Dynamic ultrasound radiation force in fluids
  The subject of this paper is to present a theory for the dynamic radiation force produced by dual-frequency ultrasound beams in lossless and non-dispersive fluids. A formula for the dynamic radiation force exerted on a three-dimensional object by a dual-frequency beam is obtained stemming from the fluid dynamics equations. Dependence of the dynamic radiation force to nonlinear effects of the medium is analyzed. The developed theory is applied to calculate the dynamic radiation force exerted on solid elastic (brass and stainless steel) spheres by a low-amplitude dual-frequency plane wave. The dynamic radiation force is calculated by solving the dual-frequency plane wave scattering problem for solid elastic spheres. Results show that the dynamic radiation force on the analized spheres presents fluctuations similar to those present in the static radiation force. Furthermore, analysis of the static and the dynamic radiation forces on the stainless steel sphere shows that they have approximately the same magnitude. 
Glauber T. Silva, Shigao Chen, James F. Greenleaf, and Mostafa Fatemi