Enhanced Micromixing Using Surface Acoustic Wave Devices: Fundamentals, Designs, and Applications

Abstract

Microfluidics-based mixing methods have attracted increasing attention due to their great potential in bio-related and material science fields. The combination of acoustics and microfluidics, called acoustofluidics, has been shown to be a promising tool for precise manipulation of microfluids and micro-objects. In general, achieving robust mixing performance in an efficient and simple manner is crucial for microfluidics-based on-chip devices. When surface acoustic waves (SAWs) are introduced into microfluidic devices, the acoustic field can drive highly controllable acoustic streaming flows through acoustofluidic interactions with micro-solid structures, which have the advantages of label-free operation, flexible control, contactless force, fast-response kinetics, and good biocompatibility. Therefore, the design and application of various SAW micromixers have been demonstrated. Herein, we present a comprehensive overview of the latest research and development of SAW-based micromixers. Specifically, we discuss the design principles and underlying physics of SAW-based acoustic micromixing, summarize the distinct types of existing SAW micromixers, and highlight established applications of SAW micromixing technology in chemical synthesis, nanoparticle fabrication, cell culture, biochemical analysis, and cell lysis. Finally, we present current challenges and some perspectives to motivate further research in this area. The purpose of this work is to provide an in-depth understanding of SAW micromixers and inspire readers who are interested in making some innovations in this research field.

Keywords: acoustic mixing; interdigital transducer; microfluidics; micromixer; surface acoustic wave.

Figure 1

Schematic representation of acoustofluidic micromixers. (a) Bubble micromixer. (b) Sharp-edge micromixer. (c) Resonant acoustic micromixer.

Figure 2

Simulated results illustrating the acoustic process of SAW-based micromixer. (a) The SAWs propagate along the substrate surface and transform into leaky SAWs at the solid–fluid interface, radiating the acoustic pressure into the microfluidic channel at the Rayleigh angle. (b) Leaky SAWs drive bulk recirculation (acoustic streaming flow) in the microchannel. (c) Schematic illustration showing the energy of the SAWs radiating into the microfluidic channel.

Figure 3

Various IDT designs: (a) standard bidirectional straight IDTs, (b) split IDTs, (c) single-phase unidirectional transducer (SPUDT), (d) floating electrode unidirectional transducer (FEUDT), (e) chirped IDT, (f) slanted-finger IDT (SFIT), and (g) focused IDT.

Figure 4

Microfluidic mixing via SAWs generated by single straight IDT. (a) A straight IDT generates SAW to drive ASF within a continuous-flow straight microchannel. (b) SAWs on a LiNbO₃ chip to generate fast mixing flows in a microfluidic well [95]. (c) Mixing inside a nanoliter sessile droplet using an intense ASF induced by SAWs [96]. (d) Mixing nanoliter droplets between a glass and a piezoelectric substrate using SAWs [97]. (e) A reusable micromixer platform using SAW device. The anti-symmetric higher-order lamb waves are generated on the thin glass plate to radiate compressional waves into the microchannel to induce fluid motion [98]. (f) Acoustofluidic mixing technique inside a single-layered microfluidic channel. High-frequency SAWs generated from the IDT placed right beneath the channel mixed the fluid flow under the influence of strong ASFs [99]. (g) SAW-induced heating platform to enhance the vapor-mediated solute Marangoni effect for rapid mixing of sessile droplets [100].

Figure 5

Microfluidic mixing via SAWs generated by single non-straight IDT. (a) Using focused SPUDT to generate focused SAWs for intense micromixing in a microliter droplet [101]. (b) Focused SAWs launched perpendicular to the flow to drive channel flow mixing [102,103,104]. (c) A conductive liquid-based focused SAW (CL-FSAW) device for mixing. The PDMS channel contains electrode channels as focused IDTs for focused SAW generation and a main fluidic channel [105]. (d) A dome-shaped chamber-based SAW (DC-SAW) device for mixing [106]. (e) A flexible platform for performing various protocols on the chip. The mixing process was enhanced by focused SAWs, and the customized delivery of fluid into the mixing chamber was controlled by a single-layer valve [107]. (f) SAWs generated by an SFIT. The lateral position of the excitation SAWs can be fine-tuned by adjusting the frequency, allowing for more precise control of the mixing region [108,109].

Figure 6

Microfluidic mixing via SAWs generated by multiple IDTs. (a) Active mixing technique using dual acoustic streaming field induced by transversely arranged SAWs in a microfluidic channel [110,111]. (b) Similar to (a) but using parallel arrangement [110]. (c) Mixing technique using three-dimensional dual SAWs (3D-dSAWs) generated from two focused SPUDTs of top and bottom piezoelectric substrates [112]. (d) Two aligned IDTs on droplet actuation to achieve efficient mixing of a sessile droplet [113]. (e) Similar to (d) but using two offset IDTs [113]. (f) Mixing technique using asymmetrically aligned focused SAWs to enhance sensitivity of microarray electrode detection [114]. (g) Enhanced micromixing driven by dual eccentrically focused SAWs (DEF-SAWs). The DEF-SAWs are excited by two eccentrically arranged, coplanar, focused IDTs patterned on the surface of a 500 μm-thick LiNbO₃ substrate [115].

Figure 7

The wide applications of SAW-based micromixers.