Electric Field and Humidity Trigger Contact Electrification

1.6 MB·English

by  Yanzhen Zhang

Electric Field and Humidity Trigger Contact Electrification
Yanzhen Zhang1, Thomas Pa¨htz2,3,∗ Yonghong Liu1,† and Xiaolong
Wang1, Rui Zhang1, Yang Shen1, Renjie Ji1 & Baoping Cai1
1. College of Electromechanical Engineering, China University of Petroleum, 266580 Qingdao, China
2. Institute of Physical Oceanography, Ocean College, Zhejiang University, 310058 Hangzhou, China
3. State Key Laboratory of Satellite Ocean Environment Dynamics,
Second Institute of Oceanography, 310012 Hangzhou, China
Here, we study the old problem of why identical insulators can charge one another on contact.
We perform several experiments showing that, if driven by a preexisting electric field, charge is
5
transferredbetweencontactinginsulators. Thishappensbecausetheinsulatorsurfacesadsorbsmall
1
amounts of water from a humid atmosphere. We believe the electric field then separates positively
0
from negatively charged ions prevailing within the water, which we believe to be hydronium and
2
hydroxideions, such that at the point of contact, positive ions of one insulator neutralize negative

n ions of the other one, charging both of them. This mechanism can explain for the first time the
a observation made four decades ago that wind-blown sand discharges in sparks if and only if a
J
thunderstormis nearby.

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1 PACSnumbers: 65.40.gp,68.35.Md,72.20.-i,72.80.Sk,73.25.+i,73.40.Ns,82.30.Fi

]
t
f I. INTRODUCTION [26]. On the top of severaldunes, he saw sparks without
o
branchesextending fromthe groundstraightupto a few
s
. Contact electrification, which describes the phe- meters height in the air and clearly distinguished them
t
a nomenonthattwocontactinginsulatorscanacquireelec- from thunderstorm lightning. However, at days without
m tric charges when they are separated, is responsible for nearbythunderstorm,butsimilarlystrongwinds,Kamra
- numerous mysterious natural phenomena, such as the [26] mysteriously did not observe any such sparks. Re-
d generationofelectrifiedparticlesinwind-blownsandand cently, Ref. [15] also made similar observations in the
n
dust [1–4], lightning near volcanic dust plumes [5], and laboratory: The authors fluidized a particle bed filling a
o
c devastating explosions in grain and coal plants [6]. Con- glass jar. With fluidization the bed particles started to
[ tact electrification is also extensively exploited in indus- collidewitheachother. Theythenacquiredlargeelectric
trial applications, such as electrophotography [7], Laser charges when a pre-existing electric field generated by a
1
printing [8], 3D printing [9, 10] and electrostatic separa- van de Graaff generator was applied on the jar, but did
v
5 tions [11]. Despite this huge importance of contact elec- not so when the generator was turned off. The relative
5 trificationfornatureandindustry,itsunderlyingphysics air humidity in these experiments and Kamra’s observa-
2 remain elusive [12–16], even though it has been studied tion was of comparable magnitude. Ref. [15] proposed
3 since ancient Greece. The main difficulty is to under- and simulated a charge transfer mechanism to explain
0 stand how insulators, which by definition have no free their laboratory experiments: Under the presence of a
1. charge carriers, can charge one another on contact. In sufficiently strong pre-existing electric field, oppositely
0 fact, many complex contact electrification mechanisms, charged charge carriers gather at opposite hemispheres
5 based on electron transfer [17, 18], ion transfer [13], of the insulators before collision, so that at a collision
1 transfer of charged material [19], asymmetric partition- of two insulators the charge carriers within the colliding
:
v ingofhydroxideions[20,21],andnanochemicalreactions hemispheres neutralize each other, which charges both
i [22, 23] have been proposed, but the scientific debate colliding particles. The simulations further considered
X
remains controversial [19, 24, 25]. For instance, charge thatparticlesareneutralizedwhentheyhitthegrounded
ar transfersbetweeninsulatorsoftendependonthe contact bottom,whichallowednet-chargingtheagitatedparticle
mode(e.g.,pointcontact,areacontact,rubbingcontact) cloud, even though the charge transfer mechanism con-
andotherspecifiedconditions[24],with theconsequence serves charge. These simulations were in quantitative
thatcertainmechanismsmaybepredominantforcertain agreementwiththelaboratoryobservations,andthusof-
conditions, but negligible for other conditions. fered a possible explanation for Kamra’s [26] observa-
Moreover, certain phenomena, which are associated tions for the first time [2]. However, since Ref. [15] nei-
with contact electrification, remain insufficiently ex- ther specified the identity of the involvedchargecarriers
plained: For instance, in 1971 Kamra made the fas- northemannerinwhichthey movewithinthe insulator,
cinating observation of electric activity when a thun- a complete picture explaining Kamra’s [26] observations
derstorm blew over gypsum sand dunes in New Mexico has still been missing.
