Stephen Berkebile and James R. Gaier

Glenn Research Center, Cleveland, Ohio

Adhesion in a Vacuum Environment and its

Implications for Dust Mitigation Techniques

on Airless Bodies

NASA/TM—2012-217723

September 2012

AIAA–2012–3465
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Stephen Berkebile and James R. Gaier

Glenn Research Center, Cleveland, Ohio

Adhesion in a Vacuum Environment and its

Implications for Dust Mitigation Techniques

on Airless Bodies

NASA/TM—2012-217723

September 2012

AIAA–2012–3465

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

42nd International Conference on Environmental Systems

sponsored by the American Institute for Aeronautics and Astronautics

San Diego, California, July 15–19, 2012
Acknowledgments

The authors thank Ken Street for discussion and coordination of synthetic noritic glass preparation, Mike Weinstein of Zybek 

Advanced Products for supplying the synthetic noritic glass, Dave Hull for glass sample cutting, Joy Buehler for glass polishing, 

Scott Panko, Frank Lam and Rick Mondry for support in the laboratory, and Nicholas Garafolo and Henry De Groh for providing 

the S0383-70 sample. Financial support was provided by NASA ETDP Thermal Control and Dust Mitigation Projects through 

the NASA Postdoctoral Program (administered by Oak Ridge Associated Universities).

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217723 1 

Adhesion in a Vacuum Environment and its Implications for 

Dust Mitigation Techniques on Airless Bodies 

Stephen Berkebile1

 and James R. Gaier 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

During the Apollo missions, the adhesion of dust to critical spacecraft systems was a greater 

problem than anticipated and resulted in functional degradation of thermal control surfaces, spacesuit 

seals, and other spacecraft components. Notably, Earth-based simulation efforts did not predict the 

magnitude and effects of dust adhesion in the lunar environment. Forty years later, we understand that 

the ultrahigh vacuum (UHV) environment, coupled with micrometeorite impacts and constant ion and 

photon bombardment from the sun result in atomically clean and high surface energy dust particles and 

spacecraft surfaces. However, both the dominant mechanism of adhesion in airless environments and 

the conditions for high fidelity simulation tests have still to be determined. The experiments presented 

in here aim to aid in the development of dust mitigation techniques for airless bodies (e.g., lunar 

surface, asteroids, moons of outer planets). The approach taken consists of (a) quantifying the adhesion 

between common polymer and metallic spacecraft materials and a synthetic noritic volcanic glass, as a 

function of surface cleanliness and of triboelectric charge transfer in a UHV environment, and (b) 

determining parameters for high fidelity tests through investigation of adhesion dependence on vacuum 

environment and sample treatment. Adhesion force has been measured between pins of spacecraft 

materials and a plate of synthetic volcanic glass by determining the pull-off force with a torsion 

balance. Although no significant adhesion is generally observed directly as a result of high surface 

energies, the adhesion due to induced electrostatic charge is observed to increase with spacecraft 

material cleanliness, in some cases by over a factor of 10. Furthermore, electrostatically-induced 

adhesion is found to decrease rapidly above pressures of 10–6 torr. It is concluded that high-fidelity tests 

should be conducted in high to ultrahigh vacuum and include an ionized surface cleaning process. 

Nomenclature 

AZ-93 inorganic white thermal control paint 

b1,b0 fitting parameters 

Fadh force of adhesion 

FEP fluorinated ethylene propylene 

PC polycarbonate 

PTFE polytetrafluoroethylene 

RMS root mean square 

Ustrike energy of strike 

UHV ultrahigh vacuum 

 

1

 NASA Glenn Research Center, NASA Postdoctoral Fellow, Oak Ridge Associated Universities. 
NASA/TM—2012-217723 2 

Introduction 

In the Review of United States Human Space Flight Plans Committee report of 2009, two of three paths 

considered to the final goal of Mars include travel first to the Moon or to “near-Earth objects ... and the 

scientifically interesting moons of Mars, ....” (Ref. 1). All of these bodies are airless bodies, with a vacuum 

environment exceeding that attainable in chambers on Earth, and are under constant bombardment by space 

plasmas, high energy particles, high energy electromagnetic radiation, and micrometeorites (Refs. 2 to 4). 

Additionally, the Moon, the moons of Mars and many asteroids are covered by a layer of regolith which 

contains a significant amount of fine to ultrafine particulate matter. Our experience with the Apollo missions 

showed that when exposed to dust in vacuum, the performance of thermal control surfaces is seriously 

degraded (Ref. 5), photovoltaic efficiency is decreased (Refs. 6 and 7), mechanical systems are 

compromised through abrasion (Ref. 5) and optical elements of cameras and telescopes are scratched or 

occluded (Ref. 8). As a result, these and other systems may fail when operating in a dusty vacuum 

environment if effective mitigation strategies are not developed and implemented. 

