Robert D. Green and Jerry G. Myers

Glenn Research Center, Cleveland, Ohio

Bonnie L. Hansen

Parks College of Saint Louis University, Saint Louis, Missouri

Preliminary Results of a Microgravity

Investigation to Measure Net Charge

on Granular Materials

NASA/TM—2003-212109

January 2003

AIAA–2003–1304
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Robert D. Green and Jerry G. Myers

Glenn Research Center, Cleveland, Ohio

Bonnie L. Hansen

Parks College of Saint Louis University, Saint Louis, Missouri

Preliminary Results of a Microgravity

Investigation to Measure Net Charge

on Granular Materials

NASA/TM—2003-212109

January 2003

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

41st Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 6–9, 2003

AIAA–2003–1304
Acknowledgments

The following people at NASA Glenn Research Center are to thank for their help in the design, construction, and

analysis aspects of the prototype rig: Glenda Yee, Leonard Gumennik, Art Birchenough, Juan Agui, Masami

Nakagawa, Dick Delombard, and Eric Bauman. Also thank you to the Office of Physical and Biological Research

(OPBR) for funding this effort.

Available from

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Hanover, MD 21076

National Technical Information Service

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Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

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Aeronautics and Space Administration.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TMó2003-212109 1 

PRELIMINARY RESULTS OF A MICROGRAVITY INVESTIGATION 

TO MEASURE NET CHARGE ON GRANULAR MATERIALS 

Robert D. Green and Jerry G. Myers 

National Aeronautics and Space Administration 

Glenn Research Center 

Microgravity Science Division 

Cleveland, Ohio 44135 

Email: Robert.D.Green@nasa.gov

Email: Jerry.G.Myers@nasa.gov

Bonnie L. Hansen 

Parks College of Saint Louis University 

Saint Louis, Missouri 63108 

Email: hansenbl@slu.edu

ABSTRACT

Accurate characterization of the electrostatic 

charge on granular materials has typically been 

limited to materials with diameters on the order of 

10 μm and below due to high settling velocities of 

larger particles. High settling velocities limit both 

the time and the acceptable uncertainty with which 

a measurement can be made. A prototype device 

has been developed at NASA Glenn Research 

Center (GRC) to measure coulombic charge on 

individual particles of granular materials that are 

50 to 500 microns in diameter. This device, a novel 

extension of Millikanís classic oil drop experiment, 

utilizes the NASA GRC 2.2 second drop tower to 

extend the range of electrostatic charge 

measurements to accommodate moderate size 

granular materials. A dielectric material with a 

nominal grain diameter between 106 and 

250 microns was tribocharged using a dry gas jet, 

suspended in a 5×10×10 cm enclosure during a 

2.2 second period of microgravity and exposed to a 

known electric field. The response was recorded on 

video and post processed to allow tracking of 

individual particles. By determining the particle 

trajectory and velocity, estimates of the coulombic 

charge were made. Over 30 drops were performed 

using this technique and the analysis showed that 

first order approximations of coulombic charge 

could successfully be obtained, with the mean 

charge of 3.4E-14 coulombs measured for F-75 

Ottawa quartz sand. Additionally, the measured 

charge showed a near-Gaussian distribution, with a 

standard deviation of 2.14E-14 Coulombs. 

INTRODUCTION

That an investigation of electrostatic charge on 

granular materials is of research interest is in some 

sense, surprising. Even though frictional charging 

of materials has been well known for over 

3000 years, we still do not understand its causes.1

Hence our motivation for a preliminary study to 

measure charge on granular type materials. 

A number of industrial and aerospace 

applications require a better understanding of the 

electrostatic properties of granular and aerosol 

particles. Electrostatic paint systems, inkjet and 

laser jet printing systems and the Xeroxô process 

used in photocopiers require precise control of 

electrostatically charged solid or liquid particles. 

Design of efficient, i.e. low pressure drop, air 

filtration systems involve understanding the 

electrostatic properties of dust in a pneumatic 

driven flow system; an example is 3Mís Filtreteô 

product line of filters for residential heating and air 

conditioning applications which utilizes 

electrostatic-charged fibers to achieve filtration 

down to the 0.3 micron size level.2

 The 

pharmaceutical industry routinely designs and 

operates equipment that transports and handles 

granular media in the form of fine powders and 

tablets. In the agricultural industry, application of 

pesticides involves flow of granular media in fine 

powder and liquid droplet forms and adherence to 

the target crop involves electrostatic forces 

combined with other adhesion forces. 