Here we investigate a contact electrification mecha-
nism,whichistriggeredbytheinterplayofapre-existing
∗ [email protected] electric field (E) and humidity, by experimental means.
† [email protected] As we explain in the following sections, we believe this
2
mechanism works in the following way: First, the insu- II. EXPERIMENTS WITHIN SILICONE OIL
lators adsorb water from a humid environment on their
surfaces. Then positively and negatively charged ions We carried out experiments within cells completely
dissolved in this water, we believe hydronium and hy- filled with silicone oil. These experiments are described
droxide ions, polarize and thus gather at opposite sides below(adetaileddescriptionoftheseexperimentscanbe
of the insulators due to the electric field. On contact found in Appendices A and B):
they neutralize the respective other species around the
contact domain. Finally, when separated, one insulator 1. We placedaglassbeadbetweentwochargedmetal
remains with most of the positively and the other one electrodes and observed that it bounced forth and
with most of the negatively charged ions. Apparently, back between these electrodes. We measured the
theheredescribedchargetransferalongandbetweenthe absolute value of the chargeof the glass bead after
insulators resembles the aforementioned charge transfer contactwiththeseelectrodesasafunctionofE(see
mechanism of Ref. [15]. However,since also the identity Fig. 1a). These experiments confirm that charging
of the involved charge carriers and the manner in which of the glass bead occurs during the contact with
they move along the insulator surface are specified, our these electrodes.
studies offers a first complete explanation for the afore-
2. We placed either two or four identical glass beads
mentioned observations of Kamra [26].
between these electrodes and observed that they
bounce between each other and the electrodes (see
Our study indicates that, at least for point contacts Figs.1band2,andSupplementaryMovie1). Inthe
(e.g., interparticle collisions), the mechanism we investi- caseoftwoglassbeads,wemeasuredthechargesof
gatedseemstobemuchstrongerthanothercontactelec- bothbeadsbeforeandafteracollision(seeFig.1b).
trificationmechanismsiftheelectricfieldandtherelative These experiments confirm that a large amount of
humidity are sufficiently large. Interestingly, humidity charge is transferred between the two glass beads
and thus hydronium and hydroxide ions due to the dis- at contact since the total charge of the two beads
sociationofwaterhavebeenassociatedwithalargevari- remains roughly constant.
etyofelectrostaticeffectsandevenbeenidentifiedaskey
3. While in experiments 1 and 2 the glass beads were
components in contact electrification in many previous
stored in air before placed into the silicone oil, we
studies[13,20,21,29–34]. Humidityhasalsobeenlinked
nowbakedthemforseveralhourstogetridofmost
to the pick-up of insulating particles from a grounded
of the water on their surfaces. After baking no
particle bed [35] or from a grounded conductive plane
bouncing was observed, confirming that the pres-
[36] by pre-existing electric fields. In fact, in laboratory
ence of water on the surface of the glass beads is
experiments such particles were lifted by strong electric
necessary for the their charging.
