Mahsa Azad,1 Harlan D.O. Sangrey,2 Arvin Farid,3 Jim Browning,4 Elisa Barney Smith5
Electromagnetic Stimulation of Two-Phase Transport in Water for
Geoenvironmental Applications
ABSTRACT: Air sparging is a popular remediation technology for contaminated soils. However, the
application is not efficient due to the limitations of airflow that result from the formation of random
preferential air channels. Controlling the formation of these air channels and enhancing diffusion
surrounding them can considerably improve the effectiveness of air sparging. This work is a study on
how electromagnetic (EM) waves —without a measureable increase in temperature— enhance the
transport of a nonreactive dye in water as a visible analogy of air sparging (i.e., airflow within
groundwater). This paper explains the details of the experimental setup and procedures required to
conduct the EM-stimulation experiment as well as electric-field mapping and digital imaging of dye
transport for the purpose of digital visual analysis. Several antenna designs and the way they direct the
transport mechanism are studied. The results of EM-stimulation tests with no measureable temperature
increase show that EM waves enhance and direct the dye transport in accordance with the EM source
(transmitting antenna) and its radiation pattern. The rate of transport of the dye is studied and compared
for unstimulated and EM-stimulated tests. Because of the small size of the dye molecules and existence
of an alternating electric field, dielectrophoresis is the most likely potential transport mechanism.
However, the existence of other competing factors dominating dielectrophoresis interferes with this
type of study. Future modifications in the experimental design seem to have the potential to improve
the investigation of the transport phenomenon.
KEYWORDS: Air Sparging, Remediation, Electromagnetic Stimulation, Diffusion, dielectric Spectroscopy,
Viscous Fingering.
1
Graduate Research Assistant, Department of Civil and Environmental Engineering, Boise State University, MS 2060,
1910 University Drive, Boise, Idaho 83725-2060; PH (208) 392 - 2848; FAX (208) 426-2351, Email:
mahsaazad@u.boisestate.edu.
2
Graduate Research Assistant, Department of Civil and Environmental Engineering, Boise State University, MS 2060,
1910 University Drive, Boise, Idaho 83725-2060; PH (208) 841-5148; FAX (208) 426 – 2351, Email:
harlansangrey@u.boisestate.edu.
3
Assistant Professor, Department of Civil and Environmental Engineering, Boise State University, MS 2060, 1910
University Drive, Boise, Idaho 83725-2060; PH (208) 426 - 4827; FAX (208) 426 – 2351, Email:
arvinfarid@u.boisestate.edu.
4
Associate Professor, Department of Electrical and Computational Engineering, Boise State University, MS2075, 1910
University Drive, Boise, Idaho 83725-2075; PH (208) 426 - 2347; FAX (208) 426 – 2470, Email:
jimbrowning@u.boisestate.edu.
5
Associate Professor, Department of Electrical and Computational Engineering, Boise State University, MS 2075, 1910
University Drive, Boise, Idaho 83725-2075; PH (208) 426 - 2214; FAX (208) 426 – 2470, Email:
ebarneysmith@u.boisestate.edu.
1
Introduction
Environmentally hazardous spills of chemicals/petrochemicals and gasoline are a serious
threat to groundwater. There is an increasing need for cleanup of these spills, and this need is
not expected to diminish in the foreseeable future, as large numbers of underground
chemical/petrochemical and gasoline storage tanks around the country and world are aging
and are either beginning to leak or have been leaking for years (EPA, 1988). These
contaminants can seep into the ground and, without any treatment, can spread to groundwater
and drinking water. The ability to treat large areas of spillage in an efficient and cost-effective
manner is necessary for the treatment of contaminated sites. The ex-situ method of
excavation and removal of contaminated soils is very expensive and destructive. Instead,
there are several less-destructive remediation technologies with different levels of
effectiveness.
In-situ air sparging (AS) is a remediation technique for removing organic contaminants
by a combination of volatilization and aerobic biodegradation processes (Johnson et al.,
1993). This procedure involves the injection of contamination-free air into the saturated zones
of the subsurface to enable hydrocarbons to transform from the dissolved to the vapor phase.
At the same time, oxygen is transferred from air to the contaminated groundwater, which, in
turn, promotes the biodegradation of volatile organic compounds, or VOCs (Hinchee, 1994).
The contaminated air is then vented through the unsaturated zone. Air sparging is most often
used together with soil vapor extraction (SVE). The SVE system creates a negative pressure
in the unsaturated zone through a series of extraction wells to control the vapor plume
migration.
2
Air sparging does, however, have limitations. Ji et al. (1993) found that in a medium- to
fine-grained water-saturated soils, air flows in randomly formed detached channels. The area
surrounding these air channels, called mass transfer zones (MTZ), is the region through
which the volatilization occurs. This would result in a VOC concentration gradient causing
VOCs to diffuse toward the air channels (Braida and Ong, 2001). As a result, diffusion is the
transport mechanism in air sparging treatment outside air channels. Chao et al. (1998)
compared the cleanup rate outside and inside air channels and concluded that the cleanup
outside air channels is negligible. As a result, several portions of contaminated regions would
not be exposed to the injected air and would remain untreated.
