Tiny rubber spheres used to make a programmable fluid

Building a robot that could pick up delicate objects like eggs or blueberries without crushing them took lots of control algorithms that process feeds from advanced vision systems or sensors that emulate the human sense of touch. The other way was to take a plunge into the realm of soft robotics, which usually means a robot with limited strength and durability.

Now, a team of researchers at Harvard University published a study where they used a simple hydraulic gripper with no sensors and no control systems at all. All they needed was silicon oil and lots of tiny rubber balls. In the process, they’ve developed a metafluid with a programmable response to pressure.

Swimming rubber spheres

“I did my PhD in France on making a spherical shell swim. To make it swim, we were making it collapse. It moved like a [inverted] jellyfish,” says Adel Djellouli, a researcher at Bertoldi Group, Harvard University, and the lead author of the study. “I told my boss, 'hey, what if I put this sphere in a syringe and increase the pressure?' He said it was not an interesting idea and that this wouldn’t do anything,” Djellouli claims. But a few years and a couple of rejections later, Djellouli met Benjamin Gorissen, a professor of mechanical engineering at the University of Leuven, Belgium, who shared his interests. “I could do the experiments, he could do the simulations, so we thought we could propose something together,” Djellouli says. Thus, Djellouli’s rubber sphere finally got into the syringe. And results were quite unexpected.

The sphere has a radius of 10 mm, and its 2-mm-thick silicone rubber walls surround a pocket of air. It was placed in a container with 300 ml of water. When the pressure in the container started to increase, the sphere, at 120 kPa, started to buckle. Once it started to buckle, pressure remained relatively steady for a while, even though the volume occupied by the fluid continued dropping. The liquid with a sphere in it did not behave like water anymore—it had a pronounced plateau in its pressure/volume curve. “Metafluids—liquids with tunable properties that do not exist in nature—were theorized by Federico Capasso and colleagues, who wanted to achieve a liquid with negative refractive index. They started with optics back then, but looking at the behavior of water with this rubber sphere in it, we knew what we had was a metafluid,” says Djellouli.