A single beam acoustic device, with its relatively simple scheme and low intensity, can trap a single lipid droplet in a manner similar to optical tweezers. Forces in the order of hundreds of nanonewtons direct the droplet toward the beam focus, within the range of hundreds of micrometers. This trapping method, therefore, can be a useful tool for particle manipulation in areas where larger particles or forces are involved. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3206910͔Optical trapping of small objects has been used to quantitatively investigate the mechanism of biological particles. 1,2 Optical tweezers 3 typically output forces on the order of piconewtons, and displace micrometer-sized particles up to hundreds of nanometers from the trap. Despite its precision and wide range of biological applications, however, its use has been limited to either optically purified samples or shallow regions in a medium. The high energy of focused lasers, in addition, has often induced photodamage on target particles. 4 In acoustics, Wu 5 introduced the term of acoustic tweezers and showed that latex spheres or clusters of frog eggs could be trapped by applying two opposing sound waves at 3.5 MHz. Here we present a single beam acoustic trapping that is similar to optical tweezers and differs from Wu's 5 dual beam method, based on our theoretical findings in ray acoustics regime: the tight focusing, the acoustic impedance match of particles with surrounding medium, and the particle size greater than the ultrasonic wavelength. [6][7][8] Note that only the transverse trapping was demonstrated in the present experiment because an axial focusing as tight as the lateral focusing could not be achieved.A 30 MHz lithium niobate ͑LiNbO 3 ͒ single element transducer was built to trap lipid droplets and driven by sinusoidal bursts. Note that the transducer was allowed to move only in the direction perpendicular to the beam axis ͓see Fig. 1͑a͔͒. The excitation frequency was varied from 23 to 37 MHz, and its peak-to-peak voltages were 22, 32, and 41 V pp . Due to the need for sharp intensity gradients, the transducer was press focused 9 to obtain an F-number of 0.75. The transducer was mounted on a three-dimensional motorized positioner ͑LMG26 T50MM, OptoSigma, USA͒, and a series of control commands was sent to the positioner by the customized LABVIEW controller ͓see Fig. 1͑b͔͒. To evaluate generated forces by the transducer, the acoustic peak pressures at different frequencies were measured at the focus, by a calibrated PVDF needle hydrophone having an active element size of 40 m ͑HPM04/01, Precision Acoustics, U.K.͒, as shown in Fig. 2͑a͒.Oleic acid lipid droplets of 126.0Ϯ 5.6 m in diameter were synthesized using droplet-based microfluidic devices, a robust method of forming monodispersed droplets in the nanometer to micrometer size range. 10 A single droplet was used as a target particle because of its good acoustic impedance match with the de-ionized water. The droplets were then carefully loaded underneath an acoustica...