To resolve this problem, either the air-channel formation needs to be controlled, or the
diffusion rate outside the channels needs to be raised in a controlled manner. Vermeulen and
McGee (2000) have examined electromagnetic (EM) bioheating and have demonstrated the
potential of this process in enhancing the removal of VOCs by the generated heat. However,
only preliminary work was performed, and the process was not controlled. They discussed
the use of in-situ heating without looking at the negative environmental impacts due to the
overheating of organisms. There is also no study of the impact of radio-frequency (RF)
stimulation other than through heating.
The major goal of this work is to use EM waves at frequencies at which heat generation
is minimal (less than 1–2 °C). An advantage of EM waves over other enhancement methods
is the fact that lower-energy EM waves can be applied at a lower range of frequencies
(MHz), where free and even bound water molecules are forced to oscillate yet will not raise
the water temperature due to water’s high heat capacity. This is because, according to the
dielectric spectroscopy of water molecules and different relaxation theorems such as the
3
Maxwell-Wagner relaxation theorem (Hilhorst, 1998), in lower ranges of frequency the
values of the dielectric permittivity of water molecules are higher, which means water
dipole molecules better align with the electric field. Therefore, oscillating molecules by
applying an alternating electric field with minimal heat generation would be more
successful at a lower MHz range of frequency. The fluctuations of water molecules would
enhance any transport mechanism within saturated regions of the soil. Therefore, several
transport mechanisms can be enhanced such as diffusion in saturated media. As a result,
EM stimulation can improve airflow in saturated soils and, therefore, expedite the cleanup
process using air sparging or similar technologies.
Scope
The hypothesis behind this work is that stimulation of the media expedites diffusion and
that a relationship of the diffusion rate with different frequencies and the power levels of EM
stimulation may exist.
This study is the preliminary step in the investigation of the effect of EM waves on
transport mechanisms. The diffusion of a nonreactive dye in water was used as a visible
analogy to air sparging in saturated soils at the laboratory scale. Water was used as the
medium of study to enable the visualization of this effect. The effect of EM waves is studied
through a comparison of the rate of change of concentration in different locations of the
medium.
Although this experiment is qualitative as well as quantitative, some details of the
experimental setup must be refined for more successful understanding of the governing
phenomenon as well as modeling and computation purposes in the future. The study of the
4
effects of EM waves on the transport of air in water-saturated media (glass beads of a specific
diameter) would be the next step of this research in the future.
Summary of Experimental Method
A nonreactive inert dye was injected into a medium of water to diffuse while the
temperature was recorded constantly; no temperature change occurred. Meanwhile, a number
of pictures were taken for visual analysis of the diffusion process. Taking images at
consistent time intervals and using the intensity of the images to measure the dye
concentration enable the transport rate to be determined. The same procedure was then
repeated while the medium and dye in the test apparatus were stimulated using a monopole
antenna cased in a chlorinated polyvinyl chloride (CPVC) pipe, submerged in the medium.
The stimulation was repeated for different magnitudes of power to find a possible correlation
between the power magnitude and dye diffusion.
The electric-field component of the EM waves was measured and mapped at specific
slices for a better understanding of the relationship between the diffusion and stimulation
(since the electric-field magnitude is expected to be the possible controlling factor). The most
well-known governing phenomenon for the dye transport in water under alternating electric
fields is dielectrophoresis. According to dielectrophoresis, small particles (1-100 μm in
diameter) experience a dielectrophoretic force due to the electric-field variation. For this
reason, the effects of the potential dielectrophoretic forces are studied in this work to
investigate the mechanism controlling the dye transport. However, as will be shown later, a
more organized electric-field variation is required to eliminate other competing factors and
evaluate dielectrophoresis for this type of study in the future.
5
Theories behind Dye Transport
Diffusion
Diffusion can be described by a partial differential equation, which incorporates variables
influencing the physical mechanism of matter transport. The theory that every particle moves
in a random direction depending on its interactions with the surrounding particles is at the
root of these models. Although the movement of individuals is random, if there is a gradient
in the concentration of particles, there will be a net flux in the system. This gradient will be
reduced until the net flux of the system causes the gradient to approach zero. According to
Fick’s theory, the temporal change in concentration at each point is related to the divergence
of the concentration flux change. Hence, for a homogeneous three-dimensional medium, the
second Fick’s law is represented in Eqn. 1 (Cussler, 1997).

C 
 .( DC )
t
(1)
where
C = concentration of the diffusing material (e.g., g/m3),
t = time (s),
D = diffusion coefficient, in a tensor format (e.g., m2/s).
