biological cells and, yes, even frosting and chocolate.

After each layer regains is structure, the nozzle of the DIW printer returns to add another layer, stacking the complex fluids to build intricate structures and components that can be used in robotics, medicine, architecture, aerospace engineering, and even geriatric care.

"DIW is a really exciting process because the hardware is inexpensive and works with a range of materials that are relevant in so many different fields," Compton said. "In our review, we tried very hard to summarize this broad field that includes all those different areas of application and really identify what the fundamental state of the art is."

The physics puzzles underlying DIW have to do with the delicate competition between the liquid-like and solid-like nature of the inks, particularly when loaded with solid particles or needle-like fibers and forced to flow through the tight constriction of the deposition nozzle.

Each 'ink' needs to behave as a liquid long enough to dispense through the print nozzle without clogging it—which anyone who has ever iced a cookie knows can be a frustrating challenge—but then immediately bind to previous layers like a paste and hold its shape like a solid. For many applications, including architecture and robotics, the 'ink' also needs to solidify or cure into a final product that can support a variety of mechanical loads.

Throughout the history of DIW, achieving these goals has often required a mixture of physics understanding, guesswork, and trial-and-error attempts. Compton, Sauret, and Ray hope their review leads to predictive design frameworks that will make DIW a more reliable and precise process.

"There's a lot of focus right now on just developing new materials that you can print with this method," Compton said. "That definitely serves a purpose, but it would be nice to see the underlying physics studied in more detail—to get a more unified understanding of the process. I'm excited and hopeful to see such work stimulated by this paper."

More information:
Alban Sauret et al, Fluid Mechanics Challenges in Direct-Ink-Writing Additive Manufacturing, Annual Review of Fluid Mechanics (2025). DOI: 10.1146/annurev-fluid-100224-111013

Provided by University of Tennessee at Knoxville