This article reviews acoustic microfiuidics: the use of acoustic fields, principally ultrasonics, for application in microfiuidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.

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Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

Techniques that can dexterously manipulate single particles, cells, and organisms are invaluable for many applications in biology, chemistry, engineering, and physics. Here, we demonstrate standing surface acoustic wave based “acoustic tweezers” that can trap and manipulate single microparticles, cells, and entire organisms (i.e., Caenorhabditis elegans) in a single-layer microfluidic chip. Our acoustic tweezers utilize the wide resonance band of chirped interdigital transducers to achieve real-time control of a standing surface acoustic wave field, which enables flexible manipulation of most known microparticles. The power density required by our acoustic device is significantly lower than its optical counterparts (10,000,000 times less than optical tweezers and 100 times less than optoelectronic tweezers), which renders the technique more biocompatible and amenable to miniaturization. Cell-viability tests were conducted to verify the tweezers’ compatibility with biological objects. With its advantages in biocompatibility, miniaturization, and versatility, the acoustic tweezers presented here will become a powerful tool for many disciplines of science and engineering.

https://www.pnas.org/content/pnas/109/28/11105.full.pdf

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves

https://www.repository.cam.ac.uk/bitstream/1810/264501/1/Fu_et_al-2017-Progress_in_Material_Science-VoR.pdf

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

Fluid manipulations at the microscale and beyond are powerfully enabled through the use of 10–1,000-MHz acoustic waves. A superior alternative in many cases to other microfluidic actuation techniques, such high-frequency acoustics is almost universally produced by surface acoustic wave devices that employ electromechanical transduction in wafer-scale or thin-film piezoelectric media to generate the kinetic energy needed to transport and manipulate fluids placed in adjacent microfluidic structures. These waves are responsible for a diverse range of complex fluid transport phenomena—from interfacial fluid vibration and drop and confined fluid transport to jetting and atomization—underlying a flourishing research literature spanning fundamental fluid physics to chip-scale engineering applications. We highlight some of this literature to provide the reader with a historical basis, routes for more detailed study, and an impression of the field's future directions.

https://users.monash.edu/~lyeo/Dr_Leslie_Yeo/Surface_Acoustic_Wave_Microfluidics_files/annurev-fluid-010313-141418.pdf

Surface Acoustic Wave Microfluidics

Quantum information can be stored in micromechanical resonators, encoded as quanta of vibration known as phonons. The vibrational motion is then restricted to the stationary eigenmodes of the resonator, which thus serves as local storage for phonons. In contrast, we couple propagating phonons to an artificial atom in the quantum regime and reproduce findings from quantum optics, with sound taking over the role of light. Our results highlight the similarities between phonons and photons but also point to new opportunities arising from the characteristic features of quantum mechanical sound. The low propagation speed of phonons should enable new dynamic schemes for processing quantum information, and the short wavelength allows regimes of atomic physics to be explored that cannot be reached in photonic systems.

https://science.sciencemag.org/content/sci/346/6206/207.full.pdf

Propagating phonons coupled to an artificial atom

The Remez exchange algorithm is used as a computation tool for the design of finite impulse response linear phase digital filters. Given a desired frequency response specification, it furnishes a finite set of impulse response coefficients for the digital filter synthesis that yield an optimum approximation to the desired frequency response. This algorithm can be applied to lowpass finite impulse response(FIR) linear phase digital design as well as to a variety of bandpass filter responses, including (a) optimum single passband filters, (b) multiple passband filters, (c) bandstop filters, and (d) equalizer filters with non-symmetric amplitude response. While the lowpass filter design is not applicable to SAW filters, it transpires that the other bandpass filter types can be applied to SAW implementation. The frequency response of a digital filter is different from that obtained for a classical analog type in that the digital one repeats periodically at the multiples of the sampling frequency. FIR linear phase filters represent stable structures because their impulse response is finite, while their transfer function contains only zeros and no poles in the complex frequency domain. One optimization technique for designing such filters involves minimax criteria, whereby the maximum error in the frequency response of the filter is minimized. This technique is used in the Remez algorithm in minimizing the ripple of an equiripple Chebyshev amplitude response.

Finite Impulse Response Design Techniques for Linear Phase SAW Filters

Publisher Summary This chapter reviews the principles of operation of SAW comb filters and low-loss SAW filters employing single-phase unidirectional transducers (SPUDTs ). SAW comb filters can find application in diverse signal processing applications, such as for (a) range cells in linear frequency modulated continuous wave radar, (b) circuitry for counting and indicating television channels, and (c) multimode oscillator circuits for frequency-agile radar. One limitation suffered by conventional SAW comb filters is that they have inherently large insertion losses. As a result, these comb filter types are limited to use in signal processing stages where signal levels are high. This is because the signal-to-noise performance of a processing stage is reduced by the value of the insertion loss of the circuit employed. SAW comb filters and the Lewis type of SPUDT design both employ ladder-type inter digital transducer constructions, where the IDT fingers are clustered in periodically-spaced groups or “rungs.” The SPUDT construction additionally employs sets of SAW reflection gratings judiciously interspersed within these finger groupings. By the selection of the relative periodicities of the rungs in the input and output IDTs, the SPUDT device can be designed for operation as either a low-loss narrow-band filter or as a low-loss wideband comb filter.

Comb and Single-Phase Unidirectional Transducers

This chapter focuses on real-time SAW convolvers for programmable matched filtering, such as in spread-spectrum communications. This chapter examines three types of real-time convolvers. The first of these is the three-port monolithic waveguide SAW convolver, whose operation is based on acoustic interactions on the surface of the piezoelectric substrate. This type is currently leading the three-port SAW convolver field in applications calling for bandwidths of 100 megahertz or less. The other two real-time SAW convolvers incorporate a silicon semiconductor in close proximity to a lithium niobate substrate such that their operation is dependent on the acoustoelectric interactions of SAW-induced electric fields at the silicon surface. In the order of evolution, these last two are the three-port SAW/ silicon convolver and the more intricate four-port SAW/silicon convolver. This four-port structure finds application in wide-band radar and spread-spectrum communications, such as for packet radio applications. The choice of whether to use a SAW convolver or a linear SAW filter for matched filtering depends on a number of factors. On the credit side, the SAW convolver can provide for a fixed or programmable matched filtering of lengthy coded sequences, well beyond the present range of fixed or programmable transversal SAW filters.

Colin Campbell

Papers

Finite Impulse Response Design Techniques for Linear Phase SAW Filters

Comb and Single-Phase Unidirectional Transducers

Real-Time SAW Convolvers