Coalescence of levitating droplets in a bichromatic acoustic field

The lab-on-a-drop concept implemented under acoustic levitation conditions has emerged as a valuable tool in various fields of science and engineering. Among the multitude of tasks that can be performed using acoustic levitation, controlled droplet coalescence is currently one of the greatest interest. However, existing methods often complicate automation of this process and suffer from several drawbacks, including low reproducibility and sample loss. To overcome these limitations, we developed a bichromatic acoustic levitator operating at two frequencies (25 and 40 kHz). The coalescence process comprises two stages: droplet holding in the 40 kHz field and controlled merging upon activation of the 25 kHz field, which destabilizes the trapping potential. This approach enables independent control of each droplet through spatial separation of acoustic potentials. Experimental results demonstrate that the proposed method achieves successful coalescence in approximately 95% of cases, followed by stable trapping of the merged droplet. These findings create opportunities for broadening the range of reactions and physicochemical processes that can be studied in contactless conditions, including combustion and chemical synthesis.