Anti-symmetric flexural modes for the detection of humidity variation

Antisymmetric flexural (ASF) modes are guided acoustic modes propagating along the apex line of a wedge with particle motion that is antisymmetric about the midplane bisecting the apex angle. This paper describes experimental observations in applying ASF modes for the detection of moisture variation. In our setup, the wedge tip is coated with a thin layer of hygroscopic film for the absorption of moisture, so the phase velocity of the ASF mode is influenced by the changing humidity of the surrounding air. Enhanced by the wedge tip geometry, mass-loading effects on the ASF velocity resulting from the absorption of humidity are pronounced in a low-frequency regime of 0.2 to 2.0 MHz. Numerical models based on finite element analysis are constructed to explain the humidity sensing mechanism.