Microfluidic acoustic trapping force and stiffness measurement using viscous drag effect

Area of Science:

• Acoustic manipulation
• Microfluidics
• Biophysics

Background:

• Acoustic tweezers enable non-invasive manipulation of microscale objects.
• Calibrating acoustic trapping forces is crucial for precise control of moving particles in microfluidic devices.
• Previous studies have limited calibration of acoustic forces for dynamic targets in microchannels.

Purpose of the Study:

• To experimentally determine the acoustic trapping force and trap stiffness for a moving droplet in a microfluidic channel.
• To calibrate the trapping force against known viscous drag forces generated by fluid flow.
• To investigate the influence of transducer excitation parameters on trapping efficiency.

Main Methods:

• Utilized a 24MHz focused acoustic transducer to trap 70μm droplets in a 500μm microfluidic channel.
• Employed Chebyshev-windowed chirp coded excitation (18-30MHz) for acoustic beam transmission.
• Calibrated trapping force to viscous drag force from syringe pumps, varying pulse repetition frequency (PRF), duty factor (DTF), and input voltage (Vin).

Main Results:

• Minimum trapping force increased with higher input voltage and PRF.
• At PRF=0.1kHz and DTF=30%, minimum trapping force ranged from 2.2nN (22Vpp) to 3.8nN (54Vpp).
• At Vin=54Vpp and DTF=30%, minimum trapping force increased from 3.8nN (0.1kHz) to 6.7nN (0.5kHz).
• Trap stiffness (k) increased with higher Vin and PRF, indicating stronger trapping capabilities.

Conclusions:

• Higher driving voltage and PRF result in stronger acoustic traps for moving droplets.
• Acoustic trapping force and stiffness can be precisely controlled by adjusting transducer excitation parameters.
• This method shows potential for non-invasive manipulation of individual moving targets in microfluidics.