Tiny Spheres Key to Tunable ‘Smart Liquid’

Tiny Spheres Key to Tunable ‘Smart Liquid’

Programmable liquids could aid robot grippers, shock absorption, acoustics, and more

Illustration of a cartoon child bouncing on a liquid-filled ball

Pneumatic and hydraulic systems use pressurized gases and liquids, respectively, to carry out countless crucial mechanical tasks. Each has distinct pros and cons: pneumatic systems are springy and thus less vulnerable to shock damage; hydraulic ones offer more precision and power. This is because gases are compressible, whereas liquids are not. But “there’s nothing in between, naturally speaking,” says Adel Djellouli, an applied physicist at Harvard University. “Our idea was to occupy this middle space and make artificial liquids with programmable compressibility.”

In Nature, Djellouli and his colleagues describe testing a new “meta fluid” composed of tiny, air-filled silicone rubber spheres suspended in liquid. This fluid’s compressibility, viscosity and transparency are all programmable. Potential applications include smart shock absorbers, advanced e-inks and programmable sound baffles.

The mechanism is simple: At certain pressures, the spheres buckle and let the liquid become more compressed. The spheres’ size, thickness, softness and number can be adjusted to tune various properties under pressure. Djellouli and his team demonstrated the fluid’s versatility by putting it in a hydraulic robot gripper to grasp objects of differing size and fragility: a glass bottle, an egg and a blueberry. The researchers used the same amount of fluid in the system to close the gripper on each object without crushing it.

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“We tuned the springiness of our fluid so that whether it’s something big and rigid or small and fragile, the gripper can safely grab it,” Djellouli says. This strategy could endow conventional robots with capabilities previously limited to more specialized “soft” robots.

Air in the spheres scatters light, the researchers found, making the liquid opaque like foamy water—but buckled spheres scatter less, making it transparent. Easy switching between states could contribute to quick-response e-inks. Buckled spheres also make the fluid more viscous because they can no longer roll around one another freely. This property could be useful in developing tunable vibration-damping systems, says Robert Shepherd, who builds advanced robots at Cornell University. “The more viscous something is, the more energy it dissipates,” he says.

A 2022 study described a similar fluid using collapsible, straw-shaped capsules, Shepherd adds, but with a crucial difference. “The straws were quite large, so flowing through small orifices wasn’t [possible],” Shepherd says. The new spheres are “a pretty cool advance.”

Additional abilities have yet to be investigated. The fluid’s thermodynamic properties may be useful for energy storage, and Djellouli says he is currently exploring acoustic possibilities: “You can tune a meta fluid so it blocks specific frequencies,” such as speech—and then lets them through at the flick of a switch.

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