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Numerical simulations of the Navier-Stokes, Nernst-Planck, and the Poissonequations are employed to describe the transport processes in an aqueouselectrolyte in a parallel-plate nanochannel, where surface-acoustic waves(SAWs) are standing or traveling along (piezo-active) channel walls. It isfound that -- in addition to the conventional acoustic streaming flow -- atime-averaged electroosmotic flow is induced. Employing the streamfunction-vorticity formulation, it is shown that the Maxwell stress term causesan electroosmotic propulsion that is qualitatively identical to the onediscussed in the context of alternating current (AC) electroosmosis (EOF).Differences arise mainly due to the high actuation frequencies of SAWs, whichare in the MHz range rather than in the kHz regime typical for ACEOF. Moreover,the instantaneous spatial periodicity of the EOF in the travel direction of theSAW is intrinsically linked to the dispersion relation of the latter ratherthan a free geometric parameter. This leads to a specific frequency band wherean EOF of sizable magnitude can be found. On the low frequency end, the ratiobetween the electric double layer (EDL) thickness and the SAW wavelengthbecomes extremely small so that the net force leading to a non-vanishingtime-averaged flow becomes equally small. On the high frequency end, the RCtime of the EDL is much larger than the inverse of the SAW frequency leading toa vanishing effective charge density of the EDL. For a parallel-plate channel,the EOF can be maximized by using two SAWs on both channel walls that have thesame frequency but are phase-shifted by $180^\circ$. It appears that theSAW-EOF is the dominant pumping mechanism for such a scenario. The proposedactuation might be a viable alternative for driving liquid electrolytes throughnarrow ducts and channels, without the need for electric interconnects andelectrodes.

Electroosmotic flow in small-scale channels induced by surface-acoustic waves