Experimental demonstration of tunable spectral flow of elastic localized modes

We experimentally demonstrate the tunability of a topological defect mode and its spectral flow within the bandgap of a periodic structure made of beams and cylindrical masses and hosting a topological bandgap in its frequency spectrum. A defect is introduced by incorporating a pair of piezoelectric plates into a single unit cell of the lattice, giving rise to a localized mode inside the bandgap. We experimentally observe this mode and its spectral flow in the bandgap by altering the elastic modulus of the piezoelectric plates using negative capacitance shunt circuits. Further, through finite element simulations and experiments, we show that the spectral flow can be tuned by modulating the mass of the defect cell alone. This work introduces additional degrees of freedom in the design of elastic wave-based devices for applications in acoustic logic, waveguiding, and energy harvesting.