Microfluidic channel in a soft polymer actuated by surface acoustic waves. The surface acoustic waves leak energy into the polymer and microchannel and as a consequence, a pseudo-standing wave field arises. This field leads to acoustic streaming drag forces and radiation forces on suspended microparticles.

ALE Formulation: magnitude of the Lagrangian-mean velocity (a) and of the Eulerian velocity (b) for a device with dimensions comparable to the acoustic wavelength. The bottom wall is actuated. The lines represent the streamlines of the respective velocities. The Lagrangian velocity streamlines do not cross the channel walls, indicating no mass flow across the channel walls, while the Eulerian velocity streamlines cross the channel walls as though there were an outflow across the channel walls.

Ezb Formulation: magnitude of the (a) Lagrangian velocity and (b) Eulerian velocity for a device with dimensions comparable to the acoustic wavelength. The bottom wall is actuated. The Ezbc formulation (results in this figure) yields results that we believe are erroneous. The lines represent the streamlines of the respective velocities. Here the Lagrangian velocity streamlines cross the channel walls, indicating an unphysical mass flow across the channel walls. By contrast, consistent with the boundary conditions imposed in Ezbc formulation, here the Eulerian velocity streamlines do not cross the channel. These results are not supported by experimental evidence reported in Barnkob et al. (2016).