Viscosity Metamaterials*

Metamaterials are composite materials that are typically engineered by assembling their constituents at a scale smaller than the characteristic length associated with their response. They have traditionally been used to design exotic properties including structural, optical, thermal, and acoustic properties that are otherwise not achievable from the conventional materials. Here, we introduce a new class of metamaterials- viscosity metamaterials- created by applying acoustic perturbations to a shear thickening suspension. Specifically, we show that the perturbations can drive large viscosity oscillations spanning orders of magnitude at a timescale that is much smaller than the timescale of global material flow. This behavior enables us to construct metamaterials whose viscosity is a combination of thickened high viscosity and fully dethickened low viscosity states. We show in a phase diagram that the boundaries of the viscosity metamaterial is determined by the interplay between the applied strain rate, acoustic power, and the timescales of perturbations. We envision that these novel viscosity metamaterials can be used in numerous applications ranging from soft robotics to microfluidics, and from 3D printing to engineering exotic fluids.