fields in the presence of sufficient humidity. It was spec-
ulatedthatthishappensbecausetheparticleschargevia 4. We hungtwofibersdownabar. Atthe endofeach
induction (i.e., the redistribution of electrical charge in fiber a glass bead was placed, and the distance be-
a conductor due to electric fields) due to water on their tween the two fibers was exactly one bead diam-
surfaces adsorbed from a humid environment. Indeed, eter, meaning that the glass beads were initially
the water makes these surfaces conductive, resulting in just in contact. If we activated a strong electric
anupwardly-directedliftingforceduetotheelectricfield field (E = 2.5kV/cm) directed normal to the con-
[37]. Note that this speculated lift mechanism strongly tact plain, the glass beads separated (cf. Fig. 3b),
resemblesourcontactelectrificationmechanism. Theim- whilenoseparationoccurrediftheelectricfieldwas
portant difference is the contact time: It is not a priori absent (cf. Fig. 3a) or directed parallel to the con-
clearthattheliftingmechanism,inwhichparticlesarein tact plane (cf. Fig. 3c). These experiments con-
enduringcontactswiththeirconductivesurroundingsbe- firm that a pre-existing electric field is responsible
foretheyareliftedfromthesurface,canbegeneralizedto for the charging of the glass beads, and that the
a contact electrification mechanism working for general relevant component is the one in direction normal
contacts of insulators, including comparably very brief to the contact plane. Moreover,we now varied the
contacts during interparticle collisions in midair. More- component of E in direction normal to the con-
over, it is actually unclear why adsorbed water on the tactplaneandthen,regardlessofwhethertheglass
particle surfaces makes them conductive because exper- beadsseparatedornot,manuallyremovedonebead
imental studies have shown that water films on parti- including its fiber from the setup after turning on
cle surfaces may not be continuous, but instead covered the field. After the removal, we increased the elec-
by a multitude of small water islands, even for very hy- tric field to a high value (E = 3.75kV/cm) and
drophilicparticles[27,28]. Inourstudy,weproposethat measuredtheabsolutevalueofthechargeofthere-
adsorbed water may make the insulator surfaces quasi- maining bead indirectly from its displacement due
conductive even if the water films are discontinuous be- to the Coulombforce. We found that it is approxi-
cause charge might be exchanged between these water matelyproportionaltotheelectricfieldcomponent
islands and water vapor surrounding the insulators. indirectionnormaltothecontactplane(seeFig.4).
3
FIG. 2. Experiments within silicone oil: Four glass beads
(r = 1mm) bouncing between two charged metal electrodes
(E =2.5kV/cm). Thearrowsindicatethelocationsofcharge
exchange during each bounce. All pictures show the view
from the top of thecell.
FIG.3. Experimentswithinsiliconeoil(a-c): Twocontacting
glass beads (r =1mm) placed on fibers hanging down a bar.
(a)Separationoftheglassbeadsdidnotoccurintheabsence
of an electric field. (b) Separation did occur under the in-
fluence of an electric field (E = 2.5kV/cm) directed normal
FIG. 1. Experiments within silicone oil: (a) Absolute value
to the contact plane. (c) Separation did not occur underthe
of the charges of a glass bead (r = 1mm) and a water drop
influenceofanelectricfielddirectedtangentialtothecontact
(r=1mm), bouncing between two charged metal electrodes,
plane. (d-f) Charge transfer mechanism of hydrophilicparti-
aftercontactwiththeelectrodesversuselectricfieldstrength
clesincontact with eachother. Notethatthewaterfilmwas
(symbols). The charges were estimated from a balance be-
amplified and drawn as continuous for illustration. The real
tween the viscous drag and Coulomb forces, while the solid
water film is very thin and may not be continuous.
line displays the theoretical charge of a perfectly conducting
sphere (r =1mm) [38]. (b) Positions relative to the cathode
andchargesoftwoglassbeadscollidingbetweentwocharged
metal electrodes (E = 2.5kV/cm) before and after their col- O dissolved in the adsorbed water can considerably in-
2
lision (Q1+Q2≈q1+q2). creasethesurfaceconductivityofundopeddiamond[39]).