Several potential NASA missions, in 

particular, planetary missions to either the Moon or 

Mars, require a fundamental understanding of 

granular flow and the potential electrostatic effects 

on these flows. A number of aerospace applications 

requiring granular flows support technological 

developments for the Space Science (SSE) and 

Human Exploration and Development of Space 

(HEDS) Enterprises. Reducing or mitigating dust 

contamination of space suits, equipment such as 

solar arrays, and crew habitat will be necessary for 

a manned mission to the Moon or Mars. A number 

of ISRU (In-Situ Resource Utilization) applications 
NASA/TMó2003-212109 2 

to support manned or sample return planetary 

missions involve handling and processing of 

granular materials. Chemical processing of 

planetary atmospheres, such as oxygen production 

from the carbon dioxide in the Martian atmosphere, 

will require efficient filtration of very fine 

atmospheric dust. For utilization of the planetary 

regolith, a whole host of processes will require 

direct handling of granular media. 

A number of techniques have been used to 

measure electrostatic charge on particles. Flagan3

provides an extensive review of electrical 

measurements on aerosols, focusing on the Faraday 

cup electrometer and mobility analyzers used to 

measure charge on atmospheric gas ions and very 

small aerosol particles. Cross4

 discusses the 

Faraday cup, Millikan method, deflection methods, 

and laser Doppler anemometry for measuring 

charge and mobility of individual aerosol or dust 

(solid) particles. Of these, the Millikan method 

provides the ability to measure charge on 

individual particles along with sampling 

statistically significant populations in a relatively 

short time. Millikan5

 initially developed this 

technique with silicone oil drops to first measure 

the magnitude of the electron charge, a 

fundamental physical constant. Hopper and Laby6

modified the technique, using a horizontal 

(perpendicular to gravity) instead of vertical 

electric field to improve charge measurement 

accuracy. Kunkel7,8 and Kunkel and Hansen9

measured the charge levels on quartz dust and 

ammonium chloride aerosol particles up to 

10 microns in diameter using an apparatus similar 

to Hopper and Labyís. McDonald et al.10 utilized 

Millikan cells to measure charge on fly ash 

particles (0.3 to 1.5 μm diameter) after they were 

passed through a laboratory precipitator. Cross and 

Peterson11 performed similar work on brown coal 

fly ash particles (up to 8 μm) and reported 

consistency in measurements of their rotatable 

Millikan cell with measurements in a Faraday cup. 

More recently, Polat et al.12 measured charge on 

spray droplets (~50 to 125 μm) of distilled water 

and aqueous surfactant solutions using a horizontal 

electrical field Millikan cell. 

In all this work, the particle sizes were limited 

to 10 μm for quartz sand and similar geological 

type materials (with densities ~2.5 gm/cm3

) or up 

to 100 μm for lower density aerosol particles like 

water droplets. The factor limiting this particle size 

is the gravity force which shortens the residence 

time the particle spends in a horizontal electrical 

field (generated by vertical electrode plates). This 

limits the horizontal deflection the particle 

experiences, which is necessary to make an 

accurate charge measurement. 

In this work, we illustrate a method to extend 

the maximum size of particles measured from 10 to 

100 to 300 microns in diameter. It is also shown 

that charge measurements can be made 

simultaneously on a large number of particles 

during a single experiment, allowing the range of 

population charge levels to be obtained, while 

minimizing the effects of rapid charge decay. This 

data is necessary to assess granular medium 

aggregation phenomena in fractional gravity 

environments where charge forces on larger 

particles begin to approach gravity forces in 

magnitude. 

METHODS

Background Theory

The method utilized in this technique is based 

on the balance between forces exerted on a particle, 

similar to the approach taken in Millikanís classic 

oil drop experiment. In this case, the forces under 

consideration are the electrostatic forces and the 

aerodynamic drag forces exerted on the particle. 

In the absence of gravitational and other 

external forces, a particle with coulombic charge q, 

positioned in a uniform electric field of strength E, 

will be acted on by an electrostatic force that is 

parallel to the electric field and equivalent to 

FE =q E (1) 

When suspended in air, particle movement will 

result in an aerodynamic drag force in the opposite 

direction of the particleís trajectory. If the particle 

is spherical and its velocity is slow, i.e., in the 

creeping flow (Stokes flow) regime, the drag force 

obeys Stokes Law and is equivalent to 

V D FD

2 =6π μ (2) 

where μ is the dynamic viscosity of air, D is the 

diameter of the particle and V is the steady state 

(terminal) velocity of the particle. It is noted at this 

time that the added mass and integral history 

components of the drag force and any inertial lift 

effects are neglected. 

Under steady state conditions, these forces 

should balance, i.e. the particle should achieve a 

steady state velocity that is parallel to the electric 

field when the electrostatic force on the particle 

balances the viscous drag force on the particle. 
NASA/TMó2003-212109 3 

In other words, steady state is achieved when 

FE = FD or 

V Dq E

2 =6π μ (3) 

This balance allows the columbic charge on the 

particle to be calculated as 

E

V D

q 2

6π μ

= (4) 

provided the particleís diameter and steady state 

velocity parallel to a known electric field can be 

determined. It is noted that this analysis also 

applies to a particle with a non-zero settling 

velocity (perpendicular to the electric field), as 

long as the flow around the particle remains in the 

Stokes regime. 