Under the conditions of ultrahigh vacuum (UHV, <10–9 torr) found at the surfaces of airless bodies, 

dust behaves very differently than in our atmosphere (Refs. 2 and 5). Dust adhesion properties are thought 

to be enhanced by two classes of mechanisms, increased surface energy and electrostatic attraction. 

Firstly, particles of dust are made more reactive through “space weathering” processes of ion 

bombardment and micrometeorite impacts which create smaller dust particles and freshly fractured 

surfaces. In UHV environments, the atomic cleanliness of freshly fractured surfaces is maintained and the 

population of chemically reactive surface species remains significant due to the scarcity of molecules in 

the gas phase colliding with the surface of the dust. The particle bombardment of the dust (and, with 

exposure time, the spacecraft materials) in the UHV environment results in the absence of a thin film of 

hydrocarbons and water that exists on surfaces in our atmosphere. As a result, the adhesion of dust is 

enhanced up to two orders of magnitude, primarily due to Van der Waals interactions (Ref. 9). The 

second mechanism which can enhance dust adhesion arises from electrostatic forces. For example, most 

lunar particles are composed of nonconductive minerals and glasses and will build up a equilibrium static 

charge under constant ion and photon bombardment (Refs. 10 and 11). Such effects lead to positive 

charge on the surface of the moon facing the Sun (photoelectric effect) and negative charge on the dark 

side (electrons from the solar wind) (Ref. 10). 

As a result of space weathering under UHV conditions and the mechanisms listed above which lead 

to an increased adhesion, dust mitigation strategies which work under atmospheric conditions cannot be 

assumed to work on the surfaces of airless bodies. As the Apollo missions have shown, even tests in low 

to high vacuum environments can lead to misleading results which do not accurately simulate the lunar 

environment (Ref. 2). 

In this study we address the issues of adhesion on airless bodies by using well-defined conditions which 

attempt to simulate a lunar environment combined with simple model systems of lunar and spacecraft 

materials to determine which mechanism, electrostatic or high surface energies, causes the greatest 

adhesion. To this end, the adhesion between pins of spacecraft materials and a plate of lunar simulant glassy 

material has been investigated in a UHV environment. The materials can be tested with control of their 

surface atomic cleanliness, as well as their electrostatic charge. This study includes measurements of 

adhesion due to both Van der Waals forces and electrostatic charging induced by triboelectric charging. 

Until recently, it was unclear whether surface energy or electrostatic forces dominate for different 

particle sizes and for different types of dust. This investigation demonstrates that the electrostatic charge is 

the dominant mechanism of adhesion that can be expected in an airless body environment, and that 

particular attention must be given to issues of triboelectric charging. The triboelectric charge transfer is 

found to be sensitive to surface atomic cleanliness of the samples, and electrostatically induced adhesion is 

affected by background pressures exceeding 10–6 torr. Therefore, mitigation techniques should be tailored to 

deal with electrostatic forces. Furthermore, high fidelity tests and validation of dust mitigation techniques 

should include a step to remove surface contamination and be conducted in high to ultrahigh vacuum. 
NASA/TM—2012-217723 3 

Experimental 

This work aims to understand the adhesion forces between lunar dust and thermal control surfaces in the 

lunar environment, but the actual experiment necessarily makes several compromises. A torsion balance 

was used to measure the pull-off force (Ref. 12), so only macroscopic samples could be used. A plate of 

synthetic norite volcanic glass with a lunar-like composition (Zybek Advanced Products, Boulder, CO) was 

used in place of the lunar dust. As determined by Auger electron spectroscopy, the chemical composition of 

the synthetic noritic glass surface is approximately 48 percent O, 18 percent Si, 16 percent Ca, 12 percent 

Al, and 6 percent Mg. The synthetic glass was produced by melting noritic materials from the Stillwater 

mine and quench cooling them. This material is also used as a source for the glass content in NU-LHT lunar 

regolith simulant. Lunar vacuum was simulated in an ultrahigh vacuum (UHV) chamber which was pumped 

to a vacuum of 10–10 torr or better, but still two orders of magnitude higher than lunar vacuum. Ion 

bombardment by the solar wind was simulated by sputtering the surfaces with Ar+

 ions. 

The spacecraft materials were in the form of polished 3.2 mm diameter pins with hemispherical ends 

(radius = 1.6 mm). Most pins were polished mechanically with diamond paper down to 0.1 m. 