In the absence of external forces, the value of the diffusion coefficient for liquids similar
to the dye used in this experiment is usually in the range of 10-4-10-6 cm2/s. The diffusion
equation must take into account any external forces and their effects. One of the external
forces in any diffusion test on Earth’s surface is the resultant of Earth’s gravity and buoyant
force. EM stimulation is also an external force that can dominate diffusion.
6
It is believed that if the effect of gravity is ignored, the mechanism of the unstimulated
dye movement in water can follow the diffusion equation. Nevertheless, there are still other
factors affecting this movement such as fingering due to turbulence or convective flow.
Dielectrophoresis
Dielectrophoresis is a potential mechanism behind EM stimulation. It is a phenomenon
where uncharged particles in a background medium can experience a force created by an
electric-field gradient (Pohl et al., 1971). The RF electric field induces a polarization in
particles in the medium, and the particles are subject to a net force from an electric-field
gradient despite being neutral. Although dielectrophoresis is most readily observed for
particles with diameters ranging from 1 to 100 μm (Jones, 2003), the 1-5 nm dye particles
used in this experiment can still be affected. The force that can be exerted by the
dielectrophoresis is dependent on the complex dielectric constants of the particle (  *p ) and of
the surrounding medium ( m* ) . This relationship for the induced force (FDEP) is described in
Eqn. 2 (Pohl, 1978).

 ε*p  ε*m    2
3
FDEP  2 πr ε m Re *  E
 ε

 m 
(2)
where
E = electric field and
r = radius of a homogeneous sphere with no net charge (the particle).
This force is proportional to the gradient of the magnitude of the electric field squared.
When the dielectric permittivity of the particle is higher than that of the medium, the particle
will move toward the area of higher electric-field intensity. In a case where the dielectric
permittivity of the medium is higher than that of the particle, the particle will move against
7
the gradient, toward the area of lower electric-field intensity. The effects of dielectrophoresis
are frequency-dependent for each medium and particle combination.
Although the medium under study is water in this experiment, it is noteworthy that the
dielectrophoresis can govern the transport mechanism in porous media as well. However, a
modified precise study is required to estimation the dielectric permittivity of soil due to the
anisotropic nature of the material. In the case of nonclay minerals such as sand and silica
flour, the dielectric constant is independent of frequency (Arulanandan and Arulanandan,
1985). In the case of clay minerals, the dielectric constant is a function of frequency.
Nevertheless, at high frequencies (around 50 MHz), the dielectric constant is shown to be
independent of frequency (whether it is a clay mineral or a non-clay mineral), but it is still a
function of porosity, orientation, and shape of particles (Arulanandan, 1991).
Material and Methods
Several diffusion tests were performed in a 40cm × 40cm × 40cm clear, acrylic cube to
study the EM stimulation. The acrylic cube was used to enable visualization of the dye
movement.
The water used as the background medium was deionized and deaerated, and the dye
used for visualization was McCormick green food color.
Stimulation and Technical Approach- The choice of frequency used for EM stimulation
and the size of the apparatus were based upon Maxwell-Wagner relaxation mechanisms of
water at various frequencies, practicability, and the restrictions imposed by the instrument.
Hence, the experiments should be performed in a frequency range between 100 MHz and
200 MHz.
The EM-stimulated tests consisted of a submerged CPVC-cased monopole antenna as the
8
source of EM stimulation. The monopole antenna was made from an RG8 coaxial cable with
70 mm of the ground shield stripped away. The length of the exposed central conductor
(stripped length of the outer conductor) of the antenna was designed based on the wavelength
of EM waves at the desired frequency. In an attempt to place the EM source at the center of
the box, the center of the extended dielectric/center conductor is placed at the center of the
box.
Devices for EM stimulation include a signal generator (Agilent, E4400B), an amplifier
(RF-Lambda, 100LM8), a network analyzer (Agilent, E5071), and a spectrum analyzer
(Agilent, N9320A) for the purpose of generating signals and controlling the reflection and
transmission of the signal. To maximize the forward power output into the testing medium
and reduce the reflection back into the amplifier, a matching network was used to match the
impedance of the testing medium to that of the source impedance (i.e., of the amplifier = 50
Ω).
The entire system was placed in a large Faraday cage to contain the EM waves to prevent
interference from outside the experiment and to prevent radiation from the structure
(Jamieson et al., 2007). A wooden frame with an aluminum screen was assembled and
carefully grounded with a removable door for access. However, to create asymmetry the door
was not used during testing.
Dye Injection - Using a pipette, the dye was delivered to the center of the acrylic injection
table in the water tank. The pipette is taped to a vertical CPVC pipe passing through the
middle of the injection table to avoid any agitation in the water. The top of the acrylic table
was concave to allow the dye to be introduced into the middle of the test medium without
9
immediately flowing away. A syringe was used to inject into the pipette a carefully measured
amount of dye (1.2 g), which exited onto the concave portion of the injection table.
