Abstract:

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, acoustic, thermal and mechanical properties have been realized, with the potential to revolutionize fields ranging from energy harvesting and conversion to sensing and imaging. Although these materials are typically based on a periodic array of solid building blocks, recent studies have demonstrated that the mixing of carefully designed units into a fluid medium can also yield remarkable properties. Such “metafluids” have enabled reconfigurable and adaptable photonic fluids, negative-acoustic-index materials and unconventional thermodynamic properties. Unlike solid metamaterials, metafluids can take the shape of their container, flow, and do not require a precise arrangement of their building blocks. These characteristics make metafluids a promising platform to enhance the functionality of numerous systems that interact with fluids during operation.

Inspired by these recent advances, here we show that by mixing highly deformable shells into an incompressible fluid we can realize a metafluid with programmable elastic response, optical behavior and viscosity. We show that the reversible buckling of the shells radically changes the characteristics of the fluid and provides exciting opportunities for expanding its functionality. First, we experimentally demonstrate both at centimeter and micrometer scale that the buckling of the shells endows the fluid a highly nonlinear behavior. Then, we numerically study how the shells geometry affects such nonlin- ear response. Finally, we harness the nonlinear fluid behavior to develop smart robotic systems, highly tunable logic gates and optical elements with switchable reponse. Further, we demonstrate that shells buckling also affects the fluid viscosity, making the flow in the laminar regime dependent not only on the pressure difference between two points but also on the absolute value of pressure at these points. As such, the proposed metafluid provides a promising platform to enhance the functionality of existing fluidic devices by expanding the capabilities of the fluid itself.