Ultrasonically levitated

Fig. 17.5 Ultrasonically levitated microdroplet dye laser. Left. Photograph of a lasing levitated microdroplet. Right. Schematics of ultrasonic field with the microdroplet being trapped at a node in the ultrasonic field. Reprinted from Ref. 11 with permission. 2008 Optical Society of America...

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). 

This section discusses the most interesting acoustic levitation approaches for use in analytical chemistry and describes devices for sample positioning in the Ievitator and reagent delivery, ultrasonic levitators and the coupling of acoustic levitators to detection and separation systems. Also, it discusses future prospects for acoustic levitation. 

In 1997, Welter and Neidhart [72] developed one of the earliest applications of ultrasonic levitation in analytical chemistry by using an Apos BA 5 acoustic ievitator from... 

One experimental system for the determination of analytes in ultrasonically levitated samples is an automated set-up that can be used in conjunction with spectrometric detection using the method of standard additions and/or microtitration with spectroscopic detection of the end-point. Although the potential of this combined approach remains to be explored, acoustic levitation appears to be a highly promising choice for microanalyses [124]. 

E. H. Trinh, R. G. Holt, and D. B. Thiessen, The dynamics of ultrasonically levitated drops in an electric field, Phys. Fluids 8,43, 1996. 

Trinh, E. H. (1994). Experimental study of streaming in ultrasonic levitators. Physics of Fluids, 6, 3567-3579. 

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. 

The importance of sample delivery is frequently overlooked in the studies on acoustic levitation, even though precision in the analyses relies heavily on an appropriate choice of the sample volume. Delivery systems are closely related to the nature of the sample (liquid, suspension, solid or gas). Any type of system e.g. a micropipette, capillary, or microsyringe) can be used to position a drop from a liquid or slurry sample in an ultrasonic Ievitator. 

Figure 8.11. Portable Raman spectrometer coupled to an acoustic levitator. A — Dantec ultrasonic acoustic levitation device, B — Raman fIbre-optIc probe, C — control unit for the acoustic levitation device, D — quartz halogen light source and E — InPhotonIcs portable 785-nm Raman spectrometer. (Reproduced with permission of the American Chemical Society, Ref [120].)...

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... 

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. 

Samples can be removed from the levitator with the same instruments that are used to position them in the nodes of the standing ultrasonic wave (viz. syringes, microlitre pipettes and glass capillaries). Microlitre pipettes are the most efficient for removing whole samples. 

The equilibrium position, for this case (in which the particle is small compared to the acoustic wavelength), is independent of the size of the particle, but dependent upon the acoustic contrast factor and the strength and frequency of the ultrasonic field. This offers a means of characterizing the acoustic field, the pressure amplitude of which is often difficult to determine in microsystems, by balancing acoustic and gravitational forces on an acoustically levitated particle. Particles that are not neutrally buoyant will have a minimum acoustic energy density required to hold the particle against buoyancy forces. 

Modification of the levitator was based on an acoustic levitator commercially available from Tec5 (Oberursel, Germany), equipped with an ultrasonic transducer and a wave generator at 58 kHz. The optional climate chamber was not able to provide temperatures in the range required for this study (below 0 °C). Hence, it was necessary to develop a cooling mechanism for the levitator, which resulted via a process chiller in the idea of an electrical cooling device. 

During all drying experiments using the levitation system, the deformation of the droplet to form hollow/donut shaped granules was observed. It has to be clarified if this is an effect of the drying itself or of the surrounding properties, for example, the ultrasonic field or the generation/injection process of the droplet into the levitator. 

Figure 1. Experimental apparatus A. ultrasonic acoustic levitation device showing cells trapped in a node B. Raman fibre optic probe C. control unit for acoustic levitation device D, Quartz halogen light source E. InPhotonics portable 780 nm Raman spectrometer...

The most recent work on SINNMR has resulted in the development of the first dedicated acoustic/NMR probehead. This accepts special SINNMR sample tubes that contain a piezoelectric transducer (interchangeable so that a range of acoustic frequencies from 1 to 10 MHz can be used) at its base. This configuration permits the ultrasonic irradiation of particles in less dense support liquids so that the particles can be levitated by the acoustic field and... 

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