The chemical production of a single bubble acoustically collapsing at audible frequencies

The vast majority of the studies concerned with the chemical response of collapsing bubbles has been carried out at frequencies larger than 20 kHz. However, solid evidence of the inefficacy of low frequency sound waves in driving bubbles to chemically active collapse have never been produced. In this work, we have investigated gas/vapor micro-bubbles in water acoustically forced by sound in the audible spectrum (i.e. < 20 kHz). Particular focus is given to both mass and heat transfer phenomena, which have revealed to be of dramatic importance in inertially collapsing bubbles driven at very low frequencies. The chemical production is investigated by considering the case study of the production of gaseous ammonia (NH₃) in bubbles initially containing molecular nitrogen (N₂). Our study suggests that, although the larger extent of vapor segregation cushions the collapse at very low frequencies, the bubble response in terms of peak temperatures and pressures is comparable to that obtained with higher frequencies. Production of ammonia appears to be possible only above a certain amplitude threshold which is found to increase with frequency. The results suggest that at low enough power, the production of ammonia should be observable only at low frequencies.

*The authors acknowledge the EC-RISE-ThermaSMART project. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 778104.