Seminário proferido pelo pesquisador Thiago Burgo (Unicamp) em 26 de agosto de 2015 para cerca de 20 estudantes e professores, na seção UCS (Caxias do Sul, RS) do Instituto Nacional de Engenharia de Superfícies.

1. Friction Force and its Relationship to the
Electrostatic Charges at Interfaces
Dr. Thiago A L Burgo

2. Outline
Unicamp
• Friction coefficient induced
by electrostatic charges
– Coefficient of rolling
resistance x Electrostatic
potential (EP)
– Friction angle x EP
– Friction at nanoscale
• Lateral Force Microscopy
(LFM)
Argonne
Na/onal
Laboratory
• Tribocurrent and
macroscopic friction force
– Dependence on the
atmosphere
– Friction force fluctuations
– Electrostatic adhesion
• Nanomechanical Mapping on
tested PTFE
– Triboemission
• Concluding Remarks
• Electrostatic Discharge (ESD) and Technological Challenges

3. Explosions triggered by electrostatic discharge
https://www.youtube.com/watch?
v=6lKUsUycBNA&spfreload=10
www.firesciencetools .com

4. In October 2013, a fire of
great proportions hit
Copersucar facilities.

5. • For nanotechnology
– Electrostatic force is even
larger than the inertial
force, for micromachine
parts made of insulators.
– The electrification of the
insulator is not well
understood, especially at
the micro-scale.
• Safety and technologies
– Dust explosions
– Fires
– Pharmaceuticals
– Polymer recycling
– Electrospinning
– Solid paint
– Electrocopying
– Toner
Consequences

6. Challenges
Faced
By
Solar
Energy
Use
Calle et al. Active dust control and mitigation technology for lunar and Martian exploration. Acta
Astronautica 69, 1082-1088 (2011).

7. Electrodynamic
Repulsion
Courtesy of Carlos Calle, Electrostatics and Surface Physics Laboratory - NASA

8. Surfaces
under
rela<ve
mo<on:
Triboplasma
Heinicke G.
Tribochemistry.
(1984)
Matta et al. J.
Phys. D: Appl.
Phys. (2009)
Camara, et al.
Nature (2008)
Burgo, et al.
Polym. Degrad.
Stabil. (2014)
enerated. All these
na follow a certain
plained by Suh [3].
city, normal load,
nt of hydrocarbons
,5], are correlated
s. According to
emission of both
les was measured
ls, ceramics, and
conditions with
ould be considered
y discovered by
s mostly composed
rding to the most
noemission is an
g from mechanical
Fig. 1. General scheme of triboemission [7].
K. Kajdas / Tribology International 38 (2005) 337–353

9. How
powerful
is
the
triboplasma?
!
Camara et al. Correlation between nanosecond X-ray
flashes and stick–slip friction in peeling tape. Nature,
455, 1089-1093 (2008).
X-Rays!
X-ray Fluorescence
(XRF) technology
Tribogenics

10. What
creates
sta/c
electricity?
Williams, M.W. What creates static electricity? American Scientist 100, 316-326 (2012)
• Complex and unexpected charge distribution in
every material scanned by Kelvin electrodes
(EFM, KFM/SEPM, Macro-Kelvin):
– Cardoso et al., Langmuir 1998, 1999
– Galembeck et al., Polymer 2001
– Gouveia et al., J. Phys. Chem B, 2005, 2008
– Soares et al., J. Braz. Chem. Soc. 2008
– Gouveia et al., J. Am. Chem. Soc. 2009
– Ducati et al., Langmuir 2010
– Bernardes et al., J. Phys. Chem. C 2010
– Burgo et al., J. Electrostatics 2011

11. Polymer
Ethanol
2
hours
Clean
Polymer
drying
Electrical
poten<al
scanning
with
Kelvin
electrode
OR
EXPERIMENTAL
Balance
Polymer
P:
1,5
kPa
RPM:
5000
Time:
3
s
PE
foam
Shaking
table
Polymer
Glass
spheres
or
pellets
de
PTFE
Amount:
4
g
Time:
60
min
1-­‐
Cleaning
2-­‐
Charging
3-­‐
Charge
mapping
X
Y
Polymer

12. 1 cm
Bipolar Segregated
Domain

13. Formation free-radicals is followed by electron transfer:
from the hydrocarbon free-radicals to the more
electronegative fluorocarbon radicals. Ions are segregated
due to the chain size, following Flory−Huggins theory
and superseding weak electrostatic interactions between
highly spaced charges.
!
Burgo et al. Triboelectricity: macroscopic charge patterns formed by self-arraying ions on polymer
surfaces, Langmuir, 28(19), 7407-7416 (2012).

