Trapping of a mie sphere by acoustic pulses: effects of pulse length

The acoustic counterpart of optical tweezers shows great promise as a single-particle manipulator using a highly focused acoustic beam. Understanding the dependence of the trapping performance of the acoustic beam on the acoustic pulse length may facilitate its development and extend the applications. Herein, we propose a ray-based model for the time-course simulation of instantaneous forces exerted on single Mie spheres by highly focused acoustic pulses of arbitrary lengths. The simulations considered single fat/lipid spheres with a density of 950 kg/m3 and speed of sound of 1450 m/s, suspended in water and located on the beam axis. Simulation was used to establish the spatial and temporal pressure data of pulsed acoustic fields transmitted from a 100-MHz transducer with a half-power bandwidth of 50% and an f-number of 1. The instantaneous intensity vectors were calculated to represent rays for estimating forces exerted by consecutive wave-particle interactions. The results suggest that short acoustic pulses can exert negative forces pulling spheres beyond the focus in the direction opposite to that of wave propagation. Varying the excitation pulse duration has no effect on the region where the exerted forces are averagely negative. Lengthening the excitation pulse duration rapidly increases the amplitude of the average force. A smaller sphere experiences a greater average force when the spatial length of a transmitted acoustic pulse is comparable to the sphere diameter. The amplitude of the instantaneous force can be maximized as long as the acoustic pulse length is longer than the sphere diameter. Regulating the relation between acoustic pulse length and sphere size may be advantageous in particle sorting applications.