Acoustic band gap materials, so-called phononic crystals, are introduced as a new platform for sensing material properties in small cavities. The sensor employs specific transmission windows within the band gap to determine properties of one component that builds the phononic crystal. The dependence of the frequency where transmission takes place is correlated to material properties, specifically to the sound velocity of a liquid. This value is related to several parameters of practical interest like the concentration of one component in a mixture or conversion rate in a microreactor. The capability of the concept will be demonstrated with a one-dimensional arrangement of solid plates and liquid-filled cavities and a two-dimensional periodic arrangement of liquid-filled holes in a solid matrix. The properties of 1D phononic crystals will be analysed in terms of the effective acoustic impedance and the resulting transmission behaviour and experimentally verified. The transmission properties of the 2D phononic crystal will be modelled with the layer multiple-scattering theory. Similar features which can be employed for sensing purposes will be discussed.

http://iopscience.iop.org/article/10.1088/0957-0233/20/12/124014/pdf

Phononic crystals for liquid sensor applications

his paper presents a systematic overview on the recent works on surface acoustic wave (SAW) driven microfluidics. SAW microfluidics is based on acoustic streaming induced by leaky SAW radiation into a liquid. The development of this field attracts attention from microfluidic research community due to its rapid actuation, programmable capability, simple and yet efficient operation. In our paper, SAW microfluidic applications are categorized into droplet-based applications and continuous-flow applications. Droplet is actuated into unique behaviours depending on the applied SAW power. A wide range of droplet based applications have been employed utilizing these behaviours. In continuous-flow system, applications are further categorized based on the interaction of travelling SAW and standing SAW with the bulk liquid. Finally, future perspectives of SAW driven microfluidics are discussed

/pdf/surface-acoustic-wave-driven-microfluidics-a-review-3pqa502o5d.pdf

Surface Acoustic Wave Driven Microfluidics – A Review

The propagation of surface acoustic waves (SAWs) along solid?liquid interfaces depends sensitively on the properties of the liquid covering the solid surface and may result in a momentum transfer into the liquid and thus a propulsion effect via acoustic streaming. This review gives an overview of the design of different SAW devices used for the sensing of liquids and the basic mechanisms of the interaction of SAWs with overlaying liquids. In addition, applications of devices based on these phenomena with respect to touch sensing and the measurement of liquid properties such as density, viscosity or the composition of mixed liquids are described, including microfabricated as well as macroscopic devices made from non-piezoelectric materials. With respect to the rapidly growing field of acoustic streaming applications, recent developments in the movement of nanolitre droplets on a single piezoelectric chip, the rather macroscopic approaches to the acoustic pumping of liquids in channels and recent attempts at numerical simulations of acoustic streaming are reported.

https://iopscience.iop.org/article/10.1088/0022-3727/41/12/123002/pdf

Sensors and actuators based on surface acoustic waves propagating along solid–liquid interfaces

Surface acoustic wave (SAW) excitation of an acoustic field in a trapezoidal glass microfluidic channel for particle manipulation in continuous flow has been demonstrated. A unidirectional interdig ...

Surface acoustic wave-induced precise particle manipulation in a trapezoidal glass microfluidic channel

A new acoustic microsensor principle is introduced for the first time. It is based on acoustic band gap materials, so-called phononic crystals. The sensor employs features of the band gap to determine properties of one component that builds the phononic crystal. Here the dependence of the frequency of specific resonant modes within the band gap is correlated to liquid material properties. The sensitivity to variations in speed of sound and density of the liquid confined in a small gap has been found to be much higher than achievable with traditional ultrasonic measurements.

Phononic crystal sensor

The wave propagation in an acoustic wave guide sensor filled with liquids and driven by pulsed excitation of Lamb waves on glass plates forming the boundaries of the wave guide has been analyzed with respect to the formation of different wave groups in the receiver signal. The results of model calculations of the wave propagation were compared with measured transit times. Evidence is provided for the capability of this sensor with respect to the detection of gas bubbles in liquids and the tracking of their movement.

Wave propagation in an acoustic wave guide sensor for liquids driven by pulsed excitation of surface acoustic waves

The propagation of liquids induced by acoustic streaming turned out to be a successful approach, but applications seem to be restricted to small dimensions of the liquid channel in the order of some tens of micrometers and to piezoelectric substrate materials In this contribution another approach will be described, which allows to extent the dimension of liquid channels to the order of some millimetres or centimetres and allows the use of common structural materials like metals, glass or ceramics as substrates for the propagation of surface acoustic waves: Rayleigh-like waves are excited on the interface of the substrate to the liquid by piezoelectric interdigital transducers attached to the backside of the substrate, from which sound waves are radiated off into the liquid under a characteristic angle determined by the velocities of waves in the liquid and on the substrate, respectively Results of liquid propulsion in open channels obtained with such a set up will be presented, which demonstrate the contributions of different mechanisms of momentum transfer like eg acoustic streaming due to wave propagation along the channel bottom surface, sound absorption in the liquid ("quartz wind") and sound reflection at liquid surfaces

M. Munch

Papers

Wave propagation in an acoustic wave guide sensor for liquids driven by pulsed excitation of surface acoustic waves

P5G-2 Acoustic Surface Wave Induced Propagation of Liquids in Open Channels

Measurement of the properties of liquids based on the dispersion of Lamb waves in an acoustic waveguide