An all-acoustic toolbox for performing unit operations and their combinations on levitated droplets

Automatic and contactless manipulation of droplets in mid-air has wide potential applications from materials processing to biochemistry and pharmaceutical development. To perform (bio)chemical reactions in a contactless and automated fashion, herein, we report an all-acoustic toolbox for performing unit operations (generate, hold, move and merge) on droplets with integrated optical detection. The toolbox was realized using phased arrays of ultrasound transducers in combination with pump and capillary nozzles to generate pendant droplets of required volumes and pulling them away from the tip of the nozzles using acoustic forces, then using the acoustic forces to move and merge droplets followed by quantitative measurements using the integrated optical detection. This is an unprecedented report on droplet generation by pulling pendant droplets using acoustic forces. The generation of aqueous droplets containing different percentages of dimethyl sulfoxide was possible up to 50 % DMSO. The error in the volume of aqueous droplets generated using the approach was between 5.6 and 13 % depending on the volume of the droplet and the transducer voltage. The success rate for holding droplets varied between 78 and 95 % depending on the droplet size and transducer peak-to-peak voltage. Up to six droplets in two columns of three could be generated, moved and merged. Subsequently, a sequence of unit operations was created to implement an esterase enzyme assay, which is widely used in biochemical labs and industrial processes, in levitated droplets. The reaction rate was higher in levitated droplets (2.17 to 5.21 × 10^-3 s^-1 compared to that in microtiter plates (∼2.0 × 10^-3 s^-1), which we hypothesize is a result of stirring of the droplet by acoustic streaming. Future work will focus on increasing the number of droplets that can be levitated and manipulated using our all-acoustic toolbox. The adoption of our all-acoustic toolbox for performing (bio)chemical reactions will not only increase automation but also reduce the use of single-use non-recyclable plastics such as microtiter plates and pipette tips.