The application of this approach requires that 

sufficient time exist for the particle to reach 

terminal velocity. Using Newtonís second law, it 

can be shown that the instantaneous particle 

velocity discussed above is given by the equation 

( ) [ ]

[ ]t t

E t t

V e v e

e v e D

F

v t

β β

β β

π μ

− −

− −

= − +

= − +

0

0

1

1

2 6 (5) 

where v0 represents the initial particle velocity at 

time t = 0 and β is the inverse of the characteristic 

time constant (τ) defined as 

2

1 18

ρ D

μ

τ

β = = (6) 

where ρ is the particleís density. Setting v(t) = 

0.99V, v0 = 0 and solving for t, a conservative 

estimate for the time required for the particle to 

reach 99% of the terminal velocity is given by the 

equation 

τ

β = =4.6052⋅ 4.6052 99 t (7) 

Therefore, a particle that exhibits a characteristic 

time less than 0.48 seconds should reach 99% of its 

terminal velocity within 2.2 seconds, the duration 

of the microgravity period in the NASA GRC drop 

tower. 

Experimental Apparatus

For this study, a Millikan cell chamber 

(internal dimensions 5×10×10 cm) was constructed 

from polycarbonate and nylon (see Figures 1 and 

2). Two parallel electrodes consisted of 10×10 cm 

copper plates placed 5 cm apart. An E-field 

generator (or high voltage power supply) was built 

in-house from a series of DC-DC converters (Pico 

Electronics, Inc., Pelham, New York) to provide a 

voltage range of 50 to 2900 V. Power was provided 

by a 28 V power supply on the drop rig. Above 

the cell, a tribocharging/hopper chamber was 

fabricated consisting of a cylindrical polycarbonate 

chamber with a nylon insert sloped like a chute to 

funnel the material sample into the Millikan 

(or E-field) chamber. Frictional charging 

(or tribocharging) was accomplished with a jet of 

dry air provided through a 0.25 in. stainless tube 

penetrating the center of the hopper chamber lid. 

This air jet was actuated with a solenoid valve and 

regulated from 5 to 20 psig with a pressure 

regulator. The dry air gas was stored on-board the 

drop rig at a pressure of 100 psig in two 300 cc gas 

cylinders. 

Sample Preparation

The sample granular material chosen for this 

work was F-75 Ottawa quartz sand (US Silica 

Corporation, Ottawa, Illinois), which is a wellcharacterized material commonly used in granular 

media research. The bulk sand was sieved in an 

ATM Model L3P sonic sifter (ATM Corporation, 

Milwaukee, Wisconsin) to produce the appropriate 

size ranges to be investigated. This study utilized a 

106 to 250 μm size and a 250 to 300 μm particulate 

size, for which the characteristic time constants (τ) 

are 0.25 and 0.58 seconds respectively. After 

sifting, the samples were washed in a dilute 

solution of de-ionized water and Micro-90Æ

detergent (International Products Corporation, 

Burlington, New Jersey), rinsed in deionized water, 

and dried in a conventional oven to remove any 

fines or metallic particles left over from grinding 

processes. Samples were stored in a desiccator and 

kept in tightly closed containers to minimize 

moisture pickup from the ambient air. 

Experimental Procedure

The silica samples were loaded into the 

hopper/tribocharger chamber (see Figure 2) prior to 

the drop rig being installed in its drag shield. The 

room temperature and humidity was monitored 

with a handheld sensor. Relative humidity ranged 

from 43 to 62%. One to 3 minutes prior to the drop, 
NASA/TMó2003-212109 4 

the sample was agitated with a dry air jet at 5 to 

20 psig for 30 to 60 seconds to facilitate 

tribocharging (i.e. frictional charging). A few 

seconds prior to the drop, a solenoid actuated door 

was opened to allow a stream of sample particles to 

flow into the lower E-field test chamber 

(see Figure 2). The E-field power supply was 

activated 0.2 to 0.5 seconds after drop initiation 

(i.e. start of microgravity period) to allow particles 

to decelerate in the y direction in the absence of the 

gravity force. Particles were imaged using a 

30 frames/sec Pulnix model TMC-7DSP video 

camera (PULNiX America, Sunnyvale, California) 

fitted with a Schneider 12.5 to 75 mm zoom lens. 

After acquisition, the analog image data was 

converted to digital AVI files using VirtualDub,*

 a 

video capture software program. Particle 

displacements vs. time were extracted from the 

digital video images using Spotlight,Ü

 an in-house 

image tracking software program. 