Tominimizethelikelihoodofchemicalreactionswiththe
glassbeadsurface,weredidtheexperiments1and2,but
III. IDENTITY OF CHARGE CARRIERS this time wesubjectedtheglassbeadstoa humidN at-
2
mosphere before placing them into the silicone oil. In
What are the charge carriers giving the glass beads fact, we observed exactly the same bouncing behaviors
their charges? The fact that the glass beads must have as before. Since N is a very inert gas and thus very un-
2
been subjected to a humid atmosphere prior to the ex- likely to react with the surface of the glass beads, ions
periments to receive charges in contacts indicates that dissolvedintheadsorbedwaterremainasthemostlikely
the surfacesofthe glassbeads haveadsorbedwaterfrom charge carrier candidates. This is further supported by
the atmosphere(seeAppendix Dfordifferentadsorption our measurements showing that the absolute charge of
mechanisms). Hence, the charge carriers are either ions the insulator after contact with the electrodes as a func-
dissolved in the adsorbed water or charge carriers pro- tion of E resembles that of a water drop (see Fig. 1a),
ducedbyreactionsbetweentheinsulatorsurfaceandsub- indicating that the contact electrification mechanism is
stancesdissolvedwithintheadsorbedwater(forinstance, notrelatedtochemicalpropertiesoftheglassbead. How-
4
C]10−9 ions (cf. Fig. 3f).
[ The charge transfer mechanism described above re-
Q
quires that the ions can be transported along the in-
ad, MQ/eCasu=re1m.6e5n×ts10−10E/(kV/cm) sulator surface. One possible transport mechanism is
e
b10−10 that the ions can freely move in a continuous water film
ss coating the insulator surface. However, this possibility
a
gl seems unlikely since experiments have shown that water
of films on particle surfaces may not be continuous, even
e
g for very hydrophilic particles [27, 28]. Instead particle
har10−11 surfaces might be covered by a multitude of small wa-
c
ter islands [27, 28]. We thus speculate that the ions in-
e
t
u stead might hop from water island to water island. This
ol
s might be possible because each water island might be
b
A10−12 10−1 100 apbolre. tIonefxaccht,acnhgaercgheatrrgaensswfeirthbetthweeseunrrmouetnadlinsugrwfaacteesravnad-
Horizonzal component of electric field, E [kV/cm]
surroundingwater vapor under high humidity have been
confirmed experimentally [21]. Since the lifetime of ad-
sorbed water molecules is of the order of milliseconds
FIG.4. Experimentswithinsiliconeoil: Twocontactingglass
[40],thereshouldalwaysbewatervaporsurroundingthe
beads(r=1mm) placedon fibershangingdown abar. Mea-
particles, even in our silicone oil experiments.
surements of the absolute value of the charge of the glass
beads as a function of the electric field component in direc-
tion normal to the contact plane after removal of one of the
beads. V. SENSITIVITY OF CHARGE TRANSFER TO
SURFACE MATERIAL PROPERTIES
ever, one should be aware that this resemblance might
Whatever the exact charge carrier transport mecha-
be coincidental. Concerning the identity of the ions dis-
nism might be, it is at least clear that it is the more
solved in the adsorbed water, we believe, like many pre-
efficient the more water is available on the insulator sur-
viousstudiesundersimilarcircumstances[13,21,29–32],
face. Thissuggeststhattheaforementionedchargetrans-
that the majority of them are hydronium and hydroxide
fer mechanism works better for hydrophilic materials,
ions due to the reaction 2H O⇋H O++OH−, especially
2 3 such as glass, than for hydrophobic materials, such as
in the cases in which the glass beads where stored in a
polyethylene (PE), polystyrene (PS) and Polytetrafluo-
humid N atmosphere. This is because in water satu-
2 roethene (PTFE). Hydrophilic materials are character-
rated with nothing but N , other ions seem unlikely to
2 izedbylargefreesurfaceenergies,allowingthemtoform
be created by standard chemical reactions. Nonetheless,
attractive bonds with the water molecules [40]. We esti-
itcannotbeexcludedthatotherions,suchasHCO− due
3 matedthe freesurface energyofthe insulatorsindirectly
to remnants of CO dissolved in the water or ions from
2 bymeasuringthecontactanglebetweena1µLwaterdrop
substrates with ion bonds contaminating the insulator
andtheinsulatorsurfacewithlargecontactanglescorre-
surface,aresignificantlyorevenpredominantlyinvolved.
