Flow of bidisperse suspensions under the effect of standing bulk acoustic waves

Acoustic handling of dense suspension of microparticles has relevance in lab-on-a-chip, food, and pharmaceutical technologies. So far the studies on acoustic focusing of dense suspensions treat the particles to be of the same size, i.e., monodisperse. Here, the flow of dense bidisperse suspensions of microparticles in a microfluidic channel flow exposed to a standing acoustic wave field is studied. Experiments show selective enrichment of the larger particles along the channel center where the smaller particles are depleted. A continuum-based model reveals that the particle migration is governed by the combined influence of shear-induced migration and acoustic radiation forces. The migration behavior of the suspended particles along the channel length is quantified in terms of the degree of focusing, relating to the particle migration toward the center of the channel, and the degree of segregation, relating to the separation of larger and smaller particles. It is observed that both the degree of focusing and segregation varies directly with the intensity of the acoustic field and the ratio of particle sizes and inversely with the suspension volume fraction.