The Mosaic of Surface Charge in Contact Electrification

TLDR: It is demonstrated that each contact-electrified piece develops a net charge of either positive or negative polarity, and each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions that accommodate significantly more charge per unit area than previously thought.

Abstract: When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material's surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. Whereas each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random "mosaic" of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.

Figures

Fig. 1. Possible scenarios of contact electrification and experimental KFM surface potential maps. The traditional view in (A, upper right) assumes that upon contact and separation, one surface charges uniformly positively and the other, uniformly negatively. In contrast, the mosaic picture in (A, lower right) assumes that each surface is a union of regions that have different propensities to accept or donate charge. Upon contact charging, these regions are then expected to develop different charge polarities. The mosaic picture of contact electrification is validated by the potential maps shown in (B)-(D). The map in (B) corresponds to PDMS before contact electrification. Such approximately uniform profiles were also observed for other “native” materials tested. In contrast, contact charged surfaces in (C) and (D) feature a “mosaic” of (+) and (-) regions. (C) PDMS charged negatively against another PDMS piece, (D) PC contact-charged positively against PDMS. The left column has 2D projections of the maps; on the right, the corresponding 3D maps are displayed. Color scale is adjusted to -1 V to 1 V in all images. Net surface charge densities on the contact-charged pieces (measured by Faraday cup prior to KFM measurements) were

Fig. 3. Geometric universality of charge “mosaics”. (A) A typical experimental KFM scan of electrostatic potential and the corresponding digitized map (“boundary set”) delineating regions of positive and negative potentials. (B) Illustrates calculation of the local box counting (LBC) dimension of a potential map whereby the “wiggly” lines correspond to the boundaries between (+) and (-) regions. The map is covered by grids of different box sizes (here, = 1,2,4). Next, for a given , the number of boxes, N( ), through which the wiggly lines/boundaries are passing is calculated – in the figure, these boxes are shaded in

Fig 4. Deducing charge density within the mosaic’s islands from high-resolution KFM scans of an electrified surface. (A) A typical KFM potential map (here, PDMS charged against PDMS); white line corresponds to the line over which the potential profile in (B) was taken (black line). Red, dotted curve in (B) gives a potential profile calculated above (at ~ 100 nm, corresponding to the elevation of the KFM tip) a plane presenting two oppositely charged, nearby “islands”. Theoretical potential fits the experimental profile assuming that each of the nanoscopic islands comprises ca. 500 elementary charges (corresponding to surface charge ~ C/cm2). Scale bar in (A) corresponds to 100 nm.

Fig. 5. The microscopic basis of charged mosaics. (A) Confocal Raman microscopy images of (left) an uncharged PDMS piece (using a gold coated silicon wafer as background); (middle) positively charged and (right) negatively charged PDMS pieces contact-charged against one another (in both cases, uncharged PDMS piece was taken as a background). Scale bars =2 m. (B) Total Raman spectra (R= Raman Intensity) of the regions mapped in (A). The peaks at 1642 cm-1 and 1710-1730 cm-1 for SiCH2COOH (marked with “*”), and between 1730-1830 cm-1 for SiCOOH and RCOOOH where R=Si

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Edinburgh Research Explorer
The mosaic of surface charge in contact electrification
Citation for published version:
Baytekin, HT, Patashinski, AZ, Branicki, M, Baytekin, B, Soh, S & Grzybowski, BA 2011, 'The mosaic of
surface charge in contact electrification', Science, vol. 333, no. 6040, pp. 308-312.
https://doi.org/10.1126/science.1201512
Digital Object Identifier (DOI):
10.1126/science.1201512
Link:
Link to publication record in Edinburgh Research Explorer
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Peer reviewed version
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Science
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Science 15 July 2011, Vol. 333 no. 6040 pp. 308-312.
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1
The mosaic of surface charge in contact electrification.
H. T. Baytekin, A. I. Patashinski, M. Branicki, B. Baytekin, S. Soh, B. A. Grzybowski*
Department of Chemistry and
Department of Chemical and Biological Engineering,
Northwestern University
2145 Sheridan Rd, Illinois 60208, USA.
* Correspondence to grzybor@northwestern.edu
ABSTRACT: When dielectric materials are brought into contact and then separated,
they develop static electricity. For centuries, it has been assumed that such
contact charging derives from the spatially homogeneous material properties (along the
material’s surface), and that within a given pair of materials, one charges uniformly
positively and the other, negatively. We demonstrate that this picture of contact charging is
incorrect. While each contact-electrified piece develops a net charge of either positive or
negative polarity, each surface supports a random “mosaic” of oppositely charged regions
of nanoscopic dimensions. These mosaics of surface charge have the same topological
characteristics for different types of electrified dielectrics, and accommodate significantly
more charge per unit area than previously thought.