Polycarbonate (PC) was polished with AlOx paper and then chemically polished with a methylene 

chloride vapor. Mylar and Tedlar were used in the form of sheets spanned over an aluminum pin of the 

above curvature with no further treatment. Soda-lime glass was melted to the desired curvature. Silicone 

rubber S0383-70 (Parker Hannifin) was obtained in the form of an o-ring seal with tube radius of 1.3 mm 

and ring diameter 9 mm. AZ-93 paint (zinc orthotitanate in a potassium silicate binder) was applied to an 

aluminum pin (AZ Technologies, Huntsville, AL) and used as-received. All samples were rinsed in pure 

ethanol prior to placement in the UHV chamber. A list of the materials used can be found in Table 1 next 

to the root-mean-square surface roughness as measured by a Veeco/Wyko NT 1000 optical profilometer 

and the Young’s moduli of the materials. 

The experimental details have been described fully previously (Ref. 13), but the concept is 

straightforward. The hemispherical end of the pin being measured is driven towards the glass plate with a 

speed of 9.3 m/s. It continues to be driven until the plate is pushed back which twists the torsion wire. 

After being displaced to a load of 0.1 to 0.5 mN, as measured by a differential variable reluctance 

transducer (DVRT) which measures the position of a balance arm on the opposite side of the plate, the 

drive motor is halted, and the pin and plate remain in contact for a specified, but variable, period of time, 

usually 15 to 30 sec. Then the pin is withdrawn from the plate at the same speed. After passing the torsion 

balance equilibrium point, the wire tends to pull the plate and pin apart. Separation occurs when the 

spring force equals the adhesion force. Examples of the raw data can be found in Reference 13. 

TABLE 1.—APPROXIMATE RMS ROUGHNESSES OF PLATE AND 

PIN SURFACES BEFORE ADHESION MEASUREMENTS IN THE 

UHV CHAMBER AND YOUNG’S MODULUS OF MATERIALS 

(FROM ENGINEERINGTOOLBOX.COM) 

Material RMS roughness, 

nm

Young’s Modulus, 

GPa

Noritic glass plate <30 50 to 90 

PC 90 2.6 

PTFE 130 0.5 

FEP 250 0.5 

Ti-6-4 300 114 

Al-6061 170 69 

Soda-lime glass --------- 50 to 90 

Mylar (boPET) 110 2 to 2.7 

Tedlar (PVF) 150 --------- 

Silicone rubber S0383-70 620 0.001 to 0.005 

AZ-93 --------- --------- 
NASA/TM—2012-217723 4 

A variation on the procedure was used to apply an electrostatic charge. The pin was placed 50 m 

from the plate. It was then quickly brought into contact with the plate in a striking motion and retracted. 

Under many conditions this resulted in the deposition of charge on the pin, the plate, or both. This motion 

resulted in one large and several smaller bounces between the pin and plate. The strength of the strike was 

registered by the greatest displacement of the balance due to the strike. This displacement was then 

converted into potential energy from which the kinetic energy of the strike was assumed. The magnitude 

of charge deposition was inferred from the measured electrostatic adhesion force. The adhesion force was 

measured three to ten times after a strike at the strike position and was generally found to have little 

variation for any particular strike. The standard deviation of the adhesion measurements for a single strike 

is given as error bars in the figures. 

Two types of adhesion forces could be observed, electrostatic and non-electrostatic (Van der Waals, 

chemical), which were distinguished by their functional dependence on separation and therefore their 

range. Measurements of the adhesion force were made under different loads, typically in the range of 0.1 

to 0.5 mN. The duration of the loads was 30 sec for the measurements of non-electrostatic adhesion 

whereas the loads were typically maintained 15 sec for cases of electrostatic adhesion. No dependence on 

the load duration or magnitude was seen for electrostatic adhesion. The adhesion force values given 

throughout this report are generally the average of three or more contacts with the standard deviation 

shown as error bars. 

The surfaces of the materials were cleaned by bombardment with 8.5 A/cm2

 beam of Ar+ ions 

accelerated to 2 keV from an ion gun (sputter cleaning). This process removed atmospheric contamination 

due to water and organic compounds from mineral surfaces and created a number of broken bonds, both 

of which leave the surface in a state similar to that which dust particles have in an airless body 

environment. Metals were sputtered until no improvement in the C and O surface contamination could be 

discerned using an Auger electron spectrometer before each measurement. Plastic materials were 

sputtered for 30 sec to 5 min each day before measurements. No dependence on sputter duration was seen 

after the initial sputter cycle. The synthetic volcanic glass was cleaned through ion bombardment prior to 

all measurements, unless explicitly stated. 

