Acoustophoretic Mobility and Its Role in Optimizing Acoustofluidic Separations.

In the separation sciences, sample species are separated according to their physicochemical properties, the nature of the selective field, and, if present, the properties of the medium in which they are dissolved or suspended. Separations may be carried out on a continuous basis in microfluidic devices or split-flow thin channel (SPLITT) devices by selectively transporting species in a direction transverse to the direction of flow of the suspending fluid. Separation is achieved in the so-called transport mode according to relative differences in mobility of the species under the influence of the applied field. Gravitational, centrifugal, thermal gradient, magnetic, electric, and dielectric fields may all be used for continuous SPLITT fractionation. We present here the theory for optimizing the operation of the relatively new technique of acoustic SPLITT fractionation for the continuous separation of non-Brownian materials. The theory is based on a quantitatively defined acoustophoretic mobility that is consistent with the generalized concept of mobility proposed by Giddings. Until now, acoustophoretic mobility has almost exclusively been used as a qualitative descriptor for velocity induced by an acoustic field. The quantitative definition presented here will contribute to the advancement of all forms of acoustofluidic separations.