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Acoustic Levitation: A Powerful Tool to Model Spray Processes

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Abstract

The acoustic levitation was used as a model system for spray drying processes within DFG Project SPP1423-Process Spray to elucidate the drying behavior of single droplets and the formation of the particle morphology in more detail. The gas flow characteristics inside the levitator were analyzed with computational fluid dynamics simulations. The obtained data were validated with experimental values and showed that the evaporation behavior of levitated droplets can correspond to sprayed droplets. Different reactive and nonreactive substance systems were investigated in single droplet experiments. The influence of process parameters like gas temperature, relative humidity, and droplet size on the drying behavior and particle morphology was elucidated. It was possible to track the conversion of N-Vinyl-2-pyrrolidone in droplet polymerization by using Raman spectroscopy and subsequent principal component analysis. It was found that the first principal component corresponds to the first drying stage and describes the evaporation of water, whereas the second principal component described the polymerization. The crystallinity of the obtained PVP particles increased when temperature and humidity were decreased. The polymerization of partially neutralized acrylic acid revealed the duality of polymerization and crystallization of monomer. Regarding the particle morphology, it was found that a higher amount of sodium acrylate led to a smoother particle surface. One reason for this result is that if sodium acrylate precipitates, it defines predominantly the particle structure. Another reason for this trend in particle morphology is a lower gas pressure within the particle due to a lower polymerization rate, which leads to fewer cracks within the shell. Mannitol served as a nonreactive model system. The drying rate increased with elevated gas temperatures and smaller initial droplet diameters. These experimental results corresponded well with the simulation carried out by Grosshans et al. Additionally, a categorization of morphological properties was introduced. This shows an increasing particle roughness at high temperatures and high initial mass fractions of mannitol. At high relative humidity, metastable, supersaturated mannitol solutions were formed during the drying process.

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.

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Abbreviations

AA:

Acrylic acid

arctanh:

Arcus tangens hyperbolicus

CFD:

Computational fluid dynamics

cos:

Cosinus

DN:

Degree of neutralization

DoE:

Design of experiments

ESA:

European Space Agency

Eq.:

Equation

Fig.:

Figure

NaA:

Sodium acrylate

NASA:

National Aeronautics and Space Administration

NVP:

N-Vinyl-2-pyrrolidone

PC(A):

Principal component (analysis)

PExp:

Particle expansion

PVP:

Polyvinylpyrrolidone

SEM:

Scanning electron microscopy

sin:

Sinus

Tab.:

Table

VA-044:

2,2′-Azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride

A :

Surface [m2]

c :

Concentration [mol L−1]

d :

Droplet diameter [m]

F :

Force [N]

k :

Wave number [m−1]

l :

Distance between transducer and reflector [m]

m :

Mass [kg]

p :

Sound pressure [Pa]

r :

Axis length [m]

r.h.:

Relative humidity [%]

T :

Temperature [°C]

t:

Time [s]

v :

Particle velocity [m s−1]

u :

Gas velocity [m s−1]

V:

Volume [m3]

Y :

Mass fraction [−]

z :

Distance between transducer and pressure node [m]

β :

Evaporation rate [m2 s−1]

λ :

Wave length [m]

ε :

Numerical eccentricity [−]

Φ :

Porosity [−]

ρ :

Density [kg m−3]

0:

At the start of experiment

1:

Main

2:

Side

d:

Droplet

ellipsoid:

Ellipsoidal

end:

At the end of experiment

exp:

Experimental

g:

Glass transition

initiator:

Initiator

m:

Mannitol

max:

Maximal

monomer:

Monomer

lev:

Levitation

shell:

At the time of shell formation

sim:

Simulated

wb:

Wet bulb

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Acknowledgments

The authors gratefully acknowledge the financial support for this work within the DFG Project SPP 1423-Process Spray “Experimentelle Untersuchungen an schwebenden Einzeltropfen als Modellsystem für das bessere Verständnis versprühter Tropfen und der Strukturausbildung bei der Herstellung von pulverförmigen Feststoffen”.

We wish to thank all our cooperation partners for the lively exchange and their scientific expertise in many discussions.

Prof. Drusch, Prof. Schwarz, CAU Kiel, TU Berlin.

Prof. Nieken, University of Stuttgart.

Prof. Simon, University of Stuttgart.

Prof. Gutheil, University of Heidelberg.

Dr. Fries, Fraunhofer IKTS Dresden.

Prof. Steckel, CAU Kiel.

Prof. Schuchmann, Dr. Gaukel, KIT.

Prof. Walzel, TU Dortmund.

Dr. Bräuer, Prof. Schlücker, University of Erlangen.

A special thank goes to Prof. Dr. U. Fritsching and Dr. B. Giernoth for the coordination of this program.

Furthermore, we would like to thank R. Walter from the Biozentrum Grindel und Zoologisches Museum, University of Hamburg for the SEM measurements.

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Authors and Affiliations

  1. Institute for Technical and Macromolecular Chemistry, University of Hamburg, Hamburg, Germany

    M. Junk, A. H. Halfar, M. Griesing, R. Sedelmayer, J. Laackmann, L. Cepelyte, W. Pauer & H.-U. Moritz

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  1. M. Junk
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Correspondence to H.-U. Moritz .

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  1. Institute of Materials Science, University of Bremen, Bremen, Germany

    Udo Fritsching

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Junk, M. et al. (2016). Acoustic Levitation: A Powerful Tool to Model Spray Processes. In: Fritsching, U. (eds) Process-Spray. Springer, Cham. https://doi.org/10.1007/978-3-319-32370-1_4

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