Acoustofluidic Particle Trapping in a Structured Microchannel Using Lateral Transducer Modes

Acoustofluidics uses acoustic waves to manipulate particles or fluids in microfluidic systems, offering a contactless and gentle method suitable for medical, biological, and pharmaceutical applications. This approach draws particles to pressure nodes of standing (ultrasonic) waves, but the particles must differ mechanically from the surrounding fluid. This principle can be applied in acoustic traps, also known as acoustic tweezers, which use acoustic forces to trap particles in place against a flowing fluid or to move them around in a well-controlled manner.

The authors designed and fabricated an acoustofluidic particle-trapping device that uniquely relies on lateral length modes of attached piezoelectric plate transducers for ultrasonic actuation. The core of the trap is a disc-shaped chamber acting as an acoustic resonator. The device achieved performance comparable to traditional devices that rely on transducer thickness modes by generating a two-dimensional standing wave inside this trap. This method operates at lower frequencies, making it well-suited to trapping larger and even greater numbers of particles.

The study demonstrated the method’s effectiveness by capturing microplastic beads and living human cells from an aqueous solution. As the solution flows through the trap, particles get caught in the trap, thereby being filtered out of the liquid, and larger clusters of particles form inside the trap. These aggregates can reach sizes of several tens of nanoliters in volume (equivalent to tens of millions of cubic micrometers), containing thousands of particles and cells. The device achieved a 100% filtering efficiency for the plastic beads—provided that the trap is not overfilled and the flow velocity remains below a specific limit.

An interesting secondary effect emerged: particle trapping was also possible within the channel sections outside the actual trap, although less effectively. This effect is attributed to the discontinuities introduced by the disc-shaped traps, which form scattering sites that cause partial acoustic wave reflections. As a result, the straight channel sections between two consecutive traps behave as somewhat effective resonators, in which we can generate standing acoustic waves that exhibit multiple pressure nodes along the channel where particles accumulate.

Using lateral modes of a transducer instead of thickness modes offers key advantages. Lower-frequency bending modes in the free transducer plate are suppressed as soon as the transducer is in contact with the microfluidic chip, thus leaving the lateral mode as the dominant excitation mechanism. This effectively means the absence of interfering harmonics for the lateral mode, which leads to more reliable device operation. Furthermore, unlike the strongly coupled thickness modes, the lateral mode's weak coupling with the microfluidic chip preserves its resonance. These thickness modes, standard in transducer plates, are significantly disrupted when the transducer is attached to the chip.

The proposed lateral-mode acoustofluidic device exhibits high predictability, reproducibility, and operational stability, making it a promising alternative for advanced particle manipulation. This device has applications beyond medical and pharmaceutical fields, such as enriching particles for detection in sparsely populated fluids like seawater for microplastic or microorganism analysis.