Unique flow transitions and particle collection switching phenomena in a microchannel induced by surface acoustic waves

We present an experimental approach for controlled switching between uniform flow for pumping and vortical flow for mixing in a microchannel fabricated onto a piezoelectric substrate. For particle laden fluids, this arrangement permits a choice between transport and alignment of microparticles. Using surface acoustic waves with amplitudes beyond 1 nm, the transition from uniform to mixing flows occurs when the acoustic wavelength in the fluid is reduced to a dimension smaller than the channel width, i.e., $λf≥Wch$ for uniform flow and $λf<Wch$ for mixing flow. On the other hand, using relatively weak surface acoustic waves with amplitudes below 1 nm, particles in an initially homogeneous suspension agglomerate into equally spaced lines with a separation of $λf/2$⁠. Switching the transducer between its fundamental resonant frequency $f0$ and its first harmonic frequency $f1+∼2f0$ causes a switch between uniform and mixing flow, while switching between large and small amplitude excitation allows one to choose whether to collect the particles in the flow along nodal lines parallel to the channel. These results are uniquely achieved without requiring the microfabrication of complex microchannel architectures and control schemes; the switching is simply achieved by adjusting two parameters: the acoustic excitation frequency and amplitude.