It has recently been demonstrated that it was possible to individually trap 70 μm droplets flowing within a 500 μm wide microfluidic channel by a 24 MHz single element piezo-composite focused transducer. In order to further develop this non-invasive approach as a microfluidic particle manipulation tool of high precision, the trapping force needs to be calibrated to a known force, i.e., viscous drag force arising from the fluid flow in the channel. However, few calibration studies based on fluid viscosity have been carried out with focused acoustic beams for moving objects in microfluidic environments.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/μm, 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/μm. 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.
展开▼
机译：最近已经证明，可以通过24 MHz单元件压电复合聚焦换能器单独捕获在500μm宽的微流体通道内流动的70μm液滴。为了将这种非侵入性方法进一步发展为高精度的微流体颗粒操纵工具，需要将捕获力校准为已知力，即由通道中的流体流动引起的粘性阻力。然而，很少有针对流体在微流体环境中移动物体的聚焦声束进行基于流体粘度的标定研究。本文通过实验确定流的声俘获力（Ftrapping）和陷阱刚度（或顺应性k）微流体通道中的液滴。将分页器校准为由注射泵产生的粘性阻力。切比雪夫（Chebyshev）窗状chi编码激励序列（扫描频率范围从18 MHz到30 MHz）被用来驱动换能器，从而使光束能够通过通道/流体界面传输，从而询问通道内的液滴。根据脉冲重复频率（PRF），占空比（DTF）和换能器的输入电压幅度（Vin）来确定最初固定漂移液滴所需的最小力（Fmin，陷印）。在PRF = 0.1 kHz和DTF = 30％时，Fmin的陷波从Vin = 22 Vpp时的2.2 nN增加到Vin = 54 Vpp时的3.8 nN。在固定Vin = 54 Vpp和DTF = 30％的情况下， Fmin ， 捕获 的范围可以从3.8 nN在 PRF = 0.5 kHz时为0.1 kHz至6.7 nN。这些发现表明，较高的驱动电压和更频繁的光束传输都会产生更强的陷阱以保持液滴运动。刚度 k 可以通过测量捕获力（ F trapping ）对应于液滴从陷阱中心的位移（ x ）。通过绘制 V in 中某些值的 F trapping – x 曲线（22 / 38/54 V pp ）在 DTF = 10％和 PRF = 0.1 kHz时， k 被测量为分别为0.09、0.14和0.20 nN /μm。在 V in = 54 V pp ， k 的情况下，变量 PRF 从0.1到0.5 kHz em>从0.20增加到0.42 nN /μm。结果表明，对于给定的位移 x，较高的 PRF 导致更顺应的陷阱形成（或更强的 F trapping ） 。因此，结果表明，通过调整换能器的激励参数，这种声捕获方法有可能作为微流体中单个移动目标的非侵入性操纵工具。
展开▼