Antenna Positions- The first stimulating source used a CPVC-cased vertically positioned
monopole antenna. The EM stimulation consisted of 50 W of RF power at 154 MHz. The
stimulated transport pattern resulted in the dye moving up the CPVC pipe to the surface of
the water and dispersing from the pipe, as seen in Fig. 1(a). Further tests were needed to show
that the effect on the dye movement and diffusion was the result of the stimulation and not
from other factors such as the boundary or an inclined injection table. The next step was to
inject the dye away from the antenna to make sure that no attractive mechanism between the
body of the antenna casing (CPVC) and the dye existed.
A dye cup and holder was built 9 cm away from the antenna to space the injecting pipette
away from the antenna, seen in Fig. 1(b). These tests were referred to as spaced injection tests
and were conducted with 50 W of stimulating power at 154 MHz.
Again, the dye moved toward the antenna, then up the CPVC casing and toward the
surface to disperse. To ensure that thermal convection was not a source of the dye motion, the
temperature of the water was measured near the antenna. No temperature change was
measured. In order to prove that the effects were not due to small thermal changes and to
prove that the RF fields were controlling the motion, the antenna was bent 90° within the
enclosure.
This horizontally positioned monopole antenna has a completely different radiation
pattern. Since the dye moves downward first and then toward the wall that the antenna is
pointing to (negative Y-direction on Fig. 1(c)) in a pattern following the direction of the
antenna tip, the effects on the diffusion pattern are clearly not the result of thermal effects
10
near the antenna. This test again used 50 W of stimulating power at 154 MHz. Fig. 1(c)
shows the dye diffusing toward the wall the antenna is pointing to (i.e., negative Y-direction
of Fig. 1(c)) with very little dye diffusing to the opposite side. The contrast in the diffusion
patterns shows the clear effect of the EM stimulation and the radiation pattern of the antenna
on the diffusion of the dye in water. The tests were repeated and showed strong consistency.
FIG. 1 — EM-stimulated diffusion for the 40 cm x 40 cm x 40 cm cubic water medium
with: a) vertically polarized CPVC-cased monopole antenna; dye moving toward the
antenna body, b) 9 cm-spaced injection test; dye moving toward the antenna body, c)
horizontally polarized monopole antenna; dye moving toward the wall that the antenna is
pointed toward.
To ensure no mechanism or random factor (e.g., nonlevel injection table) other than the
RF electric field affected the transport, the acrylic box and entire setup, except the EM
source, were rotated 90° around the injection area. The result confirmed that the field
radiation pattern is the dominant factor affecting the transport. The dye would move to the
wall the antenna was pointing to. It was determined that the horizontal antenna oriented
perpendicular to the image provides the best opportunity to quantify the changes in the dye
transport pattern from the EM stimulation. Therefore, from this point on, test results were all
11
obtained with a horizontally positioned antenna. Two tests for each of 50 W, 30 W, 10 W,
and 0 W (unstimulated) of power were performed at a frequency of 154 MHz. Due to the
amplifier’s limited range of operation, the maximum applied power possible is 50W. The
other two applied powers (10 W and 30 W) were selected to provide a uniform trend in the
variation of power.
Imaging- Digital imaging was used to measure dye concentration and analyze the
transport of dye. Images were captured in RAW format at 1-minute time intervals using a
Cannon Rebel T2i, 18-megapixel, digital camera. To ensure consistent and stable images, a
tripod supported the camera, and a remote capture program was used to control and download
images.
To ensure a uniformly lit background, a light box was built along with a diffusive filter.
The light box consists of a wooden frame with three florescent tube lights attached to the
inside and back of the box. To diffuse the light, a layer of white Lycra was stretched to a
plywood backing. To diffuse the light further, a second frame covered with another layer of
white Lycra was used. These two layers of Lycra were spaced approximately 20 cm apart to
diffuse the light enough to reduce drastic backlight gradients to an acceptable level although
not enough to completely eliminate variations in intensity. Fig. 2 represents the structure of
the lighting box and Lycra layers.
The pixel values of the two-dimensional (2D) digital images correlate to the integral of
the dye concentration along the line of sight corresponding to each pixel in the image. The
images are broken down into a matrix, which could be manipulated to determine the
properties of the dye transport.
12
FIG. 2 — The 61.5 cm x 56 cm lighting frame with three horizontal
florescent lights and a cover layer of Lycra as well as the additional 52 cm
x 48.5 cm frame with a layer of Lycra to make the lighting condition
consistent and uniform during the imaging of the experiment.
Calibration- Several of images were captured to calibrate the concentration of the dye.
Before the injection of dye, ten images were taken and averaged to represent the background.
Ten levels of dye concentration were introduced to the medium, with five pictures captured
for each level. Dye was introduced into the water and allowed to reach a fully diffused steady
state. The average background was subtracted from all these images, eliminating the
background for analysis.
Once the background has been subtracted, leaving just the dye in the images, analysis of
the dye can begin. A relationship between each uniform concentration and all pixel values
across each image was established for a range of concentrations, but any pixel values above
the calibrated concentrations were given the maximum calibrated value. Assigning the
maximum calibrated value would affect the results, but mostly during fingering of the dye.
