Fully automating liquid processing within an acoustic levitation system

Small volumes of liquid can be manipulated in various analytical biology, chemistry and pharmacology techniques, and robotic systems have been developed to replace manual liquid handling, but droplet contact and therefore cross-contamination has yet to be eliminated. Acoustic pressure is a contactless method, using sound waves to levitate and manipulate droplets; however, current acoustic systems lack automated insertion and ejection. A team from Spain and Brazil has developed an acoustic levitator that integrates four fluidic operations, including insertion and ejection, within a single automated system.

“The idea was to do everything automatically,” said co-author Marco Andrade. The team’s acoustic levitator system was composed of an array of commercially available ultrasonic transducers operating at 40 kilohertz, placed above a plane reflector. Half of the transducers in the array generated one focal point within a standing wave, and the other half another point, both trapping and independently adjusting the position of individual droplets. An inlet for droplet insertion and outlet for ejection were integrated into the reflector.

At the inlet, a square pulse stimulated the piezoelectric buzzer and nozzle to inject droplets into the levitator. The team demonstrated the capabilities of their system by the injection and transportation of two droplets, their merging, followed by levitation over the outlet and subsequent ejection.

To calculate the acoustic pressure distribution between the transducer and reflector, the group combined a modified matrix model with the Gor’kov potential equation. These numerical simulations agreed well with experimental droplet positions.

“The idea now is to design a new system with many other inlets and outlets and to manipulate many more droplets,” said Andrade. They also want to start moving toward applications, such as conducting chemical reactions in midair.

Source: “Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator,” by Marco A. B. Andrade, Thales S. A. Camargo, and Asier Marzo, Review of Scientific Instruments (2018). The article can be accessed at https://doi.org/10.1063/1.5063715 .