A Metaliquid with Programmable Properties

Metamaterials, the properties of which depend on structure rather than composition, are finding uses in a variety of engineering applications, from acoustics to medical devices. But the problem is that they’re all solids. If they could be liquids, they could more easily conform to a broader variety of shapes while preserving easily manipulated properties—which is what researchers at Harvard University have unlocked with their development of a metafluid. 

What's equally important is that researchers can manipulate a few of the metafluid’s properties such as opacity, viscosity, and springiness. The secret to achieving such a liquid lies in small elastomer spheres that range in size from 50 to 500 microns. In their work, the researchers suspended these particles in silicon oil.  

While silicon oil was the liquid of choice for this project, the spherical capsules are “agnostic of the fluid,” said Adel Djellouli, a research associate in Materials Science and Engineering at the School of Engineering and Applied Sciences at Harvard, and first author of the published paper from the research. “We put it in a liquid because we wanted to control volume but you can put it in air, water, glycerol, glue, it doesn’t matter.” 

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What remains fixed no matter the medium of suspension is the metafluid’s ability to buckle under pressure. So, when the pressure inside the liquid increases, the spherical capsules collapse and form a lens-like half-sphere. When the pressure is withdrawn, the spheres regain their original shape. 

“For now there are liquids that are highly incompressible and there are gases that are highly compressible, but there is nothing in between,” Djellouli said. “So, with our fluid we can occupy the space between liquid and gases, and we can program the pressure volume curve the way we want between the two behaviors, and even go to the solid state if we wish.” 

A fluid’s ability to flow and occupy a volume is what makes it a fluid. “So you put that material in a container, and it takes the shape of the container. And it's able to flow, which means that it doesn't have a shear modulus,” Djellouli said, “But what we are theorizing that we can do with these metafluids is that we can even go to the glassy system. And then if we lower the pressure, the absolute pressure, the spheres will expand, and eventually reach a state where they are all next to each other and form a glass and then revert back to liquid.” 
 

Applications for programmable liquids 

“Suspending structures in a fluid in a randomized way allows us to take advantage of an instability that is called buckling, to tune the properties of the fluid—how it compresses, how it looks optically, how it flows,” Djellouli said.  

When the spheres are in their original shape, for example, the liquid is opaque. But when the spheres collapse they each form a microlens of sorts, focusing light, and making the liquid more transparent. One of the applications for such a property change: e-inks that change color based on pressure. 

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The additional applications for such a metafluid are endless. Leveraging the tunable rheology of the fluid, they can have different profiles of shock absorption depending on the type of impact that the shock absorbers are subjected to. “You can think of this type of fluid as a way to mitigate or program the pressure waves that circulate in networks. And so that's a good way to potentially protect hydraulic networks in the future,” Djellouli said.  

A potentially promising application in robotics is in using the programmability of the fluid in hydraulic grippers for robotic handling. Traditionally robotic manipulation involves sensing the forces being exerted by the arm and picker, and carefully varying the pressure to reach the end goal. It would vary depending on whether the object to be picked was a blueberry, or egg, or a glass bottle. With the metafluid, “we pre-program the liquid so we can embed control into it so it can grasp all these different objects,” Djellouli said, “by going with this technique, you embed two things: first, the softness as a liquid and then also embedded control, so you don’t need an external control mechanism.” 

Next, the researchers hope to study the thermal and thermodynamic properties of the fluid as well. 

Poornima Apte is a technology writer based in Walpole, Mass.