Once the finger has dispersed, the dye will reach a concentration below this maximum value
and be recorded accurately.
Collected images were separated into the three color planes: red, blue, and green. In
calibration, the red and blue spectra are attenuated more with an increase in concentration. In
13
contrast, the green spectrum remains fairly constant. The red and blue channels were used to
measure the dye concentration in the analysis because their intensity better correlated to the
dye concentration as observed visually.
Electric Field Mapping- Mapping the test area to understand the RF radiation pattern and
correlate the RF field to dye concentration and transport is important for understanding the
stimulation mechanisms. Hence, the electric field was mapped in the test region using a threedimensional (3D) computer-controlled translation-table. Due to the existence of the injection
table and the antenna body, areas close to the center of the medium were not accessible for
measurement. As a result, probing the entire cross-sectional (i.e., horizontal XY-slice,
coordinate system is shown in Fig 1(c)) slices were not practical due to the lack of
accessibility. Electric field measurements were performed only for depth (i.e., vertical) slices
fully accessible for probing.
A 50 Ω coaxial probe connected to a spectrum analyzer was moved to nodes of a desired
3D grid. Probing the medium and recording the electric field measurements, a 3D map of the
field was constructed. The triaxial positioner was built using an aluminum frame, stepper
motors, and ball screws for positioning. The mechanical and electrical components were
designed and built in-house. Fig. 3 demonstrates the triaxial antenna positioned (translation
table) as well as the entire setup for the EM stimulation tests.
Results and Discussion
Unstimulated Tests- The diffusion without EM stimulation is essentially horizontally
symmetric around the injection area. The dye descends vertically from the injection table and
spreads in all directions along the bottom of the tank and up the sides. The fact that the
transport of the dye is random but symmetric reveals that the governing mechanism is
14
FIG. 3 — Setup and instruments for EM-stimulation test.
diffusion, as was described previously. Since there is no preferential path of movement in the
horizontal direction (in the vertical direction there is a downward movement due to gravity),
the main factor controlling the transport is the gradient of the concentration.
The results of the digital pixel analysis provide a matrix of concentrations of the dye
corresponding to different nodes in the medium at different times. Fig. 4 shows the locations
of 208 nodes in the test medium where the dye intensity was studied at 130 time intervals.
The nodes are spaced every 2 cm, and images are saved at 1-minute intervals. The result is a
matrix of 208 × 130 dye concentrations for these nodes, where 208 refers to the number of
nodes and 130 refers to the captured times. The same procedure is used for the EMstimulated tests.
EM-stimulated Tests- When the medium is stimulated using a horizontally polarized Lshaped antenna and after the dye descends from the injection table, the diffusing dye moves
toward the negative Y-direction (according to the coordinate system shown in Fig. 4) with
very little dye diffusing toward the positive Y-direction. This effect is in contrast to the
unstimulated case. It is noteworthy that no measurable temperature change was recorded. The
15
diffusion pattern shows the clear effect of the EM stimulation and the radiation pattern of the
antenna on the diffusion of the dye in water.
FIG. 4— Location of the 208 nodes used in digital pixel analysis.
Mapping Electric Field- To fully analyze the EM stimulation and its effects, the radiation
pattern and electric-field intensity within the testing medium were mapped. The field-probing
technique enables reconstruction of a map of the RF (radio frequency) electric field and the
antenna radiation pattern. Fig. 5 shows a schematic of the top view of the location of ten
selected depth (vertical, XZ or YZ plane) slices through the medium. Due to the existence of
the injection table and the antenna body, slices close to the center of the medium were not
accessible for measurement by the probe. The selection of these slices was based on the
accessibility for probing. The height of these depth-slices is 24 cm with the transmitting
antenna centered at 12 cm from the bottom of each image (in the Z direction) to enable
measurements well below and above the antenna projection on these slices. Although the
elevation of the water inside the box was 38 cm, field measurements close to box boundaries
would have been difficult to collect and would not result in accurate digital images due to the
reflections near boundary surfaces. The axes labeled at the tip of the antenna correspond to
16
the probing coordinates. The distance between different slices was designed to enable better
understanding of the electric-field pattern inside the medium. Hence, three XZ depth-slices
were measured at Y= -2 cm, -6 cm, and -10 cm. In addition, 7 YZ depth slices were selected
for the measurement of the electric field parallel to the antenna orientation at X= -12 cm, -8
cm, -4 cm, 0 cm, 8 cm, 12 cm, and 16 cm. The probe is not calibrated to measure the absolute
value of the electric field. Hence, the electric field is normalized to the maximum value
measured within the box.
FIG. 5— Schematic of locations of field mapping on depth (vertical) slices
(top view of setup).