Adhesion in UHV Environment 

Non-Electrostatic Adhesion 

The adhesion force due to Van der Waals and other short range interactions, such as chemical 

bonding, was below the apparatus detection limits (–5 N) for most of the glass and pin sample pairs. 

(Since adhesion is an attractive force it is given a negative sign. Applied loads are positive in sign.) 

Forces as high as 10 mN were applied to press the pin and plate together, and were held as long as 

10 min. The results were the same whether the volcanic glass was tested “dirty” or had been sputter 

cleaned, and whether the pin was dirty or cleaned. 

There were three exceptions which displayed some form of non-electrostatic adhesion: the PTFE and 

6061 aluminum pins and the S0383-70 o-ring. Interestingly, these three materials are different in their 

chemical and material properties, as well as the typical applications for which they are utilized. PTFE is 

often used when a non-stick surface is desired, while the silicone rubber S0383-70 is a candidate material 

for docking seals, an application in which gas-impermeable contact is sought. Al-6061 is a structural 

material which is not generally considered for its adhesive properties (or lack thereof). 

In the case of PTFE, the adhesion force ranged from –0.02 to –0.10 mN, and depended on the 

magnitude of the applied force in a roughly linear fashion for any particular contact position (Fig. 1). 

However, the adhesion varied widely between contact locations, even for similarly treated samples. 

Leaving the pin attached to the glass at a load of 1.9 mN for a period of 2 hr resulted in an increase in the 

adhesion force by a factor of 5 to 10 over 30-second-long loads. Sputter cleaning the glass increased the 

adhesion force by about a factor of 4. Cleaning both the glass and the PTFE pin increased the adhesion an 

additional factor of 2 for some positions. 
NASA/TM—2012-217723 5 

Figure 1.—Adhesion force due to Van der Waals and chemical interactions as a function of load for a) PTFE 

pin and b) S0383-70 silicone rubber o-ring on synthetic volcanic glass. Measurements of different loads at 

any particular contact position are connected by lines in a). 

S0383-70 displayed non-electrostatic adhesion on sputter-cleaned glass when the o-ring was not 

treated after insertion in the vacuum system. The adhesion behavior was fairly erratic and varied greatly 

depending on the particular contact location selected, even though these locations were all of the same 

geometry. The adhesion for first contact at any particular location ranged from –0.1 to –0.65 mN, but was 

not dependent on the applied load, as can be seen in Figure 1. Further contacts at the same position 

always resulted in a greatly reduced adhesion force which would reduce within two to four contacts to a 

steady value between 20 percent of the original value to non-measureable. If adhesion was measureable 

for further contacts, these were found to be load-dependent, as shown for a particular location in Figure 1. 

When the o-ring was sputter cleaned, the non-electrostatic adhesion was no longer measureable, perhaps 

due to chemical modification of the surface through the breaking of bonds. Note that atomic oxygen 

treatment of such samples leads to a glassy surface which decreases total adhesion under large loads 

(Ref. 14). 

The 6061 aluminum pins adhesion ranged from –0.01 to –0.08 mN and exhibited behavior that was 

difficult to interpret. Contrary to expectations, the adhesion force decreased at higher applied forces. 

Although there was a wide range of adhesion values, all values greater than –0.02 mN occurred when the 

applied load was less than 0.4 mN. 

The mechanisms of non-electrostatic adhesion for these three samples can only be speculated on, but 

some evidence of the mechanisms is available from the conducted measurements. The S0383-70 

displayed the largest adhesion, which then reduced with further contacts, suggesting that the position of 

contact was changing. Indeed, some evidence of material transfer was observed, in that the volcanic glass 

plate sometimes showed a tendency to charge during Auger electron spectroscopy measurements after 

numerous contacts were made. Although no significant amount of carbon was seen to transfer to the 

surface (and Si and O were already present in large amounts in the glass), a change in the secondary 

electron emission characteristics clearly occurred. It is possible that a chemical interaction is taking place 

at the silicone/silicate interface which is stronger than the cohesive force of the silicone rubber, with 

further Van der Waals forces present after chemical modification is complete. No transfer of Al or Mg 

was seen from the glass plate to the o-ring. 

 

0 1 2 3 4 5 6

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

Applied loads, mN

Adhesion force, mN

PTFE pin, glass unsputtered

pin unsputtered, glass sputtered

pin, glass sputtered

a)

0 0.5 1 1.5 2

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

Applied loads, mN

Adhesion force, mN

S0383-70, different positions, first contact

S0383-70, same position

b)
NASA/TM—2012-217723 6 

The smaller adhesion of the PTFE to the glass is unlikely to be chemical in nature, as PTFE is one of 

the most chemically inert substances known. PTFE is subject to creep of the polymer chains due to the 

lack of cross-linking. The load- and time-dependent adhesion values observed suggest that physical 

deformation of the PTFE tip is occurring. We propose that, even though PTFE is known to have low 

Van der Waals adhesion values, by conforming to the glass surface, the roughness of the PTFE is 

overcome and a large enough contact area is created for Van der Waals adhesion to become significant. 

