A metafluid to equip robots

In the early 2000s, physicists made “superlenses” capable of discerning details up to 20 times smaller than a regular lens. The trick is to create nanoscale structures on the surface of the material that interact with light in a particular way. This device is an example of what we call “metamaterials”. The latter, depending on the configuration adopted, can manipulate electromagnetic waves, acoustic waves, etc. By relying on structures and not the intrinsic properties of their constituents, metamaterials are endowed with behaviors that are normally impossible. Most are solids, because it is the simplest way to manufacture and control the geometry of a structure. However, for certain applications, for example in systems that involve flows or must adapt to the shape of the container, it would be interesting to design “fluid” metamaterials. For around fifteen years, examples of such “metafluids” have been proposed. Recently, Adel Djellouli, a graduate of Grenoble-Alpes University and now at Harvard University, in the United States, and his colleagues developed a relatively simple system, but with complex behavior.

The researchers used silicone oil, an incompressible liquid (whose volume does not change when pressure increases), in which they suspended spherical rubber balls. These are filled with air and have a diameter which can vary from 50 micrometers to 2 centimeters. When physicists increase the pressure in the fluid, at a certain threshold, the spheres collapse and take on a concave shape. The team characterized the behavior of the system, notably its volume as a function of pressure. She found that when the pressure increases, the evolution of the system goes through a plateau because all the balls do not compress at the same time: when a ball crushes, the overall pressure drops slightly and a surplus of pressure is required to another ball flattens, and so on. The length of the plate can be adjusted simply by increasing the number of spheres in the fluid. Conversely, starting from the situation where all the balls are crushed, when the pressure is reduced, the evolution of the system follows a different “path” than when the pressure was put under pressure. This is called “hysteresis behavior” or “memory”. The balls return to their initial spherical shape as they pass through a smaller plate, thus erasing the memory of the fluid.

This system exhibits many interesting behaviors. “When the capsules are spherical, the metafluid behaves like a Newtonian fluid,” explains Adel Djellouli. This means that its viscosity only changes in response to temperature. However, when the capsules are crushed, the suspension transforms into a non-Newtonian fluid, meaning that its viscosity changes in response to a shear force – the greater the shear force, the more fluid the liquid becomes (like paint). . It is the first fluid that can pass from one state to another. »

From an optical point of view, the behavior also changes if we use balls whose diameter is of the order of a hundred micrometers. These spherical beads diffuse light in all directions, making the fluid opaque, like a glass of milk. But when the balls are deformed, the diffusion is less important and the fluid becomes transparent.

Finally, Adel Djellouli and colleagues demonstrated the programmable nature of the liquid by loading the metafluid into a hydraulic robotic gripper and having it grip a glass bottle, an egg, and a blueberry. In a traditional hydraulic system powered by simple air or water, the robot would need an external sensing or control system to be able to adjust its grip and pick up all three objects without crushing them. “With metafluid, no detection is necessary,” emphasizes Adel Djellouli. The liquid itself responds to different pressures, changing its flexibility to adjust the force of the gripper to lift any object, without any external control. »