spondingtosmallfreesurfaceenergies. Indeed,wefound
that the larger was the contact angle the more difficult
was the charge transfer between contacting insulators or
IV. CHARGE TRANSFER MECHANISM between insulators and electrodes (see Fig. 5). We con-
cluded this from quantitative measurements of the ab-
Knowing that the charge carriers are most likely ions solute charge of spherical beads after contact with the
dissolvedinwateradsorbedontheinsulatorsurface,itre- electrodes (see Appendix C) and from qualitative obser-
mainstoanswerthequestionwhathappenstotheseions vations of the bouncing behavior of objects (see Supple-
inanelectricfieldbefore,atandaftercontact. Assuming mentary Movie 2) for different dielectric materials: For
that these charge carriers can be transported along the instance, bouncing between the electrodes did not occur
insulator surface, the electric field, if sufficiently strong, for the most hydrophobic materials PE, PS and PTFE.
will makethe negatively(positively)chargedionsgather Also, when PE and PS particles came in contact with
at the hemisphere closest (farthest) from the anode (cf. the electrodes, they remained in contact and rolled on
Fig. 3d). At contact, presuming the electric field is nor- the electrodes for a while before eventually separating
mal to the contact plane, the positively charged ions of (see Appendix E for a possible mechanism). PTFE par-
one glass bead neutralize the negatively charged ions of ticles, which aremore hydrophobicthan PE andPS, did
theotheronewithinasmallwaterbridgeformingaround not separate at all from the electrodes after contact. In
thecontactpoint(cf. Fig.3e). Whentheglassbeadssep- fact, by measuring the average thicknesses of the water
arate,oneglassbeadremainswithmostofthepositively covering the surfaces of PTFE and silicon oxide parti-
and the other one with most of the negatively charged cles (as a representative for hydrophilic materials) as a
5
the absence of contactelectrification,for instance due to
previously described mechanisms, because the charging
mayhavebeentoo weakto be noticedinourexperimen-
tal setups. However, they do imply that the charging is
muchweakerincomparisonto the casesinwhichbounc-
ing occurs and glass bead displacement is significant, re-
spectively. This means that, at least for point contacts
(e.g., interparticle collisions), the mechanism we investi-
gatedseemsto be muchstrongerthanothermechanisms
if the electric field and the relative humidity are suffi-
ciently large. This particular conclusion is supported by
a recent study on contact electrification [41], which ex-
perimentallyinvestigatedthebuild-upofelectricfieldsin
an agitated particle bed in the absence of a pre-existing
electric field. This study reported that the agitated bed
doesnotbuildupanelectricfieldiftherelativehumidity
is too large (> 45%). Assuming the correctness of our
interpretationof howour contactmechanismworks,this
can be easily explained since our contact electrification
mechanismtendstodissipateexistingelectricfields. This
means that any attempt of the agitated bed to build up
an electric field is countered by the tendency of the field
tobe dissipateddue toourmechanism. Thistendency is
FIG.5. Difficultyofchargetransfer(qualitatively)versuscon-
the stronger the larger the humidity, explaining the ex-
tactanglebetweena1µLwaterdropandtheinsulatorsurface
istence ofacriticalhumidity beyondwhichelectricfields
forseveraldielectricmaterialsmeasuredwithanOpticalCon-
cannot be built up anymore.
tact Angle Measuring Device. Inset: Number of monolayers
ofwater moleculescomposing thewater filmson thesurfaces
of silicon oxide and PTFE, measured with an ellipsometer,
versusair humidity.