Due to the smoothness of the glass, the mechanism of adhesion is not likely to be from mechanical 

interlocking. 

Triboelectric Charging 

Electrostatic adhesion was induced through triboelectric charge transfer between the spacecraft 

materials and the glass plate. Figure 2 shows the electrostatic adhesion force for individual strikes each 

with a particular strike energy, as calculated from the balance deflection due to the strike. As is shown in 

Figure 2, this triboelectric charge transfer increased with strike energy over the range of the strike kinetic 

energy, 1 to 15 J, which indicates that electrostatic adhesion forces increased with the amount of charge 

transferred upon striking the pin to the plate. Comparisons of adhesion data are more clearly illustrated in 

Table 2, which shows the average adhesion per unit strike energy. This value was determined by dividing 

the adhesion measured in a particular experiment by the kinetic strike energy applied. Thus, materials 

with higher values converted more of the kinetic energy into charge separation, which resulted in greater 

electrostatic adhesion. 

Figure 2.—Measured electrostatic adhesion force as a function of the kinetic energy of striking the pin against 

synthetic volcanic glass. Note that all logarithmic axes are equal for the three graphs. 

10-1100101

10-3

10-2

10-1

100

101

Adhesion force [mN]

Strike energy [J]

10-1100101

Strike energy [J]

10-1100101

Strike energy [J]

unsputtered Al-6061

sputtered Al-6061

unsputtered Mylar

sputtered Mylar

unsputtered PTFE

sputtered PTFE

unsputtered Tedlar

sputtered Tedlar

unsputtered FEP

sputtered FEP

unsputtered glass

sputtered glass

unsputtered PC

sputtered PC

unsputtered S0383-70

sputtered S0383-70
NASA/TM—2012-217723 7 

TABLE 2.—MEASURED AVERAGE ELECTROSTATIC ADHESION FORCE PER KINETIC 

ENERGY OF THE STRIKING OF THE PIN AGAINST SYNTHETIC VOLCANIC GLASS 

Material Triboelectric 

affinity,

nC/J*

Average adhesion force per strike energy, 

N/J 

as is sputter cleaned 

Ti-6-4 --- ‡ ‡ 

Al 6061 --- –1.8 –7.4 

Glass (soda lime) +25 –7.2 –10.1 

AZ-93 --- ‡ ‡ 

Polycarbonate –5 –16.5 –84.4 

Tedlar --- –30.9 –105 

Mylar –40 –116 –240 

Silicone rubber S0383-70 –72 –173 –43.9 

PTFE –190 –0.9 –2410 

FEP –190 –37.6 –3020 

‡ Indicates non-detectable adhesion below –0.005 mN, i.e. about –0.3 N/J.

* from Triboelectric Table (Bill Lee, Alpha Lab Inc., 2009).

A clear difference is seen for most materials between the low to non-measureable charge transfer of 

the state “as inserted” into the UHV chamber and the high charge transfer of the “sputtered” samples. 

Sputter cleaning was conducted for 15 sec to 2 min for any particular polymer, and until a clean sample 

was obtained for the metals. As noted elsewhere (Ref. 15), such sputter treatment corresponds to an 

exposure of a few hours on the lunar surface. The exception to this rule was the S0383-70, which had the 

largest average charge transfer in the unsputtered state. Once sputtered, the triboelectrically-induced 

adhesion values were decreased, reminiscent of the decrease seen in the adhesion values after sputtering 

for non-electrostatic adhesion in the previous section. 

Table 2 lists the materials in order of their triboelectric affinity. A positive value tends to give up 

electrons and become positively charged, and a negative value tends to gain electrons and take on a 

negative charge. The synthetic volcanic glass probably has a triboelectric affinity near that of soda lime 

glass. Electrons are so mobile in metals that they do not retain charge. Note that the adhesion forces, 

particularly for the cleaned materials, are well correlated to the triboelectric affinity. 

The strike energy values of 1 to 15 J correspond roughly to the kinetic energy of spherical silica 

particles 0.4 to 1 mm in diameter when dropped onto a surface from the height of one meter in Earth’s 

gravity. If such a particle strikes PTFE or FEP, the electrostatic adhesion induced will be 1000 times the 

force of gravity acting on that particle (or 10 to 100 times the force of gravity when striking other polymer 

materials). The situation will be similar on the lunar surface, despite having only 1/6 of the Earth’s gravity. 

