van’t Oever et al. have in their recent Applied Optics paper reported an elegant way of optically probing the acoustic field in a water filled silicon glass microchannel, excited at resonance. In contrast to most prior work in the acoustofluidic field, their method relies on a purely optical effect and not an acoustophoretic response of suspended tracer particles. The experimental set-up utilises a phase-shifting Michelson interferometer where the image data collection is designed to spatially resolve the pressure field within the field of view of the imaging system. The pressure variation in the standing wave is resolved by the phase-locked and strobed illumination of the microchannel: as light travels through the water filled microchannel the optical path length differs between regions of varying pressure amplitude coupled to variations in the refractive index so that a varying interference fringe pattern can be imaged. The varying optical path length/fringe pattern distribution is then used to calculate the corresponding pressure amplitude distribution as well as the acoustic energy density inside the microchannel. The Q-value of the acoustic resonator is also derived from the interferometry measurements.
The interferometer allows measurement of the pressure amplitude distribution orthogonal to the direction of the microchannel illumination. Out-of-plane resonances are not possible to monitor with the proposed set-up. It should, however, be noted that many acoustofluidic systems rely on only setting up standing waves in plane with the microfluidic channel, for which the reported pressure field imaging method would be very valuable. The reported data agrees well with other acoustically-based results, and the measurement accuracy of 20% is set by the pressure variations and hence optical path length variations within the 500 um glass layer. The impact of the pressure variations in the glass layer could be reduced by employing a thinner glass cover.
The beauty of this interferometric acoustic-field-monitoring system is the simplicity of performing the measurements and that such measurements can be done on particle-free solutions, which is in contrast with traditional methods based on particle imaging velocimetry on the acoustophoretic response of the suspended microparticles. Such simple and rapid methods for characterizing the acoustic fields in microchannels should offer significant value in the engineering quest of developing numerical models to predict the spectral response of acoustofluidic systems, and in the long run offer design criteria for optimal acoustofluidic chip development and manufacturing.
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