Thermo-acoustic radiation of free-standing nano-thin film in viscous fluids

In this paper, the theory of thermo-acoustic wave emission and propagation is further generalized to consider media viscosity effects. The fully coupled thermo-acoustic field theory and solution for a free-standing nano-thin film in viscous fluid are further developed where fluid viscosity was usually ignored in many early works on thermo-acoustic interaction. The effects of heat loss, heat capacity and heat exchange are also considered in this investigation. In addition to examining the double frequency effect when a sinusoidal alternating current acts on the nano thin film, we have successfully decoupled the thermal wave propagation from the acoustic effect by reformulating the governing differential equations. With appropriate simplification, an analytical prediction for near-field and far-field acoustic pressure response with viscosity effect is derived. We illustrate in some practical examples that the analytical predictions are in excellent agreement with experiments. Further referring to some past works, an analytical solution of higher-order accuracy with respect to the wavenumber omitted in the previous studies is obtained. Although the viscous effect may have limited influence in the vicinity of the thermo-acoustic source, it is vital for the subsequent propagation of acoustic waves. This work may have shown great potentials for the design of thermo-acoustic projectors for new underwater sonars as compared with the electro-acoustic traditional designs that are induced by membrane vibrations.