Ultrasonic levitation technique

The ultrasonic levitation technique was introduced in the 1930s and does not rely on any specific properties of the sample except size and mass, and has been used in bioanalytical and analytical chemistry applications6-10. The acoustic levitator consists of an ultrasonic transducer and a solid reflector supporting standing waves (see Fig. 17.5). 

In contrast to most levitation techniques such as levitation in electrostatic or magnetic fields, acoustic levitation requires no specific properties of the sample, so almost every substance, whether solid or liquid, can be acoustically levitated. The maximum possible diameter for a levitated sample is a function of the ultrasonic wavelength and turns out to be about one-half the wavelength under ambient atmospheric conditions. Usually, levi-tators are operated by ultrasound frequencies between 15 and 100 kHz resulting in wavelengths from 2.2 to 0.34 cm. 

Sample levitation can be accomplished in different ways, one of which is by using ultrasonic energy. The phenomenon by which small samples of solids, liquids or suspensions can be levitated at the nodal points of a standing ultrasonic wave was first described by Bucks and Muller in 1933 [70]. The flexibility and potential of acoustic levitation in various fields are widely documented, mainly by studies in the analytical and bioanalytical fields [71-73]. Therefore, levitation can be considered a mature technique. Its development warrants inclusion of a dedicated section in this chapter to describe its fundamentals and compare the advantages and limitations of acoustic levitation with other levitation modes. The devices used for this purpose and the potential applications of each mode are also discussed. 

Ultrasounds have been used to assist in the insertion of samples into solid-liquid analytical systems for some time. The earliest ultrasonic nebulizers and automatic slurry sampling systems were reported in the 1980s. However, the actual potential of ultrasounds for increasing the efficiency of sampling systems remains to be explored. Thus, a recently developed technique based on acoustic levitation has been found to substantially... 

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