The characterisation of the fluid motion induced by the acoustic streaming effect is of paramount interest for novel microfluidic devices based on surface acoustic waves (SAWs), e.g. for a detailed description of the achievable mixing efficiency and thus the design of such devices. Here, we present for the first time a quantitative 3D comparison between experimental measurements and numerical simulations of the acoustic streaming induced fluid flow inside a microchannel originating from a SAW. On the one hand, we performed fully three-dimensional velocity measurements using the astigmatism particle tracking velocimetry. On the other hand, we derived a novel streaming force approach solving the damped wave equation, which allows fast and easy 3D simulations of the acoustic streaming induced fluid flow. Furthermore, measurements of the SAW amplitude profile inside the fluid filled microchannel were performed. Based on these results, we obtained a very good agreement between the velocity measurements and the simulations of the fluid flow demonstrating the importance of comprising the actual shape of the SAW amplitude profile for quantitatively reliable simulations. It is shown that the novel streaming force approach is a valid approximation for the simulation of the acoustic streaming induced fluid flow, allowing a rapid and simple estimation of the flow field of SAW based microfluidic devices.

High-resolution in situ measurements provide deep insights into acoustophoresis induced by surface acoustic waves, revealing the correlation between acoustic fields and 3D particle positions, energy conversion processes and localized heating effects.

The active manipulation of particle and cell trajectories in fluids by high-frequency standing surface acoustic waves (sSAW) allows to separate particles and cells systematically depending on their size and acoustic contrast. However, process technologies and biomedical applications usually operate with non-spherical particles, for which the prediction of acoustic forces is highly challenging and remains a subject of ongoing research. In this study, the dynamical behavior of prolate spheroids exposed to a three-dimensional acoustic field with multiple pressure nodes along the channel width is examined. Optical measurements reveal an alignment of the particles orthogonal to the pressure nodes of the sSAW, which has not been reported in literature so far. The dynamical behavior of the particles is analyzed under controlled initial conditions for various motion patterns by imposing a phase shift on the sSAW. To gain detailed understanding of the particle dynamics, a three-dimensional numerical model is developed to predict the acoustic force and torque acting on a prolate spheroid. Considering the acoustically induced streaming around the particle, the numerical results are in excellent agreement with experimental findings. Using the proposed numerical model, a dependence of the acoustic force on the particle shape is found in relation to the acoustic impedance of the channel ceiling. Hence, the numerical model presented herein promises high progress for the design of separation devices utilizing sSAW, exploiting an additional separation criterion based on the particle shape.

Coexisting particle patterns with different geometrical features and particle trapping locations nota prioriassociated with acoustic potential wells are presented.

To date, the efficiencies of proton exchange membrane exchange fuel cells (PEMFCs) are limited by the water flooding issue. Water monitoring systems, which are a crucial step to overcoming these flooding-related problems, are mostly either invasive or compromise on the temporal resolution and field of view. Thus, we propose an ultrasonic-Lamb-waves-based, real-time, nondestructive water monitoring system. Briefly, ultrasonic transducers are mounted on the back side of bipolar plates (BPPs) exciting Lamb waves along flow channels incorporated in BPPs. Echo signals from water droplets in the channels are also received by the transducers. Thus, with the knowledge of Lamb wave propagation velocity, water droplets are spatially resolved by time-of-flight of each droplet echo. Meanwhile, the energy of each droplet induced echo wave packet is used to quantify the local flooding status. We have implemented a flexible and generic system adaptable to various flow field designs. The working principle was demonstrated for ex-situ conditions with a BPP with a 25 cm ² active area. A water sensitivity of at least 50 nL was realized, allowing for studying droplet and slug flows in PEMFCs. A 1.3 mm spatial resolution and a 2 kHz temporal resolution were simultaneously achieved. The high-performance water monitoring opens new horizons to study dynamic water evolution in channels of PEMFCs using cost-effective instrumentation, which may pave the way towards more efficient high-power PEMFCs with increased lifetimes.

Guided elastic waves are a truly cross-disciplinary key enabling technology. For more than five decades, surface acoustic wave (SAW) and bulk acoustic wave devices find widespread applications. Nowadays, different types of guided elastic waves cover the wide spectrum of applications spanning from quantum technologies to the life sciences, from controlling single excitations to macroscopic collective states in condensed matter. Six years after the first 2019 SAW roadmap, we believe it is time to make a step back and take a fresh look at the status of the field and its future challenges. Since the first roadmap in 2019, the spectrum clearly expanded and this new edition presents a current snapshot of the status of this vibrant field and prospects for potential future developments.