Metafluids

full paper here: https://www.nature.com/articles/s41586-024-07163-z

Normally, all matter exists as either a solid, a liquid, or a gas. Gases can be compressed, whereas liquids cannot; solids have a definite shape, but gases and liquids conform easily to a container. Combined, liquids and gasses are called fluids, because they flow.

In this work, we blur the lines between these states by developing a ‘metafluid’, where we add gas-filled shells to a liquid. We thus combine solid shells that are filled with gas in a liquid.

Why? Thin shells implode when you suck volume out of them, in a phenomena that is called buckling. Why is this interesting? Well, the relationship between internal volume and internal pressure is highly nonlinear. We will use this nonlinearity!

If you make the shells out of a e.g. metal, they fail catastrophically when they buckle. However here we make them out of a rubber, so you can buckle them reversibly. They just bounce right back!

The questions is: How does a liquid, filled with rubber shells, that itself are filled with air, behave when you compress the fluid externally?

We show that for a single shell, you can sum up the nonlinear pressure volume curve of the shell (the one that is nonlinear!) with the pressure volume curve of the gas in the shell (PV=nrt :: ideal gas law). The nonlinearity is thus preserved!

If you now put multiple shells in a single container and compress it (decrease volume), we see that after an initial rise, the pressure plateaus where all the individual shells buckle, and rises again after the last shell is buckled.

This process is reversible, but when increasing the volume back again, we see a plateau are a lower pressure level emerging. This is because the buckling pressure is higher than the unbuckling pressure of a spherical shell.

We show that this theory holds across different length scales. Making large shells (radius 1cm) is easy using molding, however for small shells (radius 250 μm) we used a microfluidic manufacturing process, similar to blowing bubbles.

The main questions that still remains: Why do we want to have such a metafluid? What can we do with it, that we cannot do with normal fluids? Well, we show three applications in the paper!

FIRST, We make use of the nonlinear compressibility characteristics to make a ‘dumb’ gripper that with one input (‘grasp’ or ‘no grasp’) can hold diverse objects of different sizes, stiffness, fragility, without crushing them. The metafluid inside does the ‘calculations’.

SECOND, the metafluid has optical properties. Below a certain pressure (the buckling pressure of the shells) it is cloudy. However when the metafluid is put under sufficient pressure, and all shells are buckled, light scattering is reduced and you can see right through.

THIRD, and most interestingly, metafluids don’t flow like normal liquids! Normally, the flow rate is determined by pressure difference between two points. For the metafluid, also the absolute pressure is important, as this determines if the shells are spherical or buckled.

To conclude, “Metafluids are as multifunctional as a swiss army knife — metafluids can do it all.” From the opinion piece by Pierre-Thomas Brun : https://www.nature.com/articles/d41586-024-00888-x