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

Fundamentals of Surface Acoustic Waves and Devices: Introduction. Basics of Piezoelectricity and Acoustic Waves. Principles of Linear Phase SAW Filter Design. Equivalent Circuit and Analytic Models for a SAW Filter. SomeMatching and Trade-Off Concepts for SAW Filter Design. Compensation for Second-Order Effects in SAW Filters. Designing SAW Filters for Arbitrary Amplitude/Phase Response. Interdigital Transducers with Chirped or Slanted Fingers. IDT Finger Reflections andRadiation Conductance. Techniques, Devices and Mobile/Wireless Applications: Overview of Systems and Devices. SAW Reflection Gratings and Resonators. Single-Phase Unidirectional Transducers for Low-Loss Filters. RF and Antenna-Duplexer Filters forMobile/Wireless Transceivers. Other RF Front-end and Inter-stage Filters for Mobile/Wireless Transceivers. SAW IF Filters for Mobile Phones and Pagers. Fixed-Code SAW IDTs for Spread-Spectrum Communications. Real-Time SAW Convolvers for Voice and Data Spread-Spectrum Communications. Surface Wave Oscillators and Frequency Synthesizers. SAW Filters for Digital Microwave Radio, Fiber Optic, and Satellite Systems. Postscript. Subject Index.

Surface Acoustic Wave Devices for Mobile and Wireless Communications

This chapter reviews the characteristics and response of a two-port surface acoustic wave (SAW) resonator. It also presents the applications of standing surface waves in the implementation of very narrow-band filters formed by the SAW resonator structures. These structures can be configured to act as narrow-band resonant circuits by arranging for incident and reflected travelling wave components to interfere with each other in a coherent manner. The operation of waveguide resonators can then be examined in terms of the resonant modes and standing wave patterns. SAW resonators can be configured electrically as one-port or two-port networks. Their operation is based around the judicious use of SAW reflection gratings to form resonant structures. The elements of the reflection gratings are both periodically spaced and normal to the SAW propagation direction for affecting the narrow-band performance. Conventional electromagnetic waveguide resonator action results from the constructive interference of electromagnetic waves within a cavity bounded by metal reflector plates. However, this simple boundary implementation cannot be used for SAW resonators because of the physical nature of a surface acoustic wave.

The Two-Port SAW Resonator

This chapter discusses the operation of SAW transducers at harmonic frequencies. It further describes about instrumentation relating to the use of impulse response measurements as a useful tool in determining the sources of degradation in frequency response characteristics. This is demonstrated with respect to the gigahertz SAW delay lines under review. Unlike conventional L-C filters, SAW filters can be made to operate at harmonic frequencies. These filters are capable of operating efficiently at least up to the 11 th harmonic. This harmonic use is attractive for two reasons. First, there is a fabrication cost advantage that enables gigahertz SAW filter IDTs to be fabricated without sub-micron lithographic techniques or resorting to the use of costly electron-beam lithography. Second, the use of harmonic techniques can be beneficial in situations where the performance of the SAW filter at the fundamental frequency is degraded by bulk wave interference. SAW filters can be made to operate at selected harmonic frequencies by choosing an appropriate combination of IDT geometry and finger metallization ratio η. The sources of distortion in the frequency response characteristic of a SAW device can often be readily identified by examining its impulse response. This can be obtained by applying an inverse discrete Fourier transform (IDFT) to the measured frequency response data. Some network analyzers have a built-in IDFT processor that allows the user to conveniently examine both the impulse response and the frequency response. In other network analyzers, external IDFT software can be incorporated for this task.

Harmonic SAW Delay Lines for Gigahertz Frequencies

This chapter highlights the basic properties of surface acoustic waves and their generation by an interdigital transducer (IDT) located on the free surface of a piezoelectric substrate. The excitation of an IDT on a piezoelectric substrate can lead to the generation of bulk acoustic waves as well as surface waves. Bulk waves incident on the receiving IDT induce voltages. These interfere with the voltage signals due to received surface waves. The resultant voltage due to both sources can degrade the in-band performance specifications on amplitude, phase, or group delay response. Surface acoustic waves can be generated at the free surface of an elastic solid. In the SAW devices, the generation of such waves is achieved by the application of a voltage to a metal-film interdigital transducer (IDT) placed on the surface of a piezoelectric solid. Bulk acoustic waves can be characterized in terms of three modal types of mechanical excitation. One of these is a compressional wave, termed the longitudinal bulk wave, that is polarized in the direction of the acoustic wave propagation vector. The other two wave components, termed transverse or shear waves, have their vibrational modes. The chapter also outlines the acoustic bulk wave excitation by an IDT as a highly undesirable effect in SAW filter design that can cause severe degradation of filter performance and can also limit the fractional bandwidth obtainable with normal IDT structures.

Basics of Acoustic Waves and Piezoelectricity

This chapter discusses the techniques for implementing phase coding or frequency coding using linear SAW transducers. The versatility offered to coding applications by linear SAW devices is demonstrated for three distinctive applications. These relate to transducer configurations for (a) binary phase shift keying (BPSK), (b) quadrature phase shift keying (QPSK), and (c) continuous phase shift modulation (CPSM). Continuous phase shift modulation is also known as minimum shift keying (MSK). The general purpose of signal detection is to establish the presence or absence of a signal in the presence of background noise. To enhance or optimize the strength of the signal relative to the background noise environment, some type of pre-detection filtering is required. When the added noise is white noise of constant spectral density over the band of interest, the signal-to-noise ratio is optimized by the use of a matched filter. This capability is particularly useful in the detection of coded waveforms, where it is merely necessary to establish the presence or absence of a pulse in a specific bit-sequence interval.

Colin Campbell

Papers

Surface Acoustic Wave Devices for Mobile and Wireless Communications

The Two-Port SAW Resonator

Harmonic SAW Delay Lines for Gigahertz Frequencies

Basics of Acoustic Waves and Piezoelectricity

Coding Techniques Using Linear SAW Transducers