A metafluid with multistable thermodynamic properties

The thermodynamic properties of fluids play a crucial role in many areas, particularly in the energy and refrigeration industries. These widespread and essential cycles are leading causes of global warming, and in particular, refrigeration was recently listed as the single most polluting technology, due to fluids used within such cycles. Creating a fluid with exceptional thermodynamic properties can thus be of great practical importance, yielding possibilities for advancement in many fields.

In this work, we study the properties of a suspension composed of a multitude of lubricated multistable capsules (structures capable of transforming between different equilibrium deformation patterns) enclosing a compressible gas and immersed within another fluid. The thermodynamic properties of the suspension are determined by an interaction between the external liquid, the encapsulated gas, and the characteristic elastic energy profile of the capsules. By leveraging the elastic multistability of the suspended capsules, we can produce a fluid with multiple stable density points for a given pressure and temperature states as well as unstable regions.

We study the pressure-density relations, and stability, of a ``metafluid”, composed of a multitude of multistable gas-filled capsules suspended in a liquid. As a simple example of a multistable capsule, we examine multiple connected bi-stable elements, produced by combining two non-identical conical frusta in opposing orientations. Bi-stability is therefore achieved by switching between extended and collapsed states through the inversion of one frustum which creates a significant change in volume. We theoretically and experimentally examine the suspension’s internal energy, equilibrium pressure-density relations, and their stability for both adiabatic and isothermal processes. We show that the elastic multistability of the capsules endows the fluid with multistable thermodynamic properties, including the ability to capture and store energy at standard atmospheric conditions, not found in naturally available fluids.