Nature of acoustic nonlinear radiation stress

When a fluid is insonified with ultrasound, a flow consequence of a net stress becomes observable, which has been described as acoustic streaming, quartz wind, acoustic radiation force, or acoustic fountain. Following Sir James Lighthill's formulation of the Reynold's streaming, these phenomena have been attributed to a cumulative viscous effect. Instead, a multiscale effect, whereby the constitutive elastic nonlinearity scales from the ultrasonic to the macroscopic time, is here proposed and formulated to explain its origin. This raises an additional term in the Navier-Stokes equation, which ultimately stems from the anharmonicity of the atomic potential. In our experimental validation, this theory is consistent in water and for a range of ultrasonic configurations, whereas the formerly established viscous theory fails by an order of magnitude. This ultrasonic-fluid interaction, called nonlinear mechanical radiation since it is able to remotely exert a stress field, correctly explains a wide range of ind...