RESULTS

Approximately 30 drops were performed to 

test the prototype device and attempt to measure 

charge. Of these, 11 were deemed unsuitable for 

analysis due to mechanical problems with the 

apparatus or procedural errors. Of the remaining 

19 successful drops, 12 were analyzed to determine 

appropriate particle tracking approaches, to 

estimate charge on individual particles and to test 

for sample charge distribution. The remaining 

7 drops have not been analyzed to date. The 

experimental conditions pertaining to each of the 

analyzed drops are presented in Table 1 and 

include the particle size, granular sample volume, 

the delay from the time the drop was initiated to 

the time the E-field was activated, and the 

tribocharging period prior to the drop. Generally a 

random sample of between 15 and 20 particles was 

analyzed for each experiment. 

Figure 3 illustrates a typical sequence of 

events observed prior to and during the 

microgravity drop procedure. Figure 3(a) shows 

that just prior to the drop, a column of particles is 

formed as the particle sample flows from the 

hopper under the influence of gravity. Once the 

drop has initiated, flow from the hopper stops, the 

column of particles slows noticeably and the 

column begins to elongate as the particles with 

higher momentum continue moving downward. By 

the time of E-field activation (0.3 seconds after 

drop initiation), the particle column has stretched 

substantially as illustrated by comparing in 

 

* See http://www.virtualdub.org/ 

Ü See http://microgravity.grc.nasa.gov/spotlight/

Figure 3(b) and (c). After E-field activation, the 

particle column disperses rapidly in the lateral 

direction, as particles with various charges move in 

response to the applied electric field (Figure 3(d)). 

This continues throughout the drop (Figures 3(e) to 

3(h)) with particles moving to both the left and 

right of the initial column position. Many particles 

can be seen to have impacted and adhered to one of 

the charged electrodes on the right side of 

Figure 3(h). This sequence of events was seen in 

all the successful drops where an E-field was 

applied. 

Qualitatively, Figure 3 illustrates that the 

particles exhibit electrostatic charge and they 

respond as expected to the applied E-field. A more 

quantitative analysis was obtained by examination 

of individual particle trajectories. As a baseline, 

one drop was performed without tribocharging the 

sample and with no applied E-field (Drop #5). A 

typical particle trajectory from this drop is shown 

in Figure 4, where the x (horizontal) and y 

(vertical) displacement of the particle is plotted 

versus time (time measured relative to the time 

during the drop the first position measurement was 

obtained). Both displacements illustrate continued 

movement in both directions during the drop. 

Although the displacement in the x direction is 

fairly small, ideally, no x displacement would be 

seen. However, because no E-field was applied, 

only particles that had been sheared away from the 

column due to viscous forces could be tracked. 

Once an E-field is applied, as in drop #7 

(E-field of 100V/cm, Figure 5(a)), many particles 

respond to the induced force. Displacements of 

these particles are in some cases similar to those 

seen for no applied E-field. However, many more 

particles can be accurately tracked because of the 

break up of the particle column. The terminal 

velocity of individual particles was obtained by 

performing a linear regression on the last 0.5 to 

1 sec of the particleís measured x displacement. 

Regressions that produced correlation coefficients 

(R2

) values in excess of 0.99 were assumed to 

represent a steady state velocity for the particle. 

The solid line in Figure 5(a) illustrates charge 

calculations determined in this manner for drop #7. 

Figure 5(b) shows the charge distribution of all 

particles measured from drop #7 (no 

tribocharging). When tribocharging is undertaken 

(as in drop #8) the typical x displacement of the 

particles increases slightly (Figure 6(a)), and, as 

expected, there is an observed increase in the 

charge level distribution across the particle 

population (Figure 6(b)). This indicates that the 

action of tribocharging the material sample before 

the drop does increase the charge level in the 
NASA/TMó2003-212109 5 

sample population. Additionally, these results 

indicate that even if the charge tends to dissipate 

after tribocharging, a residual charge remains on 

the particles for an extended period (seconds to 

minutes). It is noted that the total velocity (from 

both x and y components) was used to calculate the 

Reynolds number based on estimated particle 

diameter throughout the particles trajectory. 

Generally, it was found that Re < 1 for the 

experimental conditions used in this study. 

Increasing the E-field strength effectively 

increases the typical particle x deflection and 

increases the spread of measured charge, as was 

expected (Figures 7 and 8). Although using both 

200 and 400 V/cm E-fields produced broader 

charge histograms, the most significant charge 

level remained in the region of 3E-14 coulombs. 

This indicates a good reproducibility for the system 

and of the methods employed to produce and 

measure charge. Examination of the measured 

charge from all drops (Figure 9) illustrates a mean 

charge of 3.4E-14 coulombs (standard deviation of 

2.14E-14 coulombs) and an overall distribution 

similar in shape to those found from the higher Efield experiments. 