14. Francisco, K. R., Burgo, T. A. L., Galembeck, F. Tribocharged
Polymer Surfaces: Solvent Effect on Pattern Formation and
Modification. Chem. Lett. 41, 1256-1258 (2012).
• Procedure used to transfer charges from
PTFE to LDPE using paraffin oil as transfer
agent;
• Also, ethanol can be used to suppress
charges on a previous tribocharged PTFE
surface.
Charge transfer and
electrostatic lithograph

15. Mo<va<on
and
hypothesis
• Triboeletrifica<on:
glass
beads
over
PE
or
PTFE;
• Fric<on
generates
surfaces
with
both
posi<ve
and
nega<ve
paWerns;
• Since,
coulombian
forces
(long
range)
describe
interac<ons
between
electrical
charges,
how
these
charges
affect
fric<on
on
electrified
interfaces?

16. Tribology: science of friction
• “The
science
and
technology
of
interac<ng
surfaces
in
rela<ve
mo<on
and
of
associated
subjects
and
prac<ces.”
(Peter
Jost,
1966);
• Amontons’
laws:
– Fric,on
is
propor,onal
to
normal
load
– Independent
of
apparent
contact
area
Mate, C. M. Tribology on the small scale.
Oxford University Press, 2008.
Fall, et al. Sliding Friction on wet and dry sand.
Phys. Rev. Lett. (2014).

17. Real
contact
area
and
adhesion
• Connec<ons
from
micro
to
macro
scale
is
very
difficult;
• Real
x
apparent
contact
area;
• Elasto-­‐plas<c
deforma<ons;
• Adhesion:
van
der
Waals
forces
(only???):
“…The
primary
obstacle
to
inclusion
of
sliding
triboelectrifica<on
into
our
model
is
the
mysterious
and
complex
nature
of
the
process…”
Bowden, F. P. & Tabor, D. Friction and Lubrication. 2nd ed., Oxford (1954).
Nakayama, K. Wear 194, 185-189 (1996).
Ireland, P. M. J. Electrostat. 70, 524-531 (2012).

18. Mechanical
contact:
JKR,
DMT
and
Maugis
• JKR:
Adhesion
forces
change
contact
area
• DMT:
Contact
area
remains
the
same,
but
with
addi<onal
aWrac<ve
interac<ons
• Maugis:
contact
area
is
in
between!!!
Johnson, K. L.; Kendall, K. & Roberts, A. D. Proc. R. Soc. London A (1971).
Derjaguin, B. V.; Muller, V. M. & Toporov, Y. P. J. Colloid Interface Sci. (1975).
Maugis, D. J. Colloid Interface Sci. (1992).
with adhesion
aH aH
a a
without adhesion (Hertz)
van der Waals forces

19. …and
the
Coulombic
contribu<on?
“…The primary obstacle to inclusion of sliding
triboelectrification into our model is the mysterious
and complex nature of the process…”
Ireland, P. M. J. Electrostat. (2012).

20. Coefficient
of
Rolling
Resistance
(CoRR)
x
Electrosta<c
Poten<al
A"
B" C"h
d
CoRR = h/d
glass beads
tribocharged PTFE

21. CoRR:
silanized
glass
beads
a b
θ=25˚ θ=93˚
• Rolling coefficients are strongly
modified by the surface silanitazion
of glass.
θ = 15º θ = 93º

22. Movement
restric<on
Movement is restricted on electrified interfaces!!!
Beads on top Beads withdrawn

23. Fric,on
angle:
PTFE
x
PE
• Triboelectrification
between PTFE and
PE increases
friction angles
• Some PE pellets
does not slide even
at 90º. PTFE + PE pellets
after shaking
PTFE
PTFE + PE pellets
before shaking

24. Polyethylene pellets PTFE
Fric,on
angle:
PTFE
x
PE
PTFE + PE pellets
after shaking PTFE
PTFE + PE pellets
before shaking

25. Fric<on
at
a
microscopic
level:
Lateral
Force
Microscopy
• First
verified
by
Mate
et
al.;
• AFM
plahorm;
• Deflec<on
signal
is
a
qualita<ve
measurement
of
fric<on.
http://www.doitpoms.ac.uk/tlplib/afm/lfm.php

26. α∆´ =
(1+µ2)sinθcosθ
cosθ2 − µ2 sin2θ
αW´ =
µ
cosθ2 − µ2 sin2θ
µ + =
2∆´
W´sin2θ
1
µ
LFM
calibra<on:
volts
(V)
to
units
of
force
(N)
Ogletree, D. F., Carpick, R. W. & Salmeron M.
Calibration of frictional forces in atomic force
microscopy. Rev. Sci. Instrum. 67(9), 3298-3306 (1996).

