Shell buckling for programmable metafluids

The pursuit of materials with enhanced functionality has led to the emergence of metamaterials - artificially engineered materials whose properties are determined by their structure rather than composition. Through careful design of their building blocks, metamaterials with unprecedented electro-magnetic~\cite{engheta2006metamaterials}, acoustic, thermal and mechanical properties have been realized, with the potential to revolutionize fields ranging from energy harvesting and conversion to sensing and imaging. Traditionally, these building blocks are arranged in fixed positions within a lattice structure. However, recent research has revealed the potential of mixing disconnected building blocks in a fluidic medium.

Such ``metafluids'' can show reconfigurable and adaptable photonic properties, negative acoustic indices and unconventional thermodynamic properties.

Inspired by these recent advances, here we show that by mixing highly deformable spherical capsules into an incompressible fluid, we can realize a metafluid with programmable elastic response, optical behavior, and viscosity. We show that reversible buckling of the shells radically changes the characteristics of the fluid and provides exciting opportunities to expand its functionality. First, we experimentally demonstrate both at the centimeter and micrometer scales that the buckling of the shells endows the fluid with a highly nonlinear behavior. Then, we numerically study how the shell geometry affects such nonlinear response. Subsequently, we harness this behavior to develop smart robotic systems, highly tunable logic gates, and optical elements with switchable characteristics. Finally, we demonstrate that the collapse of the shells upon buckling leads to a large increase in the suspension viscosity in the laminar regime. As such, the proposed metafluid provides a promising platform for enhancing the functionality of existing fluidic devices via expanding the capabilities of the fluid itself.