Moreover, it has been shown that the kind of charge
transfer mechanism we describe in Figs. 3d-f can lead to
function of air humidity, we found that the average wa-
a huge build-up of charge in wind-blown particle clouds
terthicknessonthesurfaceofPTFEparticlesisbetween
if the pre-existing electric field is sufficiently strong [15].
one and two monolayersof water molecules almost inde-
Infact,eventhoughthischargetransfermechanismcon-
pendentoftheairhumidity(insetofFig.5). Thisisvery
serves the total charge, a charge build-up can occur due
probablytoothin toallowefficientiontransportandex-
to settling down and neutralization of charged particles
plainswhycontactelectrificationforPTFEparticleswas
at the surface [15]. This might explain why Kamra ob-
much weaker than for hydrophilic particles. The abil-
served sparks at the top of gypsum dunes if and only if
ity of a PTFE particle to adsorb water can be increased
a thunderstorm was nearby [2, 26]: In fact, the thunder-
by immersing it in saturated sodium dodecyl benzene
stormprovideda sufficiently strongelectric fieldand hu-
sulphonate (SDBS) ethanolsolution. After doing so, the
midity (Kamra reported E ≈ 0.04kV/cm and 36−58%
contactanglebetweenthe1µLwaterdropandtheparti-
clesurfacedecreasedfromabout120◦ toabout15◦,indi- humidity) triggering our contact electrification mecha-
nism. Since,accordingtoourinterpretation,thismecha-
catingthatahydrophilicmolecularlayerwithalargefree
nismisbasedonthechargetransfermechanismdescribed
surface energy assembled on the PTFE particle surface.
in Figs. 3d-f, it led to the aforementioned huge charge
Ourmeasurementsshowthat,aftersurfacemodification,
build-up and thus to highly electrified particles, which
thePTFEparticlesbouncedbetweentheelectrodes,just
eventually discharged in sparks.
like hydrophilic particles (see Supplementary Movie 3).
VI. CONCLUSION
Finally, the fact that the charging of hydrophilic insu-
What is the relevance of the contact electrification lators in electric fields seems to resemble that of water
mechanism we investigated in comparison to previously drops (see Fig. 1a) might allow controlling contact elec-
described mechanisms? To answer this, we emphasize trification by controlling the electric fields and the com-
that the absence of bouncing of glass beads between the positions of the insulator surfaces. Exploiting this fact
electrodes in experiments 1-3 or undetectable glass bead mightopen new opportunities for industrialapplications
displacement in experiment 4. do not necessarily imply of contact electrification in the future.
6
APPENDICES
A. Detailed description of experiments 1, 2, and 3.
The experimental setup used in experiments 1, 2, and
3 is illustrated in Fig. A1. Two identical copper elec-
trodes were placed in a Perspex cell filled with silicone
oil. The width, height, and thickness of the electrodes
were 16mm, 20mm, and 3 mm, respectively. The dis-
tance between the electrodes was 16 mm, the viscosity
(µ) of the silicone oil was 100 mPa.s at 25◦C. The high
voltage power can be set to values between 0 and 10kV,
therefore the electric field (E) can be set to values be-
tween 0 and around 6.25kV/cm. A high speed camera
was used to record the bouncing movements of the glass
beadsorthewaterdropsbetweentheelectrodes. ALED
cool light was used in order to avoid heating up the sil-
icone oil. In the water drop experiments (see Fig. 1a), FIG. A1. Illustration of experiments 1, 2, and 3.
