The negative acoustic radiation force on a spherical particle in free space

The absorption power,scattering power and extinction power during the acoustic scattering process as well as the relationship between the three are analyzed.The negative absorption phenomenon caused by improper parameter selection is found through calculation.Moreover,the general expression of acoustic radiation force is derived based on the conservation law of momentum.The relationship between acoustic radiation force and acoustic energy flow is illustrated and the existence of negative acoustic radiation force is verified from both theoretical and computational aspects.The backscattering acoustic field is suppressed when the radiation force is negative.On this basis,the influence of the two common factors,the eccentricity of the particle and the viscosity of the fluid,on the negative acoustic radiation force is further studied.Using the additional theorem for spherical functions,the scattering coefficients and the acoustic radiation force expression for an eccentric sphere are derived.The results show that the eccentric distance and the particle material both significantly affect the conditions for generating the negative radiation force.Under the low frequency approximation,the sign of the final radiation force is determined by whether the positive radiation force caused by the fluid viscosity can completely offset the original negative one.The results are expected to provide a theoretical guide for developing single beam acoustic tweezers using negative radiation force to manipulate certain particles.