Radiation forces and acoustic flows in a liquid layer on an elastic substrate

The features of the formation and structure of the radiation force fields in the liquid layer created by the surface acoustic wave at the boundary of the liquid and elastic media are investigated. Various mechanisms of radiation forces and their impact on the medium and particles suspended in it are analyzed. It is shown that depending on the particle size and the density ratio, different mechanisms will be determining. Namely, the radiation force (it can be called "linear"), associated with the scattering of the acoustic field on the particle and the difference between the pulses before and behind the particle, acts on particles of relatively large sizes. Averaging leads to the appearance of a non-zero force, and the nonlinear parameters of the medium in the first approximation are insignificant here. Large particles are more involved in acoustic flows and the "linear" mechanism is predominant for them. It should be noted that this "linear" mechanism is mainly meant when talking about radiation pressure. On the contrary, for small particles (nanoparticles) this mechanism is ineffective and can be neglected in comparison with other mechanism. Nanoparticles will be captured by acoustic flows, the appearance of which is due to the nonlinearity of the equations of hydrodynamics. Such a mechanism can be called "nonlinear" because it is determined primarily by the nonlinear parameters of the medium. Acoustic flows in a liquid layer on an elastic substrate are calculated on the basis of the developed theory of radiation forces. Peculiarities of structuring ensembles of suspended particles in such flows and recoverability of the parameters of media and nanoparticles.