Fluid–structure waves in a micromachined variable impedance waveguide

Abstract

A micromachined fluid–structure system, intended to demonstrate a unique cochlear-like acoustic sensing scheme, has been fabricated and analyzed. The system consists of a 0.11-mm-deep Pyrex fluid chamber, anodically bonded to a silicon structure housing a tensioned membrane. The membrane varies in width from 0.14 to 1.82 mm. Both isotropic LPCVD silicon nitride membranes and orthotropic nitride/polyimide membranes have been fabricated. Silicone oils of either 5 or 20 cSt viscosity are used. Laser vibrometry measurements show strong fluid–structure traveling waves. Wave speeds are between 50 and 300 m/s in the 4- to 35-kHz band. These traveling fluid–structure waves exhibit maximum structural motion at a location determined by their frequency. Wave decay rate is influenced by the viscosity of the fluid and, after the waves become evanescent, by membrane orthotropy. Results from finite element computations of an orthotropic membrane coupled to a compressible viscous fluid are compared with experiment. The fluid is modeled using either a two-dimensional thin-film approximation or the three-dimensional linearized Navier–Stokes equation. Both models accurately predict the observed fluid–structure response. Nondimensional parameters controlling system performance, and regions of applicability of the models, will be discussed. [Work supported by ONR, NIH, and NSF.]