Interfacial wave between acoustic media with Willis coupling

Acoustic Willis materials, also known as acoustic bianisotropic materials, exhibit unconventional coupling between pressure and velocity, as well as momentum and strain, which have been shown to enable extraordinary control over the propagation of acoustic waves. Our work first predicts that, under proper conditions, interfacial mode can exist at the interface between two acoustic Willis materials, and the existence of the interfacial wave strongly relies on relative orientations of the Willis coupling vectors of the two materials. To demonstrate our theory, experiments are also conducted utilizing a well-designed adjustable acoustic Willis metamaterial. Our experimental results are in good agreement with simulation and theoretical analysis, although there is unavoidable acoustic attenuation in experiments due to damping effect. This work presents a new functional design with acoustic Willis materials and opens a new route to control acoustic waves.