Effects of micron scale surface profiles on acoustic streaming

Conventional models of boundary-driven streaming such as Rayleigh-Schlichting streaming typically assume smooth device walls. Using numerical models, we predict that micron scale surface profiles/features have the potential to dramatically modify the inner streaming vortices, creating much higher velocity, smaller scale vortices. Although inner streaming is hard to observe experimentally, this effect is likely to prove important in applications such as DNA-tethered microbeads where the flow field near a surface is important. We investigate here the effect of a sinusoidally structured surface in a one-dimensional standing wave field in a rectangular channel using perturbation theory. It was found that inner streaming vortex patterns of scale similar to the profile are formed instead of the much larger eight-vortices-per-wavelength classical inner streaming patterns seen in devices with smooth surfaces, while the outer vortex patterns are similar to that found in a device with smooth surfaces (i.e., Rayleigh streaming). The streaming velocity magnitudes can be orders of magnitude higher than those obtained in a device with smooth surfaces, while the outer streaming velocities are similar. The same inner streaming patterns are also found in the presence of propagating waves. The mechanisms behind the effect are seen to be related to the acoustic velocity gradients around surface features.