The influence of frequency, volume, and viscosity on the ultrasonic atomization threshold of sessile droplets.

In this research, we present a study on the atomization threshold (ATh) of sessile droplets, analyzing its relationship with the excitation frequency fexc (55-48 kHz), droplet volume Vdrop (1-100 µl), and droplet viscosity µ (1-6 mPa⋅s). The investigation focused on the atomization thresholds using ultrasonic excitation of distilled water droplets and water- polyethylene glycol (PEG)-8000 mixtures deposited on vibrating surfaces. The obtained results are compared with previously reported theoretical models. A modification to the model proposed by Alzuaga et al. [Alzuaga, Manceau, and Bastien, J. Sound Vib. 282(1-2), 151-162 (2005)] is suggested to determine the atomization thresholds of sessile droplets, incorporating the atomization droplet size equation proposed by Rajan and Pandit and an empirical constant α to account for the effect of droplet volume in this process. The results show that the relationship between the atomization threshold and viscosity does not fit well with the prediction of Eisenmenger [Acta Acust united Acust. 9(4), 327-340 (1959)] and Pohlman and Stamm [Untersuchung Zum Mechanismus Der Ultraschallvernebelung an Flüssigkeitsoberflächen im Hinblick Auf Technische Anwendungen (Investigation on the Mechanism of Ultrasonic Nebulization on Liquid Surfaces Considering Technical Applications) (VS Verlag für Sozialwissenschaften, Wiesbaden, Germany, 1965)] and Pohlman et al. [Pohlman, Heisler, and Cichos, Ultrasonicc 12(1), 11-15 (1974)] (ATh∝µ). However, the data tendency aligns with the model proposed by Alzuaga (ATh ∝ µ1/2). The results obtained in this study provide a deeper understanding of the atomization thresholds of sessile droplets through ultrasonic excitation.

On the dynamics of a big drop in acoustic levitation.

The acoustic levitation of a drop is a complex process that needs a high-intensity non-linear acoustic field; the sound pressure level has to be sufficient to raise the drop but not too large to avoid its atomization, limiting the maximum size of a levitated drop. In this paper, we present an experimental study of big drops levitation with a volume up to 166±2µl and with an effective diameter 6.82±0.03mm, figures one magnitude order larger than the maximum drop volume reported in the literature. Our acoustic levitator produces an acoustic field with a different shape than the field produced by a typical levitator. Our measurements and simulations of the acoustic field and drop dynamics suggest that the levitation of big drops is possible because the distribution of radiation pressure over the drop surface in our system differs from that in a typical acoustic levitator; its maximum value appears on the top surface of the drop and not in its equator. In addition, we determined the upper and lower limits of sound pressure necessary for the levitation of drops of various sizes that allow our system.