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.

Ultrasonic atomization of distilled water.

Experimental data on ultrasonic atomization of distilled water in a frequency range from 5 to 50 kHz are presented. A good agreement was found with the predictions of Rajan and Pandit [Ultrasonics 39, 235-255 (2001)] for the atomized primary drop size as a function of frequency. The correlation of atomization drop size for different frequencies is useful when producing nanoparticles, spray drying of suspensions, and covering of surfaces using different liquid products. Determining the displacement amplitude threshold for atomization at different frequencies is valuable in designing ultrasonic atomization systems. It is essential to measure the displacement amplitude of the atomizing surface rather that power applied to the transducer because the former is absolute while the latter depends on the efficiency of the transducer and other design parameters. As previous predictions for atomization threshold proved inaccurate, an empirical expression is proposed (based on the authors' measurements) to predict the amplitude atomization threshold for the studied frequency range.

Wettability of a surface subjected to high frequency mechanical vibrations.

Ultrasonic radiation can modify some physical properties in liquid/solid interactions, such as wettability. The dependence of solid surface wettability on its vibrational state was studied. Experiments with an interface formed by distilled water deposited on a titanium alloy and surrounded by air were carried out. It is shown that it is possible to control the apparent wettability of a given liquid/solid/gas system by applying sonic-ultrasonic vibrations of controlled amplitude at the interface. The system studied is composed of a drop of distilled water deposited on a flat titanium surface in air. The contact angle was used as an indicator of apparent wettability. It is shown that the apparent wettability of a surface is linearly dependent on the peak vibration velocity and independent of the vibration frequency. Higher vibration speed lowers the contact angle and therefore causes greater surface wettability.

Ultrasonically enhanced extraction of bioactive principles from Quillaja Saponaria Molina.

A study of ultrasonic enhancement in the extraction of bioactive principles from Quillaja Saponaria Molina (Quillay) is presented. The effects influencing the extraction process were studied through a two-level factorial design. The effects considered in the experimental design were: granulometry, extraction time, acoustic Power, raw matter/solvent ratio (concentration) and acoustic impedance. It was found that for aqueous extraction the main factors affecting the ultrasonically-assisted process were: granulometry, raw matter/solvent ratio and extraction time. The extraction ratio was increased by Ultrasonics effect and a reduction in extraction time was verified without any influence in the product quality. In addition the process can be carried out at lower temperatures than the conventional method. As the process developed uses chips from the branches of trees, and not only the bark, this research contributes to make the saponin exploitation process a sustainable industry.

A procedure for the efficient selection of piezoelectric ceramics constituting high-power ultrasonic transducers.

The most characteristic narrow-band transducer structure for high-power ultrasonic applications is the well known piezoelectric sandwich which is reminiscent of the Langevin transducer. Such structure is generally used jointly with other components in the construction of industrial high-power transducers. One of the main objectives in the design and construction of such high-power transducers is to minimize energy losses. To that purpose the selection of the piezoelectric ceramic rings forming the sandwich requires clear and specific criteria. This paper deals with a numerical and experimental procedure for the accurate selection of the piezoelectric rings constituting high-power transducers, based on the analysis of the mechanical Q, the frequency and the resonance curve. The procedure was experimentally checked by constructing and characterizing several transducer structures.