Levitated Droplets

This extraordinary small probe volume allows high efficiency detection of individual molecules but at the expense of extraordinarily low throughput. About 200000 years would be required to analyze the content of a 1-pL sample with the 

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The performance and applicability of the levitated droplet liquid dye laser are challenged by droplet evaporation and bleaching of the laser dye. In order to minimize these effects, Azzouz et al.11 used a measurement scheme, where nominally identical droplets were loaded consecutively, and each droplet was at... 

The levitated droplets and droplet dye lasers may conveniently be operated with acoustic frequencies below the critical for excitation of droplet vibrational modes, (17.4), to facilitate stable and highly spherical optical resonators. 

Charge Loss and Mass Loss Data Associated with the Explosion of Levitated Droplets of Several Hydrocarbon Compounds from Taflin et al. (1989) ... 

Fig. 22. Phase functions for a levitated droplet of hexadecane evapor data were obtained with the photodiode array of Fulton (1985). ttng...

Recently, Tang and Munkelwitz (1991) applied a variation on this theme by determining very low vapor pressures of levitated droplets from data obtained at atmospheric pressure and in an evacuated chamber. In both cases they used data from the literature obtained at temperatures much higher than room temperature together with their new data to fit the constants in vapor pressure equations of the form... 

Most of the studies of levitated droplets have involved low-vapor-pressure materials, but Tallin and his coworkers reported data for water droplets evaporating in dry nitrogen. The rapid evaporation of a water droplet requires that the experiment be automated, and this was accomplished by injecting the droplet by means of a 3,000 V dc electrical pulse applied to a flat-tipped hypodermic needle. The pulse triggered the data collection system so that phase functions and the resonance spectrum were obtained during the less than three-second duration of an experiment. From the phase function... 

All of these solutions fail for Pe = 0(1), so Davis and his colleagues (Zhang and Davis, 1987 Taflin and Davis, 1987) performed evaporative mass transfer experiments over the Peclet number range 0.01 < Pe < 4. using electrodynamically levitated droplets of hexadecane in flowing Nj and He and dodecanol in Nj. 

Internal convection is precluded by the small size of the droplets. Observations of levitated droplets with diameters of order 50j[Pg.76]

Tallin (1988) recognized that the refractive index change associated with the OCT/Brj reaction would affect optical resonances of a reacting droplet, so he carried out bromination experiments with levitated droplets using the apparatus shown in Fig. 43. 

It should be noted that the procedure described in this chapter is strictly limited to the interactions of droplets with surrounding gases, with much emphasis on the conditions that would yield powders of narrow size distributions. However, no attempt is made to describe the literature on particle formation from a single levitated droplet (7). The technique used in these studies also differs from those in which the... 

Only the two first methods allow measurement of the temperature coefficient of the surface energy. The maximum bubble pressure technique is well-adapted for metals with low and intermediate melting points and specially for oxidizable metals, while the sessile drop technique has been applied with success to measure ctlv values up to 1500°C. The drop weight method is particularly useful for very high melting-point metals because it avoids liquid contact with container materials. This is also true for the recently developed levitation drop technique that analyses the oscillation spectrum of a magnetically levitated droplet. 

Figure 8.8. Flow-through microdispenser addition to a levitated droplet. The dispenser trajectory is seen as the thin white line connecting the dispenser nozzle with the levitated droplet. (Reproduced with permission of Springer-Verlag, Ref. [68].)...

The earliest applications of acoustic levitation in analytical chemistry were concerned with the development of various steps of the analytical process. Thus, Welter and Neidhart [72] studied the preconcentration of n-hexanol in methanol by solvent evaporation and the liquid-liquid extraction of n-hexanol from water to toluene in a levitated droplet, which they found to be efficient when using GC-FID with n-pentanol as internal standard. Solvent exchange of fluorescein from methylisobutyl ketone to aqueous sodium hydroxide was also accomplished. Sample concentration in an acoustically levitated droplet prior to injection into a CE equipped with an LIF or UV detector has also been accomplished [73,118]. The target analytes (namely, dansylated amino acids) were concentrated in the levitated drop and a limit of detection of 15 nM — much lower than the 2.5 pM achieved by hydrodynamic injection without preconcentration — was achieved following CE separation and quantification. For this purpose, 36000 sample droplets 2.3 pi in volume each were sequentially positioned in the acoustic Ievitator and evaporated. This example illustrates the potential of acoustic levitation for coupling to any type of detector for micro- or nanotrace analyses. 

Derivatization of target analytes has also been performed in acoustically levitated droplets for the determination of mono-, di-, tri- and tetrabutyltin [119]. The target analytes were extracted simultaneously from acetate buffer to hexane and derivatized using NaB(C2H5)4. Then, the organic phase was transferred for separation—determination by GC-AES. The results were comparable to those provided by conventional derivatization. 

Environmental monitoring has also taken advantage of acoustic levitation for the investigation of physico-chemical processes relevant to the troposphere — mainly at temperatures below 0°C. Gas-liquid transfer of H2O2 from the gas phase to the levitated droplet was studied from in situ chemiluminescence measurements. Also, freezing of stably positioned droplets was observed by means of a microscope and a video camera, and the usefulness of this technique for simulation and investigation of cloud processes thus demonstrated. Ex situ microanalysis of sub-microlitre droplets by the use of an optical fibre luminometer also proved an effective means for investigating important physicochemical processes at the micro scale [100]. 

S. Santesson, Miniaturized Bloanalytical Chemistry in Acoustically Levitated Droplets, RhD Thesis, University of Lund, 2004. 

While not thus far exploited for LLE, it is interesting to note experiments (Bogan 2002, 2004) in which charged droplets ( 0.27 xL) are produced from a piezo-electric atomizer and are levitated in the electric field of a modified quadrupole ion trap (Section 6.4.5) operated at atmospheric pressure. Thus far the main analytical application of this technique (Bogan 2004) has involved wall-free preparation of micrometer-sized sample spots for fmol detection limits of proteins by MALDI-MS, but extension to LLE in such levitated droplets is a possibility. 

Figure 16. A drawing of the external IR optical bench used for studying acoustically levitated droplets.

Fig. 4.1 Transducer (top, 21 nun in diameter) and reflector (bottom, 40 mm in diameter) with levitated droplets within the acoustic field...

In order to carry out continuous measurements of fast processes of single levitated droplets all devices of the levitator setup were synchronized. The communication of sonotrode, camera, positioning tables, temperature and humidity probes, and microdispenser was realized with a self-developed program based on the software Lab VIEW from National Instruments. 

Since the simulated and the experimental data were in good agreement, the evaporation behavior of levitated droplets can correspond to that of sprayed droplets. The evaporation rate in the levitator depends on the gas velocity in the same manner as in spray processes. Therefore, acoustic levitation can be regarded as an adequate model for spray drying processes. 

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