Final Report
Abstract
Ultrasound may be used to create high-energy chemistry by means of acoustic cavitation
(a process of formation, growth, and implosion of bubbles in a liquid). During the
collapse of these cavitation bubbles intense heating and high-pressure areas are
generated, reaching values near 5000 k and 500 atm respectively. These two parameters
are necessary in creating chemical reactions. Since acoustic cavitation deals with bubbles
in a liquid, the bubbles shape and spherical compression are affected by buoyancy. We
will be designing a sonochemical apparatus to observe the effects that zero gravity has on
the formation of sonochemical products such as increase yield and altered geometric
properties of nano-sized metallic particles.
Test Procedure
The preflight test procedure began with the preparation of four gold chloride samples.
All four samples were extracted form the same stock solution to ensure that they were of
the same concentration. Prior to each flight, a sample was placed into the sonocation
chamber and sealed at 1 atm.
In flight, the test was carried out by automated commands from a computer. An
accelerometer was used to monitor acceleration in the vertical direction. When the
acceleration fell below 0.2g, the computer would wait five seconds and then it would
activate the ultrasonic horn. The horn would operate for seven seconds before
automatically shutting off. In the event of the pilots aborting a parabola, the horn would
automatically shut off when the acceleration increased above 0.2g. The computer
program also recorded the exact times during which the horn was operating. This data
was used to run the ground based control experiments.
Following each flight, the sample was removed from the chamber and prepared for
shipment back to the University of Washington. Analysis began four days after the last
flight. A spectrophotometer was used to measure the amount of gold chloride remaining
in each sample. A transmission electron microscope (TEM) was used to observe the
nanoparticle geometry.
A computer program written in Lab View was used to mimic each flight for the groundbased experiments, using the recorded horn operation times to reproduce the exact
sonocation times for each flight. The remaining two samples were tested and analyzed
with the procedures above.
Analysis
A spectrophotometer was used to measure the amount of gold chloride remaining in
solution. The absorption spectra obtained for each test are shown in figure 1. Gold
chloride has a characteristic absorbance peak at 290 nm. The magnitude of this peak is
linearly proportional to the concentration of gold chloride in a pure solution. During the
sonocation process a broad absorbance band appeared, presumable due to the presence of
gold nanoparticles. Absorbance is additive, so the gold chloride peak rides on top of this
broad absorption band. This complicated the analysis because it is no longer guaranteed
that the magnitude of the absorbance at 290 nm is linearly proportional to the
concentration. However, the gold chloride absorbance relative to the broad absorbance
will maintain a linear relationship with the gold chloride concentration. The absence of a
peak at 290 nm on either flight indicates that the reaction went to completion. The 290
nm peaks on the ground experiments are clearly visible, thus indicating that there is a
significant amount of gold chloride remaining in the solution.
The absorbance peak around 550 nm is characteristic of gold nanoparticles. The
magnitude of this peak indicates the concentration of particles formed. The two flight
peaks are clearly much smaller than the ground experiments, which is contrary to what
we had hoped for. However, the peak is much more broad as shown in the graph below.
This broad absorbance peak suggests a lager size distribution of the particles. While the
absorbance spectrum gives some insight into differences between nanoparticle formation,
a clear picture of particle size cannot be extracted from this information. TEM analysis
was used in order to obtain a more accurate particle size distribution.
Figure 1: This graph shows the absorbance spectrum of each sample. Flight 1 is blue, flight
2 is red, ground 1 is white and ground 2 is yellow. The nanoparticle peaks around 550 nm for
each sample are labeled and the difference can be clearly observed. The gold chloride peak is
located at 290 nm and its magnitude is related to the concentration of the solution as
mentioned above. The presence of a visible peak for both ground experiments indicates that
there is a significant amount of un-reacted gold chloride remaining in the solution.
The
absence of a peak for each flight experiment shows that the reaction went to completion.
A transmission electron microscope was used to look at the geometry of the nanoparticles
produced during the flight and ground tests. The samples were well mixed and a portion
of each solution was extracted and placed on a sample grid covered with a carbon film.
Pictures of the gold nanoparticles were taken at different magnifications. The ground
experiments showed no particles at magnification above 80,000. The flight experiments
contained nanoparticles at magnifications above 500,000 X. Using simple scaling
arguments, the magnification number and the imaged particles were used to determine
the size of the particles. The results proved conclusive that smaller particles were
produced in the zero gravity environments. These particles were on the order of 10-30
nm in diameter while the particles produced in the 1g environments were 60-100 nm in
diameter.
Results
The consistent shape of the flight and ground absorbance spectra proves the experiment
to be reproducible and the results conclusive. The fact that both flight experiments went
to near completion while the ground experiments did not indicates an increase in the
reaction rate when in a 0-g environment. Microgravity also affected the size of the
nanoparticles produced. Particles produced under microgravity conditions were, on
average, 3 times smaller than those produced under 1-g.
GROUND IMAGE
FLIGHT IMAGE
There were no other particles at higher magnifications produced on the ground. However
at Higher magnifications there were many more particles found on the flight.
The size distribution was found for the ground images, as well as the flight images, and
gives a much more quantitative result for the size difference.
FLIGHT SIZE DISTRIBUTION
Flight 1 Particle Size D istribution
60
50
Count
40
30
20
10
0
0
50
100
150
200
Particle size (nm)
250
300
35
GROUND SIZE DISTRIBUTION
G ro u n d 1 P a rt ic le s iz e d is t rib u t io n
25
20
Count
15
10
5
0
0
50
100
150
P a rt ic le S iz e (n m )
200
250
300