a pipette was used to generate water drops with a ra-
dius(r) of1mm,whichisequaltothe radiusofourglass
beads. Alltheexperimentalrunsweredoneatroomtem- with constant settling velocity within the silicone oil
perature (25◦C). (G − F + nπµrU = 0, where G is the magnitude of
b z
An experimental run was started by turning on the the gravitational force on the glass bead, F the mag-
b
electricfieldandthenplacingtheglassbead(s)orthewa- nitude of the buoyancy force on the glass bead, and z
terdrop(s),respectively,onebyone,butinfastsequence the verticaldirection). It has been shown that Eq. (1) is
without interruption, onto the surface of the silicone oil. alsofulfilledforwaterdrops,eventhoughtheyareliquid,
They sink in and it takes around 1s−2s time until the with a coefficient of n≈4 [43, 44]. Indeed, from a single
glass beads hit the bottom, which is when we started water drop falling with constant settling velocity within
capturing their movement with our camera. During the the silicone oil, we determined n=3.91. Knowing n, we
experimental run, the glass beads slide or roll along the determinedthecharge(Q)oftheglassbeadorthewater
bottom and thereby bounce between the electrodes and drop, respectively, within our electric field (E) from the
eachother. Inexperiments1 and2,the glassbeadswere electrodes through QE −nπµrU = 0, where x is the
x
exposed to air with 50% relative humidity for two days horizontal coordinate, as
beforetheexperimentalrunwasstarted,whiletheywere
baked in experiment 3. The initial horizontal movement nπµrUx
Q= , (2)
oftheglassbeadsinexperiments1and2,beforetheyhad E
their firstcollisionwith the electrode or anotherbead, is
whereby we neglected the frictional force from the slid-
due to a small amount of excess charge of the adsorbed
ing or rolling motion on the bottom because it is typ-
water [42]. Due to the absence of this charged water in
ically much smaller than the drag force. We indirectly
experiment 3, we put the glass beads initially in contact
confirmedthisbyperforminganexperimentaltestrunin
with the electrodes and/or with each other, but no hori-
which the glass beads bounced vertically and thus with-
zontal movement occured at all, and all particles settled
out wall friction instead of horizontically. In this test
down at the bottom after a few seconds.
run, we measured virtually the same charges.
From the images captured by the camera, we deter-
minedthevelocitiesoftheglassbeadsorthewaterdrops,
respectively. Once obtained these velocities were further
used to determine the charge of the glass beads or the B. Detailed description of experiment 4.
water drops, respectively, as we explain in the following:
Since silicone oil is a highly viscous fluid and the veloci- The experimental setup used in experiment 4 is illus-
ties of the glass beads are small (the oil was at rest), we trated in Fig. B1. Two identical glass beads (r = 2mm)
can calculate the fluid drag force (Fd) on a glass bead were hang down a bar using fibers with 10µm diameter
using Stokes law, and 55mm length. The glass beads were initially just in
Fd =−nπµrU, (1) contact and put between two identical copper electrodes
within a Perspex cell filled with silicone oil. The width,
where U is the velocity of the glass bead and n a shape height,andthicknessoftheelectrodeswere20mm,25mm
factor which takes into account that the glass bead is and 3mm, respectively. The distance between the elec-
not a perfect sphere (n = 6 for a perfect solid sphere). trodes was 32mm, such that the electric field could be
We determined n =6.12 from a single glass bead falling set to values between 0 and about 4KV/cm. A camera
7
FIG. B2. Sketch of glass bead displacement due to electric
field.
FIG. B1. Illustration of experiment 4.