Fig. 6 shows the contour map of the electric-field amplitude on an XZ depth slice at Y= 6 cm, described in Fig. 5. It is clear that the highest intensity of the electric field occurs
toward the bottom of the tank. The same trend was observed on other XZ depth slices as well.
However, as the probing distance from the antenna increases from 2 cm to 6 cm and then to
10 cm, the intensity of the field decreases, but not as much as it would be expected with a
propagating wave. The field intensity with a propagating wave should decrease with the
distance from the antenna, particularly outside the antenna near-field region. However, it can
be observed that a standing wave pattern has formed since the electric-field intensity does not
decrease radially away from the antenna. The Faraday cage creates a cavity region
17
encompassing the water in the measurement box and the air between the box and the cage
walls.
The standing wave is more visible in Fig. 7, which shows an example of a depth slice
between the transmitting antenna and camera (X = -8 cm), as described about Fig. 5. As seen
in Fig. 7, the maximum of the field occurs at the bottom of this slice. A similar trend to XZ
depth slices is seen on other YZ slices. It is noteworthy that in the images behind the antenna,
a pattern more consistent with a standing wave can be seen. This difference between the front
field pattern and the rear one can be attributed to the large opening in the front panel of the
Faraday cage toward the camera. Probing the electric field outside the water medium showed
that there is little electric field outside, which allowed the large opening in the front panel of
the Faraday cage for better digital image-taking. This partial cavity effect of the grounded
cage is one reason behind the inconsistent field pattern of the front and the back of the
medium.
FIG. 6— Normalized electric-field map of
an XZ depth slice at Y= -2 cm. The
antenna projection is also shown in the
map.
FIG. 7— Normalized electric-field map
of an YZ depth slice parallel to the
horizontal antenna at X= -8 cm. The
zones used in the analytical study and the
antenna projection are also shown on the
map.
18
FIG. 8— Zones 1 through 5 used for image analysis.
Evaluation of Dye Concentration- A MATLAB program was written to compare and
quantify the relative dye movement indirectly measured from digital images. The program
integrates the amount of dye in different zones of the medium on digital images. The zones
used for comparison to determine the effect of the EM stimulation are shown in Fig. 8. Table
1 lists the coordinates of the zones used in this study. Zones 1 and 5 were excluded from the
study due to the reflection from the boundary walls of the acrylic box at the imaging angle.
Zone 3 encompasses the middle of the medium where the antenna and the injection table are
located. The concentration data of this zone might not reflect the exact values due to the fact
that the dye behind the antenna and injection table does not show up on images. The rest of
the medium consists of two zones (Zone 2 and 4, respectively) on the left and right sides of
the middle zone. These two zones have the same width and length and are the zones where
Table 1- Coordinates of the zones used in the digital analysis
Start point
End Point
Y-Coordinate, cm
Y-Coordinate, cm
1
0
2
2
2
10.5
3
10.5
27.5
4
27.5
36
5
36
38
Zone
19
the study of the concentration can be performed easily with minimum interference. Because
the dye moved toward the negative Y-direction during EM stimulation and had no apparent
preferred path during unstimulated tests, Zones 2 and 4 are compared and contrasted.
EM Stimulation Effects- The average dye concentrations from Zones 2 and 4 are plotted
in Fig. 9. The figures show the concentration from the blue channels of the digital images.
The same trend was also seen for the red channels. In Fig. 9, the Zone 2 concentration, 50WZ2R1, diverges from the Zone 4 concentration, 50W-Z4R1. 50W-Z4R1 is a notation for the
50-Watt EM-stimulated test in Zone 4 (as shown in Fig. 8), and 1 indicates that this was the
first run out of 2 duplicate diffusion tests performed. The same trend is also observed when
plotting the same figure for 30-Watt and 10-Watt cases.
FIG. 9 — Zones 4 and 2, diverging concentrations under 50 Watt of EM
stimulation, first run. The spike indicates the start of stimulation.
In Fig. 9, tests start unstimulated, and then EM stimulation starts instantly. The spike seen
in the Zone 2 plot is merely a mark to show the beginning of EM stimulation. In order to
investigate the effect of EM stimulation, the same plot is shown for the unstimulated test (Fig.
10). The dye intensity in the 0-Watt (i.e., unstimulated) case shows that the dye concentration
in Zones 2 and 4 rise together, diverging little other than temporarily due to fingering. The
slope of the plot of concentration versus time is representative of the rate of increase in the
20
FIG. 10 — Zones 4 and 2, diverging concentrations in unstimulated test,
first run.
concentration in a given zone and, therefore, is proportional to the rate of transport (flux) of
the dye in that direction. The difference in the transport flux rates between Zones 2 and 4 can
be approximated by subtracting the slope of the best-fitting line of each plot for the two
zones. Using a linear trend-line function, the slope of the concentration plot is computed. The
slope is only measured after the 60th minute mark to eliminate the artificial marking spike
indicating the start of EM stimulation and the long flat portion of the curve associated with
the unstimulated period. The measured differences in the flux of dye entering Zone 4 from
the flux entering Zone 2 are consistent for each power but do not have a trend from which a
relationship between the differences in diffusion rate and power can be determined.
