Acoustic Levitation of Droplets

Acoustic droplet ejection Acoustic levitation of droplets Acoustic particle concentration Drop on demand Droplet manipulation Droplet transport by surface acoustic waves Radiation pressure Surface acoustic waves... 

Acoustic waves in liquids can give rise to so-called radiation pressure forces that can in turn drive acoustic streaming flows, deform fluid-fluid interfaces to generate droplets, or exert levitation forces on suspended drops or particles. This contribution reviews three technologically relevant examples of these effects acoustic droplet ejection, droplet transport along a solid surface using surface acoustic waves, and acoustic levitation of droplets. 

Yarin AL, Pfaffenlehner M, Tropea C (1998) On the acoustic levitation of droplets. J Fluid Mech... 

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. 

One advantage of micropumps and flow-through microdispensers is that droplet evaporation can be controlled accurately by adding a solvent in order to keep the droplet volume constant, whioh is essential for quantification purposes. Because US increases the temperature of the medium, it can facilitate solvent evaporation. This requires using a system such as an imaging detector to continuously monitor the droplet volume in order to determine the evaporation rate during acoustic levitation. Both mioropumps and microdispensers oan be coupled via FI manifolds to other units for the development of different steps of the analytical process. 

Cerenius et al. [114] evaluated the use of acoustic levitation to keep a droplet of liquid in an X-ray beam long enough to acquire X-ray diffraction spectra. Solid samples can be studied additionally by suspension in a suitable solvent. The most salient advantage of X-ray diffraction here is that it can provide more detailed information and a higher quality than the Raman spectroscopy. 

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. 

An original method involves quadrupole oscillations of drops K The drop (a) in a host liquid (P) is acoustically levitated. This can be achieved by creating a standing acoustic wave the time-averaged second order effect of this wave gives rise to an acoustic radiation force. This drives the drop up or down in p, depending on the compressibilities of the two fluids, till gravity and acoustic forces balance. From then onwards the free droplet is, also acoustically, driven into quadrupole shape oscillations that are opposed by the capillary pressure. From the resonance frequency the interfacial tension can be computed. The authors describe the instrumentation and present some results for a number of oil-water interfaces. 

Other aspects of the drop oscillation problem, such as oscillation of liquid drops immersed in another fluid [17-21], oscillations of pendant drops [22, 23], and oscillations of charged drops [24, 25], have also been considered. In particular, there are numerous works on the oscillation of acoustically levitated drops in acoustic field. In such studies, high-frequency acoustic pressnre is required to levitate the droplet and balance the buoyancy force for the experimental studies performed on the Earth. As a result of balance between buoyancy and acoustic forces, the equilibrium shape of the droplet changes from sphere to a slightly flattened oblate shape [26]. Then a modulating force with frequency close to resonant frequencies of different modes is applied to induce small to large amplitude oscillations. Figure 5.4 shows a silicon oil droplet levitated in water and driven to its first three resonant modes by an acoustic force and time evolution for each mode. 

More realistic urea decomposition experiments were performed with single UWS droplets on a quartz fiber [33]. Even contact-free experiments are possible with UWS droplets in an acoustical levitator [43]. Experiments with single UWS droplets also provide information about water evaporation from the UWS droplets as shown in Ref. [33, 43]. These data are a valuable input for modeling work, but real UWS aerosols are much smaller than the droplets used in these studies [33,43]. It is plausible that, in analogy to the TGA, DSC, and TPD experiments mentioned above [11, 36, 38, 39], smaller aerosols with faster mass transport to the surrounding gas favor the desorption of HNCO and/or urea vapor over byproduct formation inside the aerosols. 

Acoustic Levitation/FTIR Spectroscopy of Droplets and Bubbles... 

Figure 15. A diagram of the acoustic levitator assembly. A droplet or a bubble could be situated in the modes of the sound wave interference pattern.

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

The bilayer formation between p-lactoglobulin and pectin is monitored via the zeta-potential of the oil droplets. Below the isoelectric point of the protein the net zeta-potential is positive and amounted to 35 mV. Upon addition of pectin the zeta-potential decreased with increasing pectin content. At 0.2 % of pectin a plateau was reached in the zeta-potential of the emulsion droplets at about —18 and —30 mV for high and low methoxylated pectin, respectively. This pectin content did not significantly affect the viscosity of the emulsion and therefore no significant differences in the drying behaviour of the emulsions as determined by acoustic levitation... 

In summary, this work presents the acoustic levitation as a powerful tool to model spray processes. Thanks to its simple setup and experimental handling, acoustic levitation can give the opportunity to elucidate the processes within single droplets for a wide range of reactive and nonreactive spray systems. It also offers a simple way to check the suitability of systems that have not been considered for spray processes yet. 

Insertion of the droplets into the acoustic levitator was carried out by three different methods. One method was to nebulize the solution with the microdispenser MD-K-130 from the company Microdrop Technologies GmbH (see Fig. 4.4). The resulting... 

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