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We report on the first dynamic study of acoustical spring effect in a compliant cavity formed between a spherical ultrasonic transducer immersed in water and the free liquid surface located at its focus. As its optical analog, this effect is due to the mutual feedback between the cavity length L and the large acoustical power stored inside the cavity, here through acoustic radiation pressure. We use surface waves to investigate the acoustical spring effect. The amplitude of surface waves above the cavity is observed to vary with the slope of variation of the L -dependent acoustic radiation pressure exerted on the liquid surface, i.e. with the acoustic spring stiffness. The observed simultaneous back-scattering of these surface waves demonstrates that the surface response to the cavity length variations results mainly in an added stiffness, i.e., in an increase of the real part of the surface impedance above the cavity. Finally, when the liquid surface is located out of the focal plane, spontaneous surface oscillations are reproducibly observed, which may be due to a parametric instability. Graphical abstract: [ABSTRACT FROM AUTHOR]

Recent experimental developments showed that the use of the radiation pressure, induced by a continuous laser wave, to control fluid-fluid interface deformations at the microscale, represents a very promising alternative to electric or magnetic actuation. In this article, we solve numerically the dynamics and steady state of the fluid interface under the effects of buoyancy, capillarity, optical radiation pressure and viscous stress. A precise quantitative validation is shown by comparison with experimental data. New results due to the nonlinear dependence of the optical pressure on the angle of incidence are presented, showing different morphologies of the deformed interface going from needle-like to finger-like shapes, depending on the refractive index contrast. In the transient regime, we show that the viscosity ratio influences the time taken for the deformation to reach steady state. [ABSTRACT FROM AUTHOR]