Quantitative analysis of temperature dependent acoustic trapping characteristics by using concentric annular type dual element ultrasonic transducer.

Abstract This paper presents the temperature dependence of lateral acoustic trapping capability by probing the speed of sound in individual lipid droplets at a given temperature of water and measuring its corresponding displacement, a value for quantitatively evaluating a spring-like behavior of the acoustic trap with certain strength. A 20/40 MHz dual element LiNbO 3 ultrasonic transducer is fabricated to simultaneously perform both transverse trapping and sound speed measurement for each droplet over a discrete temperature range from 20 °C to 30 °C. Time of flight method is employed for pulse tracking that determines the arrival time of an echo reflected back from either a trapped droplet or a mylar film. The estimated speeds of sound in water and droplets are 1484.8 m/s and 1431.6 m/s at 20 °C, while 1506.0 m/s and 1400.6 m/s at 30 °C, respectively. As the temperature rises, the sound speed in droplets decreases at an average rate of 3.1 m/s/°C, and the speed in water increases at 2.1 m/s/°C. The average displacement varies from 150.0 μm to 179.0 μm with an increasing rate of 2.9 μm/°C, and its standard deviation is obtained between 1.0 μm and 2.0 μm over the same temperature range. Reduced sound speed as a function of rising temperature results in increased displacement, indicating that the trapping strength is adjustable by regulating ambient temperature in water as well as by changing transducer excitation parameters. Therefore, the results suggest that the temperature dependence of this trapping technique can be exploited for developing a remote manipulation tool of micron-sized particles in a thermally fluctuating environment. It is also shown that any deviated trapping strength caused by thermal disturbance near the trap can be restored to its desired level by compensating either temperature difference or trapping system condition.