2
Contact electrification (1-3), which is the transfer of charge between two surfaces that
are brought into contact and then separated, is one of the oldest areas of scientific study
dating back to Thales of Miletus and his experiments with amber charging against wool (4).
Although contact electrification has been successfully applied in several useful
technologies (e.g., photocopying (5), laser printing (6), and electrostatic separations(7)) and
chemical systems (8,9), remarkably little is known about the mechanism underlying this
phenomenon, especially in non-elemental insulators (1,10-15). In this context, it is
commonly assumed that (i) contact-charging derives from spatially homogeneous (on
length-scales larger than molecular) surface properties of contacting materials (1-3, 16-20)
and (ii) within a given pair of materials, one charges uniformly positively and the other,
uniformly negatively (1,7,15,21-24) (Fig. 1A-upper right). These assumptions, however,
make it difficult to explain numerous experimental observations whereby different particles
made of the same bulk material (25) or even different macroscopic regions of the same
sample (contact-charged (26,27) or probed using tips under bias (28, 29)) can exhibit
different charging characteristics. Here, we show that contact-electrified non-elemental
insulators are in reality random “mosaics” of positively (+) and negatively (-) charged
regions of nanoscopic dimensions (Fig. 1A-lower right). These mosaics are universal in the
sense as they comprise at least two characteristic length scales which are the same for
different materials. The mosaics accommodate significantly more charge per unit area than
previously estimated for contact electrification, but the overall/”net” charge on an
electrified surface remains relatively small due to the “compensation” between the (+) and
the (-) regions. In addition, the appearance of charge mosaics is accompanied by the
changes in surface composition and by the transfer of material between the contacting

3
surfaces. Overall, our results indicate that contact electrification cannot be attributed to and
predicted by the material’s homogeneous properties alone, as is often assumed when
constructing the so-called triboelectric series (13, 30, 31). Instead, control of contact
charging phenomena requires the control of the chemical and possibly micromechanical
properties at and near the surfaces of the contacting polymers.
The starting point of the present study is a recent observation that CE can occur between
flat pieces of identical materials (32). According to the conventional view of contact
electrification, this should not happen since the chemical potentials of the two
surfaces/materials are identical and there is apparently no thermodynamic force to drive
charge transfer. This scenario, however, assumes that CE is determined by the average
compositions and properties of the materials (reflected in the chemical potentials) and
completely neglects fluctuations from these averages. Indeed, a theoretical model
accounting for these fluctuations can explain charging between identical materials
assuming that each contacting surface is represented as a random “mosaic” of charge-
donating (D) and charge-accepting (A) regions and charge transfer occurs in places where
D and A overlap during contact (see Fig. 1A-lower right and, for further details, Ref. (32)).
While interesting in concept, the existence of charge mosaics has not been proven
experimentally, even for the case of identical materials. Furthermore, it remains to be
determined whether the mosaic picture is generalizable to contact-electrification between
different insulators and, if so, on what scales the mosaics form. If the “mosaic” model were
correct, then any contact-electrified surface should present itself as a union of (+) and (-)
regions.

4
To test this hypothesis, we used the Kelvin Force Microscopy (KFM) to image surface
potentials, , over various types of contact-electrified surfaces (e.g., polydimethylsiloxane,
PDMS; polycarbonate, PC; polytetrafluoroethylene, PTFE; silicon; aluminum; see
Supporting Online Material, SOM, Section 1). Concurrently, the overall/net charge on all
the materials before and after electrification was measured using a Faraday cage connected
to a high-precision electrometer (Keithley, 6517B). In all experiments, we verified that the
results did not depend on (i) the time of contact (for times from 2 sec to 1.5 hrs), (ii) the
pressure applied during contact (0.01 – 4.5 MPa) or (iii) the way in which the surfaces were
separated (e.g., rapidly or slowly peeled off one another). We note that experiments using
PDMS, in particular, rule out the possibility that uneven, “mosaic” charging would reflect
imperfect contact between the surfaces (since PDMS is known to come into conformal
contact with other polymers (33).
Figures 1B-D shows typical KFM maps. In all cases, the surfaces were not charged
before CE ( ~ 0; Fig. 1B). After contacting against other materials, however, the potential
maps comprised a mosaic of (+) and (-) regions. Such maps were observed for all
contacting materials (Figs. 1C and 1D), irrespective of whether the net charge was positive
or negative, indicating that the mosaic charging is a generic feature of contact-electrified
dielectrics. . As might be expected, the charges on the electrified pieces decayed with time
after contact electrification. Figure 2A illustrates that the decay of the net charge follows, to
a good approximation, first-order kinetics with the decay rate constants on the order of
~ 10
-3
s
-1
similar to those recorded by others (34). This macroscopic decay originates from
the discharging of the mosaics’ individual patches (Fig. 2B). Analysis of potential scans

