Acoustophoresis in microfluidic systems is a rapidly growing field. To make a robust acoustophoretic system, it is important to consider parameters such as the acoustic energy density inside the microfluidic channel, the optimal resonance frequencies and the modal patterns. Previously we presented a quick and simple method for in situ measurements of the acoustic energy density inside a microfluidic channel allowing for determination of the acoustic radiation force on suspended particles [1]. Here we use this method to study and investigate the different modal patterns created from acoustically actuating microfluidic device using a novel tunable-angle wedge transducer.
In the experiments the acoustophoretic performance of aligning beads (5-µm diameter) was studied as a function of the coupling angle of a tunable-angle wedge transducer. The performance was quantified by measuring the acoustic energy density in the channel by the light-intensity-based method. We studied two ways of actuating the transducer: Single frequency and frequency modulation. In addition, to investigate the geometry-dependence of the transducer-chip system when changing the angle on the tunable-angle wedge transducer, we used a planar (fixed zero angle) transducer as a reference. The tunable-angle transducer was made of a sliding aluminum wedge connected by two springs to a quarter-cylindrical fixed aluminum plate. The planar transducer was made with an epoxy backing layer allowing for broad-banded actuation. Both transducers were made of PZT crystals with their fundamental resonance frequency around 2 MHz.
We investigate the correlation between the averaged acoustic energy density Eac within a single microscope field of view and the change of transducer coupling angle. From the control experiments using the planar transducer, we obtain that the geometry change from changing the wedge angle has little effect on Eac. On the other hand, as the coupling angle is varied when driving the tunable-angle transducer a significant fluctuation in Eac is measured. Moreover, the maximum Eac for the tunable-angle transducer occurs when actuating at a single frequency at angle θ = 75°, while the obtained Eac is larger when actuating at several frequencies using modulation. Also, comparing the single-frequency and frequency-modulation actuation it can be seen that the particle manipulation is faster for the single-frequency actuation, but only at local positions along the channel. As a practical outcome of this study, we show that the angle of the tunable-angle wedge transducer can be used as an additional resonance tuning parameter besides the conventional parameter (actuation frequency). However, we also conclude that frequency-modulation is a generic and simple method for achieving uniform and robust acoustophoresis without compromising the acoustic energy density in the channel.