27. Lateral
force
microscopy
(LFM)
• Friction is largely affected
by surface charges at the
nanometer scale;
• Fractal dimension D of
friction signal is bigger
than topography signal!!!

28. Force-­‐distance
curves
(Fd)
on
tribocharged
PTFE
In Geckos, each hair produces 100 nN (due to van der Waals and/or
capillary interactions)!!! Geim, et al. Nat. Materials (2003).
Burgo et al. Friction coefficient dependence on electrostatic tribocharging. Nature Sci. Rep.
(2013).

29. Parcial
Conclusions
• Tribocharges produced by friction have a large effect
on the friction coefficients of dielectrics:
– They may exceed all other factors for mechanical energy
dissipation;
• Controlling surface electrostatics should thus open the
way to new approaches for controlling friction in
many important systems and equipment.
• Since
tribocharge
paWerns
are
fractal,
their
contribu<on
to
fric<on
coefficients
is
also
fractal.

30. Bipolar
Tribocharging
Signal
During
Fric3on
Force
Fluctua3ons
at
Metal–Insulator
Interfaces

31. Escobar, JV, Chakravarty, A, Putterman
SJ, Diamond Relat. Mater. 2013
Akbulut M, Godfrey
Alig AR, Israelachvili
J. J. Phys. Chem. B.
2006.
Tribocurrent

32. Setup

33. Tribocurrent x friction force
vacuum nitrogen
hydrogen open air
1N normal load
50 cm/s speed
PTFE x Steel

34. Tribocurrent(nA)
Frictionforce(N)
1N 2N 5N
5 cm/s 5 cm/s 5 cm/s
25 cm/s 25 cm/s 25 cm/s
50 cm/s 50 cm/s 50 cm/s

35. Results Friction force fluctuations:
ü Fluctuations of friction force
occur with certain regularity in
tribological tests and Singer has
shown that this effect is generally
caused by the presence of third
bodies (material transfer).
Singer, et al., J. Vac. Sci. Technol. A 2003.

36. Tribocurrent and transient friction
force fluctuation
Although tribocurrent signal depends on speed and load, charged
species per force ratio is constant around 10 µC/N.

37. TRACK: negatively charged
Xerox® Cyan Developer powder (iGen3,
5R706). When placed on the sample, the track
promptly repeals the negatively charged toner
particles.

38. Quantitative NanomechanicsTM (QNM)
Quantitative material properties obtained by force curve analysis at
every pixel!!!
Z Position (~2KHz)
Peak Force Setpoint

39. Force distance curves
Clean PTFE
Charged PTFE

40. Adhesion mapping on PTFE track
Strongly adhered PTFE flakes
5 mm
Pull-off force: 150 nN
for most of the pixels!
PTFE
track 5x

41. Triboemission
(triboluminescence) under
Neon atmosphere

43. Burgo, T. A. L. & Erdemir, A.
Ang. Chem. Int. Ed., 53, 2014.

44. Concluding Remarks
n Friction force fluctuations are
always accompanied by two
tribocharging mechanisms at
metal-insulator interfaces:
– injection of electrons from the
metal to PTFE subsequently
followed by material/charge
transfer from PTFE to the metal
surface;
n Friction and triboelectrification
have a common origin:
– which must be associated with the
formation of strong electrostatic
interactions at the interface.

45. Prospects:
Controlling friction?
n The nature of the fundamental processes
that give rise to friction between sliding
bodies in close proximity is a long standing
question in tribology, both theoretically
and experimentally!!!
Park, J. Y., Ogletree, D. F., Thiel, P. A. &
Salmeron, M. Electronic control of friction in
silicon pn junctions. Science 313, 186–186
(2006).

46. Electrostatically stimulated additives?
Electrostatic potential
naturally built up at
interfaces under relative
motion should attract
charged molecules.
Moreover, an external dc
source must increase ionic
migration.

47. Tribocurrent at the nanoscale?
In progress…
AFM contact modes could be combined
with techniques for monitoring the
triboelectrification of surfaces, for
example by measuring the tribocurrent,
which would result in a powerful
complementary method to AFM.
Electrometer~
Computer
Tribocurrent image

48. Acknowledgements
n Department of Energy (DOE)
n Argonne National Laboratory
– Tribology Section
– Center for Nanoscale Materials

49. Unicamp
Instituto de
Química

50. “God made the bulk; surfaces
were invented by the devil.”
Wolfgang Pauli
Thank you…