Reversible tunable topological phononic crystals based upon thermo-acoustic hydrogels

Topological insulator originally found in electronic systems has inspired an analogue in phononic crystals, which has revolutionized fundamental concepts of elastic and acoustic transmission, offering one-way propagation edge modes immune to backscattering. Nevertheless, for traditional topological phononic crystal (TPC), once the structure is made, only a particular bandgap frequency can occur. Realization of reconfigurability and frequency tuning of topological one-way transmission is challenging. To solve the problem, the TPC is made active on penetration with thermosensitive acoustic hydrogels to produce a reconfigurable bandgap of acoustic wave. We explore that at a certain frequency range, the topological edge state (TES) within the TPC can be reversibly turned on/off as changing the state of (vinyl alcohol) poly-N-isopropylacrylamide (PVA-PNI-PAm) hydrogel between hydrophilic and hydrophobic. In addition, the topology of the TPC does not vary as the TES was tuned by varying the sound group velocity of PVA-PNI-PAm rather than the structural geometry. Such a feature enables a continuous tuning of the central frequency of phononic bandgap and TES by gradually changing the state of PVA-PNI-Pam. The state transition of PVA-PNI-Pam thermal-acoustic hydrogel from hydrophilic to hydrophobic and vice versa has been experimentally realized. Our proposed proof of concept may provide a platform for intelligent acoustic devices with dynamically programmable and reconfigurable functionalities, leading to various potential applications in acoustic wave guiding and control, integrated acoustics, acoustic security, and information processing etc.