Since the potential energy of a particle depends linearly both on its height above a surface and on the 

gravitational force, the electrostatic adhesion after a fall of one meter on the Moon will be 1000/6 = 167 

times that on Earth (which is still 1000 times the force of gravity on the Moon). Particles lofted by human 

activity through a trajectory with horizontal motion will have additional translational energy. 

In order to see how the induced electrostatic adhesion might scale to smaller values, both linear and 

exponential fits were attempted. The results were less than satisfactory. An exponential fit 

1 adh 0 strike

b F  b U , with exponents b1 less than zero, seemed to fit best for the largest values of adhesion 

with sputtered FEP and PTFE, but no fit could be found which provided confidence. While there is little 

variation in the adhesion measured as the result of any particular strike (several measurements after a 

strike result in the small standard deviation shown by the error bars), there is a large scatter between 

values measured as the result of different strikes. The large scatter in the data likely arises from the 

method of striking, which, while providing an exact maximum strike energy, includes several smaller 

contacts for each data point shown. To resolve the question of scaling to smaller particle sizes, further 

measurements will need to be conducted. 
NASA/TM—2012-217723 8 

Effect of Vacuum on Charged Adhesion 

Surface cleanliness has been shown to be an important factor for the triboelectric charge transfer. 

After the initial Ar+

 ion sputter cleaning of the surfaces used in this study, the cleanliness will depend on 

the quality of the vacuum environment. Likewise, once the charge transfer has taken place, the persistence 

of the charge on the surface and, hence, the electrostatic adhesion depends on the quality of the vacuum 

environment. Mechanisms of discharging an electrically charged surface are numerous and include 

collisions with gas, conduction, electron/ion bombardment (directly through charge neutralization or 

indirectly through secondary emission), and photoelectric emission. Conduction can be aided by a layer of 

water on the surface in a humid environment or by the inner photoelectric effect creating free charge 

carriers in an otherwise largely insulating material. On an airless body, the methods of discharge will be 

limited to solar wind plasma currents, photoelectric emission, and conduction, the latter of which is 

negligible for the polymers considered here and largely irrelevant for the lunar regolith. In fact, Farrell et 

al. (Ref. 16) worry about the lack of the former two mechanisms for discharge of triboelectrically-induced 

charge in the craters of the lunar polar regions. 

In the absence of background gases and electron or ion currents, strong electrostatic adhesion was 

observed here to persist for a period of at least several days with no significant degradation in a vacuum 

of 10–10 torr. For more details, see Reference 13 where only a small drop in the strength of the 

electrostatic adhesion is seen over several hours. To test the robustness of the electrostatic adhesion with 

variation of background pressure, Argon gas and air were leaked into the UHV chamber while measuring 

the adhesion force of an FEP pin to the glass after striking. Once the charged pin has been exposed to 

Argon or air at pressures around 10–6 torr, a drastic drop in the adhesion force occurs (Fig. 3(a) and (b), 

respectively). 

The charge on the pin and on the glass plate could be observed by scanning electron microscopy as a 

large distortion in the image. It was observed that full discharge of triboelectrically induced charge areas 

could be achieved with 100s of μA/cm2

 in 1 to 2 sec. 

Figure 3.—FEP adhesion as a function of time during an increase of a) Argon pressure and b) air pressure. 

0 10 20 30 40 50

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Time, minutes

Pressure, Torr

-4

-3

-2

-1

0

Adhesion, mN

Pressure

Adhesion

a)

0 10 20 30 40 50

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Time, minutes

Pressure, Torr

-4

-3

-2

-1

0

Adhesion, mN

Pressure

Adhesion

b)
NASA/TM—2012-217723 9 

Conclusion 

In a UHV environment, triboelectric charging has been seen in this study to be the single greatest 

contributor to adhesion, particularly for atomically clean materials with large triboelectric affinities. The 

electrostatic adhesion was often orders of magnitude greater than Van der Waals forces or adhesion 

through chemical bonding, except in the case of silicone rubber. In the absence of plasma currents, such 

as in shadowed craters and sheltered areas of spacecraft triboelectric charging will dominate dust 

adhesion. In areas exposed to plasma currents or UV photons, triboelectric charging may be compensated 

(Ref. 16), but surface charge will still remain due to those same currents and photoelectric emission. As 

we have seen, charge can remain a significant source of adhesion over time scales ranging in hours and 

days in the absence of the atmospheric discharge effects of gas collisions and absorbed water. 

