Separation of suspensions and emulsions via ultrasonic standing waves – A review

• Ultrasonic standing wave systems for particle and droplet separations were reviewed.
• Microscale applications are numerous while large scale applications are emerging.
• Mathematical models used to understand ultrasonic separation systems were reviewed.
• Successful large scale micro-size separations require acoustic streaming minimization.

Ultrasonic standing waves (USW) separation is an established technology for micro scale applications due to the excellent control to manipulate particles acoustically achieved when combining high frequency ultrasound with laminar flow in microchannels, allowing the development of numerous applications. Larger scale systems (pilot to industrial) are emerging; however, scaling up such processes are technologically very challenging. This paper reviews the physical principles that govern acoustic particle/droplet separation and the mathematical modeling techniques developed to understand, predict, and design acoustic separation processes. A further focus in this review is on acoustic streaming, which represents one of the major challenges in scaling up USW separation processes. The manuscript concludes by providing a brief overview of the state of the art of the technology applied in large scale systems with potential applications in the dairy and oil industries.