The realization of microscale total analysis systems and lab-on-a-chip technologies requires efficient actuation (mixing, pumping, atomizing, nebulizing, driving, etc.) of fluids on the microscopic scale and dexterous manipulation (separation, sorting, trapping, concentration, merging, patterning, aligning, focusing, etc.) of micro-objects (cells, droplets, particles, nanotubes, etc.) in open (sessile droplets) as well as confined spaces (microchannels/chambers). These capabilities have been recently achieved using powerful acoustofluidic techniques based on high-frequency (10–1000 MHz) surface acoustic waves (SAWs). SAW-based miniaturized microfluidic devices are best known for their non-invasive properties, low costs, and ability to manipulate micro-objects in a label-free manner. The energy-efficient SAWs are also compatible with conventional microfabrication technologies. The present work critically analyses recent reports describing the use of SAWs in microfluidic actuation and micro-object manipulation. Acoustofluidic techniques may be categorized according to the use of travelling SAWs (TSAWs) or standing SAWs (SSAWs). TSAWs are used to actuate fluids and manipulate micro-objects via acoustic streaming flow (ASF) as well as acoustic radiation force (ARF). SSAWs are mainly used for micro-object manipulation and are rarely employed for microfluidic actuation. We have reviewed reports of new technological developments that have not been covered in other recent reviews. In the end, we describe the future prospects of SAW-based acoustofluidic technologies.

We demonstrate a simple and efficient device for the continuous label-free separation of microparticles using travelling surface acoustic waves (TSAW). A focusing interdigitated unidirectional transducer released high frequency (133.3 MHz) TSAW normal to the fluid flow direction to segregate 3 μm particles from 10 μm particles with a separation efficiency of 100%. The TSAW based separator does not necessitate a tight alignment of the PDMS microchannel with the transducer.

We have investigated the motion of polystyrene microparticles inside a sessile droplet actuated by surface acoustic waves (SAWs). Four distinct regimes (R1–R4) of particle concentration are identified based on the particle diameters and the SAW frequencies.

Submicron separation is the segregation of particles having a diameter difference of less than one micrometre. We present an acoustofluidic particle separator with submicron separation resolution to study the continuous, label-free, and contactless separation of polystyrene (PS) particles based on their acoustofluidic parameters such as size, density, compressibility and shape. In this work, the submicron separation of PS microspheres, having a marginal size difference, is achieved inside a polydimethylsiloxane (PDMS) microfluidic channel via travelling surface acoustic waves (TSAWs). The TSAWs of different frequencies (200, 192, 155, and 129 MHz), propagating normal to the fluid flow direction inside the PDMS microchannel, realized continuous separation of particles with a diameter difference as low as 200 nm. A theoretical framework based on the rigid and elastic theories is presented to support the experimental results.

We demonstrated the operation of an acoustomicrofluidic device composed of a polydimethylsiloxane (PDMS) microchannel and a slanted-finger interdigitated transducer (SF-IDT), for the on-demand splitting of droplets in an active, accurate, rapid, and size-controllable manner.