The acoustical radiation force in Rayleigh particles is calculated by the method of the Rayleigh scattering approach. However, in the Mie regime, the Ray acoustics approach is used to calculate the acoustical radiation force. This method, making use of a focused Gaussian ultrasonic above 20 MHz, in theory, verified the negative acoustical radiation force in one-dimension, that is, the possibility of a trapping force.  

Earlier in time, scientists had considered the attenuation of ultrasonic waves in one-dimension, calculated the effect of the attenuation on the magnitude of the axial acoustical radiation force, established the calculation model of acoustical radiation force under the case of three-dimensions, and studied the effect of the location of the particle on the three-dimensional acoustical radiation force. They used a 200 MHz zinc oxide transducer to achieve the trapping of micron-sized particles in the horizontal two-dimensions, verifying the feasibility of experimental studies of acoustic tweezers in the Mie regime.    

Based on the calculation model of radiation force exerted on arbitrarily located spheres, this research calculated the acoustical radiation forces in the lateral and axial directions. The Ray acoustics approach was utilized, considering the impacts of the ultrasonic attenuation in the particle on both lateral and axial forces. It used a highly focused 100 MHz ultrasonic beam with a Gaussian intensity distribution for simulation. As well, a fat particle whose radius was much larger than the ultrasonic wavelength was treated as the sample particle. In addition, a series of parameters in the system were analyzed, such as the influence of the ultrasonic attenuation factor and the radius, and initial position of the particle on the magnitude of the radiation force. Also, the feasibility of the acoustical trapping was discussed. 

The research result has shown that the initial position of the particle plays an important role in the magnitude of the acoustical radiation force, only when the particle is in a specific location can it receive the acoustical trapping force. Besides, the results also show that, when the ultrasonic frequency is extremely high, the ultrasonic attenuation cannot be ignored.  

This research was supported by the National Basic Research Program of China(Grant Nos. 2012CB921504, 2011CB707902), the National NaturalScience Foundation of China (Grant No. 11274166), Fundamental ResearchFunds for the Central Universities (Grant Nos. 1113020403,1101020402), the State Key Laboratory of Acoustics, Chinese Academy of Sciences (Grant No. SKLA201207), the priority academic program development of Jiangsu Higher Education Institutions and SRF for ROCS, SEMand project of InterdisciplinaryCenter of Nanjing University. 

This research result was published online: http://link.springer.com/article/10.1007%2Fs11433-013-5115-4#page-1and published on the recently issued journal of Physics, Mechanics & Astronomy, SCIENCE CHINA (Vol.56, No.7:1237–1245, July 2013, doi: 10.1007/s11433-013-5115-4).