In this paper, the acoustic trapping force (Ftrapping) and the trap stiffness (or compliance k) are experimentally determined for a streaming droplet in a microfluidic channel. Ftrapping is calibrated to viscous drag force produced from syringe pumps. Chebyshev-windowed chirp coded excitation sequences sweeping the frequency range from 18 MHz to 30 MHz is utilized to drive the transducer, enabling the beam transmission through the channel/fluid interface for interrogating the droplets inside the channel. The minimum force (Fmin,trapping) required for initially immobilizing drifting droplets is determined as a function of pulse repetition frequency (PRF), duty factor (DTF), and input voltage amplitude (Vin) to the transducer. At PRF = 0.1 kHz and DTF = 30 % , Fmin,trapping is increased from 2.2 nN for Vin = 22 Vpp to 3.8 nN for Vin = 54 Vpp. With a fixed Vin = 54 Vpp and DTF = 30 % , Fmin,trapping can be varied from 3.8 nN at PRF = 0.1 kHz to 6.7 nN at PRF = 0.5 kHz. These findings indicate that both higher driving voltage and more frequent beam transmission yield stronger traps for holding droplets in motion.
The stiffness k can be estimated through linear regression by measuring the trapping force (Ftrapping) corresponding to the displacement (x) of a droplet from the trap center. By plotting Ftrapping - x curves for certain values of Vin (22/38/54 Vpp) at DTF = 10 % and PRF = 0.1 kHz, k is measured to be 0.09, 0.14, and 0.20 nN/¦Ìm, respectively. With variable PRF from 0.1 to 0.5 kHz at Vin = 54 Vpp, k is increased from 0.20 to 0.42 nN/¦Ìm. It is shown that a higher PRF leads to a more compliant trap formation (or a stronger Ftrapping) for a given displacement x. Hence the results suggest that this acoustic trapping method has the potential as a noninvasive manipulation tool for individual moving targets in microfluidics by adjusting the transducer¡¯s excitation parameters.