Meta Fluid

Harvard researchers have created a versatile programmable metafluid that can change its properties, including viscosity and optical transparency, in response to pressure. This new class of fluids has potential applications in robotics, optical devices, and energy dissipation, and represents a significant advance in metamaterial technology. (Artist’s concept). Credit:

Scientists have developed a metafluid with programmable response.

Scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a programmable metafluid with tunable springiness, optical properties, viscosity, and even the ability to switch between Newtonian and non-Newtonian fluids.

The first of its kind, metafluid uses a suspension of tiny, elastomeric spheres – between 50 and 500 microns – that bend under pressure, fundamentally changing the properties of the fluid. Metafluid can be used in everything from hydraulic actuators to software robots, smart shock absorbers that can dissipate energy depending on the intensity of impact, to optical devices that can switch from transparent to opaque.

The research is published Nature.

“We’re just scratching the surface of what’s possible with this new class of fluids,” said Adel Djellouli, a Research Associate in Materials Science and Mechanical Engineering at SEAS and first author of the paper. “With this one platform, you can do so many different things in so many different areas.”

Metafluids vs. Solid Metamaterials

Metamaterials—man-made materials whose properties are determined by their structure rather than their composition—have been widely used in a number of applications for years. But most materials — like the metals created in the lab of Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Scientist in Electrical Engineering at SEAS — are solid.

Adjustable optics with Harvard logo under Metafluid. Credit: Harvard SEAS

“Unlike a firm metamaterials, metafluids have the unique ability to deform and conform to the shape of their containers,” said Katia Bertoldi, the William and Ami Quan Danoff Professor of Applied Mechanics at SEAS and lead author of the paper. “Our goal was to create a metafluid that not only possesses these remarkable attributes, but also provides a platform for programmable viscosity, compressibility, and optical properties.”

Using high-scale manufacturing techniques developed in the lab of David A. Weitz, Mallinckrodt Professor of Physics and Applied Physics at SEAS, the research team produced hundreds of thousands of these highly deformable spherical capsules filled with air and suspended them in silicone oil. . When the pressure inside the liquid increases, the capsules collapse to form a lens-like hemisphere. When this pressure is removed, the capsules return to their spherical shape.

Properties and applications of metafluid

This transition changes many properties of the fluid, including viscosity and opacity. These properties can be adjusted by changing the number, thickness and size of the capsules in the liquid.

The researchers demonstrated the fluid’s ability to be programmed by loading the metafluid into a hydraulic robotic gripper and picking up glass bottles, eggs, and herbs in the gripper. In a traditional hydraulic system powered by simple air or water, some kind of sensing or external control is needed to adjust the robot’s grip and pick up all three objects without crushing them.

But with metafluid, no sensation is required. The liquid itself responds to different pressures, changing its fit to adjust the strength of the holder to be able to pick up heavy bottles, delicate eggs and small blueberries without additional programming.

“We show that we can use this fluid to give intelligence to a simple robot,” Djellouli said.

The team also demonstrated a liquid logic gate that can be reprogrammed by changing the metafluid.

Optical properties and liquid states

The metafluid also changes its optical properties when subjected to varying pressures.

When the capsules are round, they scatter light, making the liquid opaque, just as air bubbles make sparkling water appear white. But when pressure is applied and the capsules collapse, they act like microlenses, focusing light and making the liquid transparent. These optical properties can be used for a number of applications, such as e-inks that change color based on pressure.

The researchers also showed that when the capsules are spherical, the metafluid behaves like a Newtonian fluid, meaning its viscosity changes only in response to temperature. However, when the capsules collapse, the suspension becomes a non-Newtonian fluid, meaning that its viscosity will change in response to the shear force – the greater the shear force, the more fluid it becomes. It is the first metafluid that has been shown to transition between Newtonian and non-Newtonian states.

Next, the researchers aim to investigate the acoustic and thermodynamic properties of the metafluid.

“The field of application for these scalable, easy-to-produce metafluids is huge,” Bertoldi said.

Citation: “Shell folding for programmable metafluids” by Adel Djellouli, Bert Van Raemdonck, Yang Wang, Yi Yang, Anthony Caillaud, David Weitz, Shmuel Rubinstein, Benjamin Gorissen, and Katia Bertoldi, April 3, 2024, Nature.
DOI: 10.1038/s41586-024-07163-z

Harvard’s Office of Technology Development has retained intellectual property related to this research and is exploring commercialization opportunities.

The research was supported in part by the NSF through the Harvard University Materials Research Center for Materials Science and Engineering grant number DMR-2011754.