DISCUSSION

Previous reports of charge measurements on 

particulate media have utilized a number of 

different techniques to initially charge and measure 

the charge distribution. Kunkel,7

 by a controlled 

release of dust particles (1 to 10 μm) into a known 

electric field, obtained Gaussian distributions of 

charge level and measured a mean net charge of 

250 electron units (4.0E-17 C) on 5 μm diameter 

quartz sand. McDonald et al.10 measured levels an 

order of magnitude higher than this for 2 μm 

diameter fly ash particles, but these particles were 

charged using a corona device. In some recent 

work, Sickenfoose et al.13 have observed a 

Gaussian distribution of charge levels for JSC-1 

dust simulant material with mean charge of 

6E-15 C for 90-106 μm size particles. Sternovsky 

et al.14 have measured charge levels of 5-6E-14 C 

for 100 to 120 μm diameters of this same material; 

charge measurements for both of these studies were 

made with a Faraday cup. Unfortunately, direct 

comparison of the current effort and those of 

previously published works is difficult because of 

different charging mechanisms, different materials, 

and typically smaller particle sizes than those 

examined in this study. Regardless of these 

limitations, the results shown for charge 

distribution are very similar (near-Gaussian) to 

those reported by other researchers. Charge 

magnitudes measured in this study are several 

orders of magnitude higher than reported in the 

literature for smaller particles. However, they 

match closely with charges measured on particles 

of similar size, but of different materials. Further 

characterization of other materials with tighter size 

distributions should provide more insight into the 

comparability of charge measurements with this 

method and those mentioned above. 

Each of the individual particle traces sampled 

in this experiment exhibited similar characteristics. 

Generally, the 2 sec microgravity period allows 

larger particles to slow considerably in the vertical 

direction, therefore, remaining exposed to the 

E-field for a longer period of time. As expected, 

this allows the particles to experience larger 

horizontal displacements than is feasible in 

horizontal E-field Millikan cells under normal 

gravity conditions. Also, the particle distribution in 

the y direction is reasonably dispersed, due to the 

stretching of the initial particle column, which 

minimizes particle-to-particle interactions that may 

interfere with the critical x-displacement tracking 

necessary for an accurate charge measurement. A 

preliminary error analysis of the charge calculation 

procedure indicates that variation in particle 

diameter is the largest contributor, with uncertainty 

in E-field magnitude and particle terminal velocity 

making much smaller contributions (<10% of total 

error). The first can be improved with better 

material preparation and selection, while the 

second may be merely an overly conservative 

estimate of the precision of the home-built E-field 

power supply. Overall, the total uncertainty of a 

charge measurement based on this approach is 

expected to be less than ±1.3E-14 C. 

A significant strength of this system appears to 

be the ability to randomly sample a number of 

particles in the same experiment simultaneously, 

which illustrates that charge level in the population 

is a significant parameter. With variations of as 

much as two orders of charge magnitude in the 

same experiment, the frictional charging approach 

used in this application may not be well 

characterized, although near-Gaussian distributions 

were evident for each individual experiment. 

Additionally, rapid charge decay may play a role in 

this large variation, but this has not been verified. 

In duplicating an individual experiment with a 

higher E-field, we found that we could obtain a 

better (i.e. broader) representation of the charge 

distribution, due to lower charged particles 

experiencing a measurable x-displacement they 

would not undergo at lower E-field settings; the 

data collected enhanced the Gaussian distribution 

on the lower end. This demonstrates that a large 

range of charge distributions can be adequately 
NASA/TMó2003-212109 6 

measured by varying the E-field setting and also 

illustrates the reproducibility. 

Although it is believed that electrostatic charge 

was successfully measured on individual grains of 

material, there are several limitations to the current 

prototype device and the employed methodology. 

The current approach assumes that the particle does 

not exceed the Stokes criteria (i.e. Re <<1) and that 

the fluid effects of ìadded massî and integral 

particle history can be neglected. It is expected that 

neglecting these effects will result in, at most, a 

~10% error on the charge measurement. However, 

if the total particle velocity is such that the Stokes 

regime is exceeded, estimates of the coulombic 

charge can still be made, but more complicated 

drag force calculations are required. Further, the 

requirement that a particle must reach a terminal 

velocity before a charge level can be estimated, 

severely limits the number and the type of particles 

that can be analyzed. It is believed that by 

including the particle acceleration and drag history 

in the derivation of particle displacement, a 

suitable estimate of the charge can be determined. 

The design of the prototype apparatus also limits 

application to certain size particles, primarily due 

to imaging limitations. Based on the current 

characterization, it is believed that refinements to 

the field of view and camera resolution will 

address these concerns. Finally, the sign of the 

particle charge was not considered in this 

preliminary analysis. Generally, particles attracted 

to the positively charged electrode were analyzed 

(i.e. negatively charged particles). However, 

qualitatively speaking, video images indicate that 

roughly a 50/50 split in charge sign is exhibited. 

This will be more quantitatively investigated in 

future efforts. 

CONCLUSIONS

A newly developed prototype device that uses 

a novel extension of Millikanís classic oil drop 

experiment in combination with brief periods of 

microgravity, illustrates that electrostatic charge 

measurements are possible on moderately sized 

(>100 microns) dielectric particles. Using this 

device, the population charge distribution for F-75 

Ottawa quartz was shown to be near Gaussian, with 

a mean charge level of 3.4E-14 C and standard 

deviation of 2.14E-14 C, which compared well 

with measurements reported in the literature for 

particles of similar size, but differing materials. 