Frequently asked questions

Q1. What have the authors contributed in "The mosaic of surface charge in contact electrification" ?

The authors demonstrate that this picture of contact charging is incorrect. 

Q2. What is the effect of the lateral charge migration on the surface of the mosaic?

In general, charge dissipation can be due to the so-called “external decay” via collisions with molecules in the surrounding atmosphere, or due to the “lateral” charge migration in the plane of the support (with concomitant “neutralization” of charges of opposite polarities) (35). 

Q3. What is the way to achieve contact between PDMS and other materials?

molecular-scale contact between PDMS and other materials provides thebasis for the family of micro-contact printing techniques, whereby molecules (e.g., alkane thiols) are transferred from PDMS stamps onto various substrates (e.g., gold) to form self-assembled monolayers that are densely packed and uniform over large areas. 

Q4. What materials were used in of their experiments?

The materials used in most of their experiments were aluminum foil (purchased from VWR international), polytetrafluoroethylene (PTFE, from McMaster-Carr, CAT# 8545K26), polycarbonate (PC, from McMaster-Carr, CAT# 8574K172) and poly(dimethylsiloxane) (PDMS). 

Q5. What was the XPS analysis of the materials used in the experiments?

X-ray photoelectron spectroscopy (XPS) analyses were performed with an Omicron ESCA probe, which was equipped with EA125 energy analyzer. 

Q6. What is the role of the RD equations?

Given the negligible role of lateral charge mobility, the RD equations can be simplified tosimple O.D.E.s and which, in conjunction with data such as thatin Fig. 2D, can be used to fit kinetic rate constants describing discharging of the (+) and (-) patches. 

Q7. What is the common method of contact between PDMS and other materials?

In this and other applications, conformal contact is most readily achieved by curing PDMS prepolymer against atomically-flat silica wafers (as in their experiments). 

Q8. What is the decay rate of a polymer?

The decay rate constants, , aredetermined from the slopes of the semi-logarithmic plots in the inset and are ~1×10-3 s-1for PDMS(+) and ~0.9×10-3 s-1 for PDMS (-). 

Q9. What is the kd’s of the spectral scan?

107, 407-410, 2003; Y. Hori, Journal of Electrostatics, 48, 127-143, 2000), kd’s are the rates of external discharge, kn represents the rate of (rapid) charge neutralization when the charges of the opposite polarities are found at the samelocation, and x = [0,L] is the domain of the problem corresponding to the directions along which the scans in Figs. 2C,D are taken. 

Q10. What is the difference between the LBC profiles and the experimental ones?

The observed shift of the LBC profiles merely indicates that for a decreasing scale of fluctuations in the examined potentials, a smaller box size has to be used in the LBC measure in order to detect the structural details of the boundary set. 

Q11. What is the lateral charge mobility in (B)?

In (B), the charge mobility is relatively large (here, D = 2× 10-16 m2/s), the profile decays and broadens with time, and its “zero” moves to ~1.5 µm. 

Q12. What is the kinetics of the discharge of a polymer?

(B) Typical KFM maps of a polymer (here, PDMS) before charging (t = 0 s), immediately after charging (t = 3000 s), and at two longer times t = 5000 and 8000 sec, when the charge within the mosaic dissipates. 

Q13. What was the XPS spectra of the materials used in the experiments?

The electroneutrality (i.e., lack of any detectable net charge) of all materials was confirmed by (1) measurements using a house-made Faraday cup connected to a high precision electrometer (Keithley Instruments, model 6517B). 

Q14. What is the effect of the scan depth?

Theintensity of these peaks decreases as the scan depth is increased from 50 nm to 2.0 m,confirming that contact-electrification affects predominantly the layer of the material near the surface – this observation agrees with multiple other studies on contact-electrification (16-19). 

Q15. What is the position of the zero?

In (C), charge mobility is significantly lower (D = 1× 10-18 m2/s), broadening is much less pronounced, and the position of the “zero” hardly changes.