An intelligent liquid with programmable properties

There are trends that are over as quickly as they appear: Bubble tea is one of them. Hardly anyone can remember this strange tea-based drink, to which gel-like balls of cornflour are added. However, Katia Bertoldi's research team at Harvard University in the US may have had bubble tea in mind when they were in the laboratory developing a new type of liquid. The so-called metafluid consists of air-filled capsules made of a rubber-like material, which are distributed in a liquid and each have a diameter of between 50 and 500 micrometres. As a result, the substance combines the properties of a solid, a liquid and a gas. When a force acts on the beads, they deform. This allows the nature of the fluid to be radically but reversibly changed. The research work was published in the scientific journal "Nature".

Metamaterials are artificially produced materials with optical, electrical or magnetic properties that do not occur in nature. They promise to conceal objects in space and time, making them invisible, inaudible or even impalpable. One exciting application is cloaking devices. However, most of these materials are solid. "In contrast to solid metamaterials, metafluids have the unique ability to flow and adapt to the shape of their container," explains lead author and Professor of Applied Mechanics, Katia Bertoldi. They can also be compressed like a gas. "Our aim was to create a metafluid that also has other unusual properties, such as programmable viscosity, elasticity and adjustable optical properties."

The team used the manufacturing techniques of another Harvard lab to produce hundreds of thousands of small, mouldable, air-filled capsules and then mixed them with silicone oil. When the pressure in the liquid increases, the capsules collapse and form a lens-like hemisphere. When the pressure is removed again, they spring back into their spherical shape. During these transitions, several properties of the liquid change, such as its translucency and viscosity. If the number, size and thickness of the capsules are changed, the strength of the reaction can also be adjusted.

In addition, the researchers report that it is the first metafluid that can demonstrably switch between the Newtonian and non-Newtonian state. The latter refers to a substance whose deformation behaviour is not linear. If a force acts on it, such a material suddenly no longer behaves like a liquid, but more like a solid.

The experts demonstrated that the liquid is programmable by filling the metafluid into a hydraulic robot gripper and having it pick up a glass bottle, an egg and a blueberry. With a conventional hydraulic system powered by air or water, the robot would have to rely on some kind of sensor or external control to adjust its grip and pick up all three objects without crushing them. But with the metafluid, no sensors are required. The fluid itself reacts to different pressures and changes its compliance to adjust the force of the gripper.

"The range of applications for such an easily producible metafluid is huge," said Bertoldi. For example, it could be used for intelligent shock absorbers that dissipate energy depending on the force of the impact, for optical devices that change from clear to opaque or for programming robots.

Spectrum of Science

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