Contact-free surface chemistry with acoustic levitation

Licentiate thesis, 2022

An ultrasonic acoustic levitator is a device that allows the implementation of contact-free studies by generating an airborne acoustic pressure field, capable of trapping, in mid-air, small volume samples (<10 μL) within the low-pressure areas (i.e., nodes). Recent studies have improved the theoretical and experimental understanding of acoustic levitation, though, the performance of the device remains often far from ideal for the implementation of physicochemical studies.

In this licentiate thesis, three different designs of multiple-transducers acoustic levitators are presented and evaluated in terms of both simulated acoustic pressure and experimental performance. It was found that by tuning the cavity length of the device, it was possible to reduce the number of transducers while generating equally high acoustic pressure fields. Furthermore, the levitator with the densest packing of transducers exhibited the best performance, in terms of stability and levitation capacity. This framework can be applied in the customization of the design for specific applications.

A highly stable acoustic levitator was utilized for determining the surface tension of aqueous surfactant solutions through a data-driven approach. Approximately 50,000 photographs of acoustically levitated droplets were used for the training of a deep neural network while the predicting evaluation was based on ~10,000 photographs. The mean absolute error of the neural network surface tension predictions was below 0.9 mN/m. The methodology presented here surpassed previous limitations related to droplet size and deformation, while generating equally high, and in specific cases higher accuracy.