Independent control of dynamic material properties by exploiting structural hierarchy and intrinsic structural gradients

Achieving high damping and stiffness is challenging in common materials because of their inter-dependent scaling. Controlling extreme mechanical waves requires synergistically enhanced damping and stiffness. We demonstrate superior damping and stiffness in vertically aligned carbon nanotube (VACNT) foams that are also independently controllable by exploiting their synthesis-tailored structural hierarchy and structural gradients. They exhibit frequency- and amplitude-dependent responses with dramatically tunable dynamic stiffness while maintaining constant damping. Developing independent control over critical dynamic properties via engineered hierarchical and gradient materials will enable the creation of non-Hermitian metamaterials and active control of mechanical waves and vibrations.