When tribocharging, or frictional charging, of the 

particle population was used prior to making 

charge measurements, the charge level was found 

to increase and the population charge distribution 

grew. While the prototype device and methods 

currently have a narrow range of application (with 

regard to particle size, particle type, etc.), many of 

the procedures utilized are well-established in 

microgravity fluid physics research, such as 

methodologies for particle tracking and data 

reduction. Future efforts will seek to extend the 

range of particle diameters that can be measured, to 

diameters larger than those dictated by the terminal 

velocity requirement. Because a particle that 

continues accelerating throughout the drop period 

(2.2 seconds) should have a unique particle 

response to the E-field, we are confident that we 

can measure charge on particles up to 500 μm in 

diameter with densities near the density of quartz 

or glass (2.5 gm/cc), and 800+ μm for materials 

with lower densities like polycarbonate. More 

diverse particle types and better characterization of 

the particle diameters are also needed. Use of 

commercially available glass beads, better sieving 

techniques, image based particle sizing techniques, 

and use of the particle deceleration in the 

y-direction (to estimate size), would allow better 

individual particle classification. A wider range of 

materials such as hematite, a significant component 

in Mars regolith, or one or more of the so-called 

Mars or Lunar simulant soils prepared by NASA 

JSC, are also under consideration for testing. 

Finally we would like to take a closer look at 

improving the precision of our measurements by 

reducing our major sources of error, particle 

diameter (using the aforementioned techniques) 

and the uncertainty in the E-field strength. In 

general, our apparatus was crude, but note that 

Millikan was able to measure the electron charge to 

4.7992 ± 0.0037E-10 esu over 85 years ago using 

an ìeye and handî method. It is hoped that with 

modern digital video imaging capabilities and 

tighter control over the particle characteristics, our 

efforts can achieve moderately similar success. 

REFERENCES

1. J.A. Cross, Electrostatics: Principles, 

Problems, and Applications, Adam Hilger, 

Bristol, England, 1987, p. 17. 

2. see 3M web site, 

http://www.3M.com/us/home_leisure/filtrete/ 

index.html 

3. R.C. Flagan, ìHistory of Electrical Aerosol 

Measurements,î Aerosol Science and 

Technology, 28, 301 (1998). 
NASA/TMó2003-212109 7 

4. J.A. Cross, Electrostatics: Principles, 

Problems, and Applications, Adam Hilger, 

Bristol, England, 1987, p. 108ñ115. 

5. R.A. Millikan, ìOn the Elementary Electrical 

Charge and the Avogadro Constant,î Phys. 

Rev., 2, 109 (1913). 

6. Hopper, V.D. and Laby, R.H., ìThe Electronic 

Charge,î Proc. Roy. Soc. A, 178, 243 (1941). 

7. Kunkel, W.B., ìThe Static Electrification of 

Dust Particles on Dispersion in a Cloud,î 

Journal of Applied Physics, 21, 820 (1950). 

8. Kunkel, W.B., ìCharge Distribution in Coarse 

Aerosols as a Function of Time,î Journal of 

Applied Physics, 21, 833 (1950). 

9. Kunkel, W.B. and Hansen, J.W., ìA Dust 

Electricity Analyzer,î The Review of Scientific 

Instruments, 21, 308 (1950). 

10. J.R. McDonald, M.H. Anderson, R.B. Mosley, 

and Leslie Sparks, ìCharge Measurements on 

Individual Particles Exiting Laboratory 

Precipitators with Positive and Negative 

Corona at Various Temperatures,î Journal of 

Applied Physics, 51, 3632 (1980). 

11. J. Cross and L. Peterson, ìEvaluation of the 

Millikan Method to Measure the Relationship 

Between the Electrostatic Charge and 

Aerodynamic Radius of Brown Coal Fly Ash,î 

Proceedings of the 8th International Clean Air 

Conference (Melbourne, Australia), 1984 

(Melbourne: Holmes). 

12. M. Polat, H. Polat, and S. Chander, 

ìElectrostatic Charge on Spray Droplets of 

Aqueous Surfactant Solutions,î Journal of 

Aerosol Science, 31, 551 (2000). 

13. A. Sickafoose, J. Colwell, M. Horanyi, and 

S. Robertson, ìExperimental Investigations on 

Photoelectric and Triboelectric Charging of 

Dust,î Journal of Geophysical Research, 106, 

8343 (2001). 

14. Z. Sternovsky, S. Robertson, A. Sickafoose, 

J. Colwell, and M. Horanyi, ìContact 

Charging of Lunar and Martian Dust 

Simulants,î Journal of Geophysical Research, 

107, (E11), 5105, doi: 10.1029/2002JE001897, 

2002. 