Differences in slope are shown in Fig. 11 and graphed versus the input RF power.
FIG. 11 — Concentration rate difference between Zone 4 and Zone 2 for
all runs and all powers.
21
Analysis of individual concentration rates of Zones 2 and 4 gives a better understanding
of the cause of the difference between the two zones. Fig. 12 shows that the rate of dye
transport into Zone 4 is fairly consistent for all the tests at different power levels. This is
somewhat inconsistant with what is qualitatively seen in Fig.11 but could be explained by the
rate of dye transport into Zone 2, shown in Fig. 13. This analysis clearly indicates that it is
not the rate (flux) of dye entering Zone 4 that has changed, but rather the rate (flux) of dye
entering Zone 2 has been retarded by the EM stimulation.
FIG. 12 — Rate (a representative of flux) of dye entering Zone 4 based on
slopes of concentrations.
FIG. 13 — Rate (a representative of flux) of dye entering Zone 2 based on
slopes of concentrations.
22
Because the dye used in the experiment is nonreactive and electrically neutral, the most
potentially
controlling
phenomenon
for
the
dye
transport
mechanism
can
be
dielectrophoresis. As expected for most matters, the dye used in this experiment has a
dielectric constant less than that of water (80). Hence, if dielectrophoretic forces are present,
they should be in the direction of the negative gradient of the electric field squared (i.e.,
toward regions of lower electric field and EM power density). However, since the electric
field does not provide an organized (i.e., monotonic in one direction) pattern, investigation of
dielectrophoresis is not feasible, especially in the Y-direction. Although in the Z-direction,
the larger values of the electric field are mostly at the bottom of the box, the interference of
the gravity forces does not allow a dielectrophoretic study in this direction. A more organized
electric field pattern (e.g., a monotonically changing electric field pattern) is required for
evaluation of dielectrophoretic effects. This requires a modified setup to provide the
described electric field pattern.
Future Research
To analyze the results in a more quantitative manner as well as a validated simulation of
the electric field, some aspects of the experimental procedure can be ameliorated. The most
important modification is to develop a more organized electric-field pattern that has a
minimum or maximum at the center of the acrylic box and monotonically changes outward
from the center. This would enable the study of dielectrophoresis. An alternative can be to
replace the Faraday cage with a resonant cavity structure. Hence, the L-shaped horizontally
polarized transmitting antenna can be replaced with a short, vertically polarized, grounded
coaxial antenna to stimulate the medium for obtaining a specific TE/TM-mode configuration
as a more recognizable field pattern.
23
There can be some improvements in injection methods as well. In order to abate fingers
for a more feasible quantitative study of diffusion, the dye can be injected into a small cup,
covered with a paper filter, placed on the injection table. The new injection method would
allow diffusion to occur with less or even no finger formation.
Conclusion
The results from EM-stimulation tests prove that EM waves can alter and direct dye
transport without any measureable thermal effect. The antenna design is a key to the transport
mechanism. For the horizontally polarized L-shaped antenna used in this experiment, the dye
is transported toward the direction the antenna is pointing (negetive Y-direction) after
descending out of the injection table. Since there was no temperature gradient measured in
the medium, the transport mechanism is expected to have been mostly controlled by the EM
stimulation only, not a strong convective flow. The movement of the dye away from the heat
source (the antenna body), proves the nonconvective flow as well.
Results show that unlike unstimulated tests, after EM stimulation, the rate at which the
dye moves toward Zone 4 (negetive Y-direction) is different from that toward Zone 2
(positive Y-direction). This difference varies with different amplitudes of input power to the
system. However, finding the correlation requires a better antenna configuration to obtain a
more organized (monotonically varying) electric-field pattern in the medium. There is a
considerable retardation in dye transport toward Zone 2 (positive Y-direction) of the box due
to the radiation pattern used. However, the results correlated to either the electric-field
amplitude or power were not fully conclusive. Drawing a conclusion requires more testing.
The resulting data from this work clearly demonstrate that the use of EM stimulation can
alter and affect the transport of the dye in the water. Since diffusion and air channel
24
formation are the main mechanisms behind remediation using air sparging, air sparging can
potentially be controlled and enhanced by the use of EM waves. The ability to create a
relationship between the EM-stimulation power (or electric field) and transport rates requires
more analysis. This work is a good basis for continuing research into the mechanism
responsible for the effects observed in this study.
Acknowledgements
This project was supported by the National Science Foundation through the
Interdisciplinary Research (IDR) program, CBET Award No. 0928703. The authors would
like to express appreciation toward the Boise State University Instrumentation office for the
fabrication and machining support of the experimental setup.