Thus dust mitigation techniques which concentrate on reducing the effects of electrostatic forces will 

be much more successful over strategies which work to reduce Van der Waals adhesion. Through the use 

of coatings with a work function matching that of regolith minerals and glasses, or by choosing materials 

whose triboelectric affinity is similar to that of the dust, triboelectric charging can be limited passively. 

For example, the implications of triboelectric charging clearly favor the silicate-based paint AZ-93 over 

uncoated silver-FEP as the choice of thermal control surfaces where dust impingement is expected. In 

fact, a recent study has shown that work function matching coatings decrease dust adhesion to FEP 

surfaces in high fidelity tests with lunar simulant particles (Ref. 17). Such areas of heavy dust 

impingement are likely those where mechanical activities will occur, as a study by Katzan about the 

sources of lofted dust on the lunar surface revealed (Ref. 11). It should be noted that the dust environment 

around asteroids is far less known than on the lunar surface. The reduction of triboelectric charging 

should certainly determine the choices of materials used where intense contact with regolith will occur, 

such as in mining operations, under any conditions. Active manipulation of the dust and spacecraft 

surface charge electrostatically, as well as other electrostatic or electrodynamic mitigation techniques 

(see Ref. 18 for an example) will likely also prove most effective at mitigating dust adhesion. 

When testing any mitigation strategies, the vacuum conditions within the test chamber and the 

monolayer surface contamination of regolith simulant and spacecraft materials are critical and will 

determine the applicability of any such test to a true airless body environment. Micrometeor impacts and 

the solar wind will ensure that the dust on the surface of any airless body will be atomically clean, while 

spacecraft materials will be cleaned by the solar wind within minutes to hours (Ref. 15). Contamination 

from water and hydrocarbons present on any surface which has been exposed to a terrestrial atmosphere 

greatly lowers the triboelectric charge transfer, and hence the adhesion. Removal of this contamination 

layer through a sputtering process or by plasma treatment will be an important step of any high fidelity 

test. Maintenance of a high to ultrahigh vacuum to prevent later accumulation of surface contaminants 

will be equally important. Maintenance of a vacuum below 10–6 torr will also be critical to avoiding 

discharge of any charged surfaces through collision with background gases. Although no significant drop 

in the adhesion was observed at 10–7 torr and below, we recommend that long term (hours) adhesion tests 

are performed and compared to adhesion loss rates in Reference 13 in order to qualify vacuum chambers 

for further tests. Ultimately, high fidelity tests and validation of dust adhesion mitigation technology must 

be made in high or ultrahigh vacuum and include a cleaning step of the regolith simulant and any exposed 

spacecraft material surfaces. 
NASA/TM—2012-217723 10 

References 

1. Review of United States Human Spaceflight Plans Committee, “Seeking a Human Spaceflight 

Program Worthy of a Great Nation,” Augustine, N. R., Chairman, 2009. 

2. Gaier, J.R., and Jaworske, D.A., “Lunar Dust on Heat Rejection System Surfaces: Problems and 

Prospects,” NASA TM-2007-214814, 2007. 

3. Gaier, J.R., “Regolith Activation on the Lunar Surface and Its Ground Test Simulation,” NASA TM2009-215648, 2009. 

4. Vander Wal, R. L., “Lunar Dust Chemical, Electrical, and Mechanical Reactivity: Simulation and 

Characterization,” The National Center for Space Exploration Research, NASA CR-2008-215431, 

Cleveland, OH, Oct. 2008. 

5. Gaier, J. R., “The Effects of Lunar Dust on EVA Systems During the Apollo Missions,” NASA TM2005-213610/REV1, 2005. 

6. O’Brien, B.J., Freden, S.C., and Bates, J.R., “Degradation of Apollo 11 Deployed Instruments 

Because of Lunar Module Ascent Effects,” Journal of Applied Physics, Vol. 41, No. 11, 1970, 

pp. 4538-4541. 

7. Katzan, C.M., Brinker, D.J., and Kress, R., “The Effects of Lunar Dust Accumulation on the 

Performance of Photovoltaic Arrays,” NASA Glenn Research Center, Document ID 19910020924, 

Brook Park, OH, 1991. 

8. Murphy Jr., T.W., et al., “Long-Term Degradation of Optical Devices on the Moon,” Icarus, 

Vol. 208, No. 1, 2010, pp. 31-35. 

9. Walton, O.R., “Adhesion of Lunar Dust,” Grainflow Dynamics, Inc., NASA CR-2007-214685, 

Livermore, CA, Apr. 2007. 

10. Sickafoose, A.A., Colwell, J.E., Horányi, M., and Robertson, S., “Photoelectric Charging of Dust 

Particles in Vacuum,” Physical Review Letters, Vol. 84, No. 26, 2000, pp. 6034-6037. 