Table 1: Experimental conditions for all analyzed drops. 

Drop # Particle 

Diameter 

(microns) 

Sample 

Size 

(ml) 

Temp

o

C 

Relative 

Humidity 

(%) 

E-Field 

(V/cm) 

E-Field 

Delay 

(sec) 

Tribocharge 

Time (sec) 

# of 

Particles 

Analyzed 

1 106ñ250 100 26 57 200 0.2 0 14 

2 106ñ250 50 26 62 200 0.2 0 5 

3 106ñ250 50 24 54 0 0 0 6 

5 106ñ250 50 27 50 0 0 0 7 

6 106ñ250 50 26 43 200 0.3 42 13 

7 106ñ250 50 22 54 100 0.3 0 16 

8 106ñ250 50 24 48 100 0.5 45 15 

12 106ñ250 50 27 45 200 0.3 45 15 

13 106ñ250 50 24 62 400 0.3 0 20 

14 106ñ250 50 26 57 400 0.3 60 20 

15 106ñ250 50 26 57 200 0.3 60 15 

30 250ñ300 30 23 46 200 0.3 60 15 
NASA/TMó2003-212109 8 

FD FE

Gas Jet 

Feed

Hopper

Plate

Spacing

Voltage on 

plates

_ +

Granular 

sample

y

x

FD FE

Gas Jet 

Feed

Hopper

Plate

Spacing

Voltage on 

plates

_ +

Granular 

sample

y

x

Figure 1. Illustration of columbic charge measurement. The particle samples are released from the hopper 

after tribocharging with the dry gas jet. As the particle moves between the charged plates, it is acted on by 

two primary forces, the force due to the electric field and the drag force. Note that the coordinate axis shown 

on the left is a indication of the x and y directions, not of the origin used in the analysis. 

(a) 

(b) (c)

Figure 2. Pictures of the columbic charge measurement device (a) as viewed from the camera location 

(b) assembled on the DIME education drop rig and (c) after being placed in drag shroud and dropped 

in the NASA-GRC 2.2 second Drop Tower. In (a), to the left is the solenoid that opens the hopper door. 

Behind the device, the back light used for illumination is seen. The sample hopper and dry gas jet 

are located above the chamber. Note the chamber is 5 cm wide, 10 cm high and 10 cm deep. 
NASA/TMó2003-212109 9 

(a) Before Drop 

(b) Initiation of Drop 

(c) E-field on (0.3 sec after initiation) 

(d) 0.33 sec after E-field activation 

(e) 0.66 sec after E-field activation 

(f) 0.99 sec after E-field activation 

(g) 1.32 sec after E-field activation 

(h) 1.65 sec after E-field activation 

Figure 3. Images acquired during typical drop experiment. Before this drop ~30mL of sand 

(250 to 300 μm) was tribocharged for ~1 minute using a dry gas jet. The electric field strength 

was 200V/cm and the ambient humidity was measured as 46%. 
NASA/TMó2003-212109 10

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Time (sec)

Position (mm)

x Position

y Position

Drop #5

T = 27o

C, RH = 50%

Diameter = 106-250 microns

Tribocharging Time = 0 secs

E-Field = 0 volts/cm 

Figure 4. Typical ìbaselineî particle trajectory exhibited during the microgravity period. This example is 

considered baseline since a zero electric field was applied and samples had not undergone tribocharging. 

Note Time = 0 in this case represents the time from which the particle tracking started. 

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Time (sec)

Position (mm)

x Position

y Position

Linear fit

Drop #7

T = 22o

C, RH = 50%

Diameter = 106-250 microns

Tribocharging Time = 0 secs

E-Field = 100 volts/cm

Est.Particle Charge = 2.2E-14 columbs

(a) 

0

2

4

6

8

10

0 2 4 6 8 10 12

Absolute Charge * 1014

(coulombs) 

Frequency

Drop #7

T = 22o

C, RH = 50%

Diameter = 106-250 microns

Tribocharging Time = 0 secs

E-Field = 100 volts/cm

(b) 

Figure 5. Results of ìNo Tribochargingî charge measurements made during a microgravity period: (a) shows a 

typical particle track and (b) shows the distribution of charge. An electric field of 100V/cm was applied to the 

particle cloud and an estimate of the particle charge was made by evaluating the particle terminal velocity in the 

x-direction by linear regression of the last 1 second of x-position data. Note Time = 0 in this case represents the 

time from which the particle tracking started. 

