Kirigami helps design new materials
In the fable The Oak and the Reed, the reed withstands the storm because of its ability to bend without breaking under the wind. This deformability of living organisms has long inspired material designers and scientists who study, for example, the behavior of an object immersed in a fluid flow. To take it a step further, a team from the Hydrodynamics Laboratory (*LadHyX) has been investigating how small cuts in a flexible material allow it to expand on its own in water to adopt a desired configuration. Their work on this topic has just been published in the journal Physical Review Fluids.
Kirigami, an art from Japan, involves cutting a sheet of paper into a specific 3D shape. "This technique has already led scientists to imagine highly flexible bandages or solar panels that can tilt to follow the path of the Sun in the sky. But kirigami had not yet been used to control the deployment of a material in fluids," explains Sophie Ramananarivo, Assistant professor at Ecole Polytechnique, who is leading a research project on this subject at LadHyX.
For the work just published, the researchers, including Tom Marzin, a PhD student and first author of the paper, used laser cutting to make fine slits in the surface of a plastic sheet. Plunged into a stream of water, these slits become pores through which the flow passes and the sheet becomes a swollen net in the direction of the flow. But the swelling is not symmetrical: as the material deforms, small blades appear. As a consequence, the force exerted by the water also has a component perpendicular to the flow. The team also built a theoretical model from the experiment to understand this phenomenon.
(a and b) A kirigami pattern is cut out of a plastic sheet. The sheet can then unfold in a stream of water. (c) Seen from above, the blades (in black), due to the geometry of the cutting pattern, cause an asymmetric swelling.
Other geometric cutting patterns can be designed, for example with carefully placed notches in the sheet, to achieve a different configuration in the flow. The eventual aim is to reverse engineer the material, i.e. to determine which patterns should be cut into the material so that it adopts the desired geometry. "What interests me in particular is how the structure introduced by the kirigami makes the material intelligent, in the sense that it adopts the right shape in the flow," explains Sophie Ramananarivo. And kirigami's cousin, origami, where the leaves are folded without being cut, is also one of her sources of inspiration.
*LadHyX: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris