Effect of microchannel protrusion on the bulk acoustic wave-induced acoustofluidics: numerical investigation

Acoustofluidics inside the microchannel has already found its wide applications recently. Acoustic streaming and radiation force are two underlying mechanisms that determine the trajectory of microparticles and cells in the manipulation. Critical particle size of viscous effects is found to be about 1.6 µm in the conventional rectangular microchannel (W × H = 380 m × 160 m) at the frequency of 2 MHz, below which the acoustic streaming dominants, and is independent of the driving voltage. In order to effectively adjust such a critical size, a approach is proposed and evaluated numerically to enhance the acoustic streaming by adding some protrusions (i.e., in the shape of a wedge, rod, half-ellipse) to the middle of the top or bottom wall. It is found that the resonant frequency and acoustic pressure will decrease and the acoustic streaming velocity will increase significantly, respectively, with the increase of protrusion height (up to 30 µm while keeping the width the same as 8 µm). Subsequently, trajectory motion patterns of microparticles have apparent changes in comparison to those inside the rectangular microchannel, and acoustic streaming can even dominate the motion of large microparticles (i.e., 10 µm). As a result, the critical particle size could be increased up to 72.5 µm. Furthermore, different protrusion shapes (i.e., wedge, rod, half-ellipse) on the top wall were compared. The sharpness of protrusion at its tip seems to determine the acoustic streaming velocity. The wedge attached to the bottom wall had higher resonant frequency and lower acoustic streaming velocity compared with the top wedge in the same dimension. The patterns of acoustic streaming and microparticle trajectory motion in the microchannel with dual wedges on the top and bottom walls are not the superposition of those of the top and bottom wedge individually. In summary, the geometry of the microchannel has a significant effect on the induced acoustofluidics by the bulk acoustic waves. A much larger acoustic streaming velocity is produced at the tip of the protrusion to change the critical size of microparticles between acoustic streaming and radiation force. It suggests that more applications of acoustofluidics (i.e., mixing and sonoporation) to microparticles and cells in various sizes are feasible by designing an appropriate geometry of the microchannel.