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Time (sec)

Position (mm)

x Position

y Position

Linear fit

Drop #8

T = 24o

C, RH = 48%

Diameter = 106-250 microns

Tribocharging Time = 45 secs

E-Field = 100 volts/cm

Est.Particle Charge = 3.2E-14 columbs

(a) 

0

2

4

6

8

10

0 2 4 6 8 10 12

Absolute Charge * 1014

(coulombs)

Frequency

Drop #8

T = 24o

C, RH = 48%

Diameter = 106-250 microns

Tribocharging Time = 45 secs

E-Field = 100 volts/cm

(b) 

Figure 6. Results of ìTribochargingî using a low electric field: (a) shows a typical particle track and (b) shows the charge 

distribution. An electric field of 100V/cm was applied to the particle cloud and an estimate of the particle charge was made 

by evaluating the particle terminal velocity in the x-direction by linear regression of the last 1 second of x-position data. 

Note Time = 0 in this case represents the time from which the particle tracking started. 
NASA/TMó2003-212109 11

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Time (sec)

Position (mm)

x Position

y Position

Linear fit

Drop #15

T = 26o

C, RH = 57%

Diameter = 106-250 microns

Tribocharging Time = 60 secs

E-Field = 200 volts/cm

Est.Particle Charge = 1.3E-14 columbs

(a) 

0

2

4

6

8

10

0 2 4 6 8 10 12

Absolute Charge * 1014

(coulombs)

Frequency

Drop #15

T = 26oC, RH = 57%

Diameter = 106-250 microns

Tribocharging Time = 60 secs

E-Field = 200 volts/cm

(b) 

Figure 7. Results of ìTribochargingî using a moderate electric field: (a) shows a typical particle track and (b) shows 

the charge distribution. An electric field of 200V/cm was applied to the particle cloud and an estimate of the particle 

charge was made by evaluating the particle terminal velocity in the x-direction by linear regression of the last 

1 second of x-position data. Note Time = 0 in this case represents the time from which the particle tracking started. 

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Position (mm)

Time (sec)

x Position

y Position

Linear fit

Drop #14

T = 26o

C, RH = 57%

Diameter = 106-250 microns

Tribocharging Time = 60 secs

E-Field = 400 volts/cm

Est.Particle Charge = 3.2E-14 columbs

(a) 

0

2

4

6

8

10

0 2 4 6 8 10 12

Absolute Charge * 1014

(coulombs)

Frequency

Drop #14

T = 26o

C, RH = 57%

Diameter = 106-250 microns

Tribocharging Time = 60 secs

E-Field = 400 volts/cm

(b) 

Figure 8. Results of ìTribochargingî using a high electric field: (a) shows a typical particle track and (b) shows the 

charge distribution. An electric field of 400V/cm was applied to the particle cloud and an estimate of the particle 

charge was made by evaluating the particle terminal velocity in the x-direction by linear regression of the last 

1 second of x-position data. Note Time = 0 in this case represents the time from which the particle tracking started. 

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12

Absolute Charge * 1014

(coulombs)

Frequency

All Drops

Particle diameter 106 - 250 microns

E-field range 100 - 400 V/m

Figure 9. Histogram of charge measurements from all drops performed, 

including tribocharged and non-tribocharged samples. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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NASA TM—2003-212109

AIAA–2003–1304

E–13760

WBS–22–101–53–01

17

Preliminary Results of a Microgravity Investigation to Measure Net Charge

on Granular Materials

Robert D. Green, Jerry G. Myers, and Bonnie L. Hansen

Electrostatic charge; Stokes' law; Millikan's oil drop method

Unclassified -Unlimited

Subject Categories: 34, 45, and 90 Distribution: Nonstandard

Prepared for the 41st Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics and

Astronautics, Reno, Nevada, January 6–9, 2003. Robert D. Green and Jerry G. Myers, NASA Glenn Research Center,

and Bonnie L. Hansen, Parks College of Saint Louis University, Saint Louis, Missouri 63108. Responsible person,

Robert D. Green, organization code 6712, 216–433–5402.

Accurate characterization of the electrostatic charge on granular materials has typically been limited to materials with

diameters on the order of 10 μm and below due to high settling velocities of larger particles. High settling velocities limit

both the time and the acceptable uncertainty with which a measurement can be made. A prototype device has been

developed at NASA Glenn Research Center (GRC) to measure coulombic charge on individual particles of granular

materials that are 50 to 500 microns in diameter. This device, a novel extension of Millikan’s classic oil drop experiment,

utilizes the NASA GRC 2.2 second drop tower to extend the range of electrostatic charge measurements to accommodate

moderate size granular materials. A dielectric material with a nominal grain diameter between 106 and 250 microns was

tribocharged using a dry gas jet, suspended in a 5×10×10 cm enclosure during a 2.2 second period of microgravity and

exposed to a known electric field. The response was recorded on video and post processed to allow tracking of individual

particles. By determining the particle trajectory and velocity, estimates of the coulombic charge were made. Over 30 drops

were performed using this technique and the analysis showed that first order approximations of coulombic charge could

successfully be obtained, with the mean charge of 3.4E-14 coulombs measured for F-75 Ottawa quartz sand. Additionally,

the measured charge showed a near-Gaussian distribution, with a standard deviation of 2.14e-14 coulombs.