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Schematic summarizing the phenomena observed in this study. (a) Application of low amplitude SAW results in the collection of microparticles along pressure nodal lines across the channel whereas high amplitude SAW drives either (b) uniform flow when (not shown) or both collection and unidirectional throughflow in narrow channels when , or (c) an oscillatory mixing flow when .
(a) An FE-SPUDT fabricated on the lithium niobate substrate in which a grooved microchannel is cut along the focused region of the transducer. (b) The measured impedance and phase spectra of the transducer, showing the fundamental SAW resonant frequency and the first harmonic frequency . (c) Amplitude of the instantaneous surface displacement perpendicular to the substrate surface when the transducer is excited by a sinusoidal signal at .
(a) Switching between particle transport and agglomeration in a wide channel with low excitation amplitudes: (aii) and (aiv) particle transport under unidirectional flow when the SAW is driven at , and, (ai), (aiii), and (av) particle agglomeration into four collection lines when the SAW is driven at . Image (avi) shows a comparison between the numerical and experimental results, showing that particles collect at along the nodes of the pressure field and why four nodal lines are generated when the SAW is driven using excitation. (avii) Using particles, we demonstrated the formation of a single collection line under excitation. (b) Switching between uniform and unidirectional throughflow and oscillatory mixing flow in a wide channel with high excitation amplitudes: (bi) no excitation and no flow, (bii), (biv), and (bvi) unidirectional flow when the SAW is driven at , and, (biii) and (bv) mixing, transverse oscillatory flow when the SAW is driven at . The estimated longest transition time for the fluid to change from one state to the next state is approximately 0.2 s. The area of the channel where all images were captured is indicated with an asterisk in Fig. 2(a) (enhanced online). [URL: http://dx.doi.org/10.1063/1.3524511.1] [URL: http://dx.doi.org/10.1063/1.3524511.2]10.1063/1.3524511.110.1063/1.3524511.2
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