References
Arulanandan, K., and Arulanandan, S., 1985, “Dielectric methods and apparatus for in-situ
prediction of porosity, specific surface area (i.e., soil type) and for detection of
hydrocarbon, hazardous waste materials, and the degree of melting ice to predict in
situ stress-strain behavior,” Patent Application, Serial No. 709,592, Regents of Univ.
of California.
Arulanandan, K., 1991, “Dielectric method for prediction of porosity of saturated soil,”
Journal of geotechnical engineering, Volume 117, Issue 2, pp. 319-330.
Braida, W., and Ong, S. K., 2001, “Air Sparging Effectiveness: Laboratory Characterization
of Air-Channel Mass Transfer Zone for VOC Volatilization,” Journal of Hazardous
Materials, B87, pp. 241-258.
Hinchee, R. E., 1994, “Air Sparging for Site Remediation,” ISBN: 1566700841 / 1-56670084-1, Lewis Publishers, Boca Raton, FL.
25
Chao, K. P., Ong, S. K., and Protopapas, A., 1998, “Water-to-Air mass Transfer of VOCs:
Laboratory-Scale, Air-Sparging System,” Journal of Environmental Engineering,
124, No. 11, pp. 1054-1060.
Cussler, E. L., 1997, Diffusion, Mass Transfer in Fluid Systems, Second Edition, Cambridge
University Press, United Kingdom.
Jamieson, K. S., ApSimon, H. M., and Bell, J. N. B., 2007, “Electrostatics in the
Environment: How They May Affect Health and Productivity,” Physics: Conference
Series, pp. 1-7.
Ji, W., Dahamani, A., Ahlfeeld, D. P., and Hill, E., 1993, “Laboratory Study of Air
Sparging: Air Flow Visualization,” Journal of Groundwater Monitoring and
Remediation, Vol. 13, No. 4, pp. 115-126.
Johnson, R. L., Johnson, P. C., McWhorter, D. B., Hinchee, R. E., and Goodman, I., 1998,
“An Overview of In situ Air Sparging,” Journal of Groundwater Monitoring and
Remediation, Vol. 13, Issue 4, pp. 127-135.
Jones, T. B., 2003, “Basic Theory of Dielectrophoresis and Electrorotation,” Journal of
IEEE Engineering in Medicine and Biology, 0739-5175, 22 (6), p. 33.
Pohl, H. A., and, Crane, J. S., 1971, “Dielectrophoresis of Cells,” Journal of Biophysical,
Vol. 11, pp. 711-727.
Pohl, H. A., 1978, Dielectrophoresis, Cambridge University, Cambridge, UK.
Hilhorst, M. A., Dielectric Characterization of Soil, Doctoral Dissertation, Wageningen
Agricultural University, Wageningen, The Netherlands, 1998
United States Environmental Protection Agency (U.S. EPA), “Underground Storage Tanks:
Technical Requirement Federal Register,” 53:37082, September 1988.
26
Vermeulen, F., and McGee, B., 2000, “In Situ Electromagnetic Heating for Hydrocarbon
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27
Table 1- Coordinates of the zones used in the digital analysis
Zone
Start point
End Point
Y-Coordinate, cm
Y-Coordinate, cm
1
0
2
2
2
10.5
3
10.5
27.5
4
27.5
36
5
36
38
Figure Captions
FIG. 1 — EM-stimulated diffusion for the 40 cm x 40 cm x 40 cm cubic water medium
with: a) vertically polarized CPVC-cased monopole antenna; dye moving toward the
antenna body, b) 9 cm-spaced injection test; dye moving toward the antenna body, c)
horizontally polarized monopole antenna; dye moving toward the wall that the antenna is
pointed toward.
FIG. 2 — The 61.5 cm x 56 cm lighting frame with three horizontal florescent lights
and a cover layer of Lycra as well as the additional 52 cm x 48.5 cm frame with a layer of
Lycra to make the lighting condition consistent and uniform during the imaging of the
experiment.
FIG. 3 — Setup and instruments for EM-stimulation test.
FIG. 4— Location of the 208 nodes used in digital pixel analysis.
FIG. 5— Schematic of locations of field mapping on depth (vertical) slices (top view
of setup).
FIG. 6— Normalized electric-field map of an XZ depth slice at Y= -2 cm.
28
FIG. 7— Normalized electric-field map of an YZ depth slice parallel to the horizontal
antenna at X= -8 cm. The zones used in the analytical study and the antenna projection
are also shown on the map.
FIG. 8— Zones 1 through 5 used for image analysis.
FIG. 9 — Zones 4 and 2, diverging concentrations under 50 Watt of EM stimulation,
first run. The spike indicates the start of stimulation.
FIG. 10 — Zones 4 and 2, diverging concentrations in unstimulated test, first run.
FIG. 11 — Concentration rate difference between Zone 4 and Zone 2 for all runs
and all powers.
FIG. 12 — Rate (a representative of flux) of dye entering Zone 4 based on slopes of
concentrations.
FIG. 13 — Rate (a representative of flux) of dye entering Zone 2 based on slopes of
concentrations.
29