11. Katzan, C.M., and Edwards, J.L., “Lunar Dust Transport and Potential Interactions with Power 

System Components,” NASA, Document ID 19920002733, NASA CR-4404, Brook Park, OH, 

Nov. 1991. 

12. Miyoshi, K., Chikayoshi, M., and Ryuchi, M., “Development of a Torsion Balance for Adhesion 

Measurements,” NASA TM-100799, 1988. 

13. Berkebile, S., Street Jr., K.W., and Gaier, J. R., “Adhesion Between Volcanic Glass and Spacecraft 

Materials in an Airless Body Environment,” 3rd Atmospheric and Space Environments Conference, 

AIAA-2011-3675, 2011. Also NASA TM-2012-217221. 

14. Banks, B.A., deGroh, K.K., Rutledge, S.K., and Haytas, C.A., “Consequences of Atomic Oxygen 

Interaction with Silicone and Silicone Contamination on Surfaces in Low Earth Orbit,” NASA TM 

1999-209179, presented at the 44th Annual Meeting sponsored by the International Society for 

Optical Engineering, Denver, Colorado, July 21, 1999. 

15. Gaier, J.R., and Berkebile, S., “Implications of Adhesion Studies for Dust Mitigation on Thermal 

Control Surfaces,” AIAA Aerospace Sciences Meeting, AIAA-2012-0875, 2012. Also NASA TM2012-217213. 

16. Farrell, W.M., et al., “Concerning the Dissipation of Electrically Charged Objects in the Shadowed 

Lunar Polar Regions,” Geophysical Research Letters, Vol. 35, 2008, pp. L19104. 

17. Gaier, J.R., Waters, D.L., Misconin, R.M., Banks, B.A., and Crowder, M., “Evaluation of Surface 

Modification as a Lunar Dust Mitigation Strategy for Thermal Control Surfaces,” 41st International 

Conference on Environmental Systems, AIAA-2011-5183, 2011. Also NASA TM-2011-217230. 

18. Calle, C.I., Buhler, C.R., McFall, J.L., and Snyder, S.J., “Particle Removal by Electrostatic and 

Dielectrophoretic Forces for Dust Control During Lunar Exploration Missions,” Journal of 

Electrostatics, Vol. 67, No. 2, 2009, pp. 89-92. 

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Adhesion in a Vacuum Environment and its Implications for Dust Mitigation Techniques on 

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Berkebile, Stephen; Gaier, James, R. 

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WBS 780896.04.06.02.03 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

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13. SUPPLEMENTARY NOTES

Stephen Berkebile, NASA Glenn Research Center, NASA Postdoctoral Fellow, Oak Ridge Associated Universities. 

14. ABSTRACT

During the Apollo missions, the adhesion of dust to critical spacecraft systems was a greater problem than anticipated and resulted in 

functional degradation of thermal control surfaces, spacesuit seals, and other spacecraft components. Notably, Earth-based simulation efforts 

did not predict the magnitude and effects of dust adhesion in the lunar environment. Forty years later, we understand that the ultrahigh 

vacuum (UHV) environment, coupled with micrometeorite impacts and constant ion and photon bombardment from the sun result in 

atomically clean and high surface energy dust particles and spacecraft surfaces. However, both the dominant mechanism of adhesion in 

airless environments and the conditions for high fidelity simulation tests have still to be determined. The experiments presented in here aim 

to aid in the development of dust mitigation techniques for airless bodies (e.g., lunar surface, asteroids, moons of outer planets). The 

approach taken consists of (a) quantifying the adhesion between common polymer and metallic spacecraft materials and a synthetic noritic 

volcanic glass, as a function of surface cleanliness and of triboelectric charge transfer in a UHV environment, and (b) determining 

parameters for high fidelity tests through investigation of adhesion dependence on vacuum environment and sample treatment. Adhesion 

force has been measured between pins of spacecraft materials and a plate of synthetic volcanic glass by determining the pull-off force with a 

torsion balance. Although no significant adhesion is generally observed directly as a result of high surface energies, the adhesion due to 

induced electrostatic charge is observed to increase with spacecraft material cleanliness, in some cases by over a factor of 10. Furthermore, 

electrostatically-induced adhesion is found to decrease rapidly above pressures of 10-6 torr. It is concluded that high-fidelity tests should be 

conducted in high to ultrahigh vacuum and include an ionized surface cleaning process.

15. SUBJECT TERMS

Thermal control; Surface properties; Adhesion; Particles; Dust; Minerals; Surface properties; Surface energy 

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