| Acoustic methods offer an enormous potential for the rapid, effective, and energy efficient manipulation of fine particles (1-100 micron) suspended in fluids. We have developed a particle-fluid separation process that involves application of a resonant acoustic field to a highly porous medium positioned within a flowing suspension. The acoustic excitation enables the porous medium to entrap suspended particles up to 100 times smaller than its nominal pore size. The process requires low acoustic power, typically results in 90% collection efficiency, and the entrapment is reversible. Detailed observations of the motion of the suspended particles within an acoustically excited porous medium reveal various types of flow phenomena that affect the particle-retention phenomena. In some instances, particle streamlines are focussed along preferred pathways resulting in the formation of chains of particles sequentially deposited at fixed positions within the porous medium. In other cases, collections of particles, which remain levitated and unattached to the elements of the porous medium, are observed. In some instances these collections follow stable closed orbits. Lastly, we observed that particles encounter strong accelerations away from certain positions within the porous medium. To elucidate the origins of the flow phenomena described above, we have completed a trajectory analysis of particles within an acoustically excited porous medium. The analysis accounts for hydrodynamic effects in the vicinity of the elements of the porous medium and the forces resulting from the scattering of the acoustic field by the porous medium. The analysis accounts for the material properties of the suspended particles and the porous medium, the intensity of the acoustic and flow fields, and pertinent geometric characteristics of the porous medium. The results of the trajectory analysis provide rationale for the three categories of flow phenomena described above. |