C. Experiments behind Fig. 5
was used to record the position of the glass beads dur- Fig. 5 and its insets contains three types of informa-
ing the experiments. The glass beads were exposed to tion. First, it showsthe contactangle betweena 1µlwa-
air with 50% relative humidity for two days before the ter drop and the insulator surface for different dielectric
experimentalrunwas started. All the experimentalruns materialsmeasuredwithanOpticalContactAngleMea-
were done at room temperature (25◦C). suring Device. Second, it shows the average thickness of
An experimental run was started by turning on the the water film for silicon oxide and PTFE as function
electric field. If the electric field component in direction of air humidity measured with an ellipsometer. Third,
normal to contact plane (E) was stronger than about it shows the difficulty of charge transfer for different di-
2kV/cm, the two glass beads separatedfrom each other, electric materials obtained from qualitative observations
otherwise they did not separate (cf. Figs. 3a-c) because ofthe bouncing behaviorofobjectsofdifferentdielectric
the attractive force (cohesion and/or adhesion) between materialsusing the experimentalsetupofexperiments 1,
the two beads was too strong. Still, even if they did not 2, and 3 described above (see Supplementary Movie 2).
separate, they acquired electric charge whose value we These qualitative observations were further backed up
determined as function of E (see Fig. 4) in the following by quantitative measurements of the charge of spherical
way: Regardless of whether the glass beads separatedor glass,Nylon,PE,PS,andPTFEbeadsaftercontactwith
not, we manually removed one bead including its fiber the electrodes (E = 2.5kV/cm). In fact, Fig. C1 shows
from the setup after turning on the field. Afterwards we the portion of the charge
increased E to a high value (3.75kV/cm) and measured
π2
the displacement (d) of the bead due to the Coulomb Q = 4πr2ǫ ǫ E (5)
theory o r
forcebycomparingthecamerapictureswithandwithout 6
electric field. From d we obtained the charge indirectly an ideally conducting sphere with radius r would obtain
from the force balance after contact with the electrodes, where ǫ is the per-
o
mittivity of vacuum and ǫ the relative permittivity of
r
QE =F =(G−F )tan(θ) (3) silicone oil. It can be seen that the more hydrophilic the
c b
material the larger is the charge after contact with the
(see Fig. B2) as electrodes (cf. Fig. 5).
(G−F )tanarcsin(d/l)
b
Q= . (4) D. Water adsorption from a humid atmosphere
E
We note that, if the electric field was weaker than The amount of water adsorbed by an insulator from
0.2kV/cm, we used longer fibers (215mm) in order to a humid atmosphere depends not only on the degree of
better detect the displacement of the glass bead. Fur- humidity, but also on the chemical properties of the in-
thermore, we wish to emphasize that we verified using sulator surface. For instance, ionic insulators, such as
the same method that the glass beads did not charge if NaCl and mica, adsorb water due to electrostatic inter-
they were not initially in contact with each other. actions between the electric fields generated by the ions
8
and much less water vapor is located near the insula-
tor surface. This makes charge transport from water
island to water island via the aforementioned exchange
withsurroundingwatervapormuchmoredifficultforhy-
drophobic particles than for hydrophilic particles. We
illustrated this in Fig. E1 through drawing isolated wa-
ter islands, while the water film on hydrophilic particles
was drawn as continuous in Figs. 3d-f to illustrate that
charge transport along it is easily possible. These wa-
ter islands, however, still contain small amounts of ex-
cess charge [47] and are either positively or negatively
charged (for illustration purposes, the charges are only
positive in Fig. E1a). On contact with the cathode, the
water island at the contact point becomes equipotential
FIG. C1. Charge of spherical beads of different dielectric
materials after contact with theelectrode.
FIG.E1. (a-d)Chargingmechanismofahydrophobicparticle
in contact with thecathode.
of the insulator and the dipole and higher moments of
the water molecule [45], while hydrophilic covalent insu-
with it (cf. Fig. E1b). Moreover, asymmetries in the
lators,such as glassand quartz, adsorbwater due to the
charges of the water islands or in the particle shape in-
generation of strong hydronium bonds [46]. Hydropho-
duce torsion, and the particle thus rolls under its action
bic insulators,such as PE,PS, and PTFE, which do not
(cf. Fig. E1c). During this rolling process, more and
adsorbwaterby eitherofthese mechanisms,canstillad-
more isolated adsorbed water domains become equipo-
sorbwaterduetoubiquitousdispersioninteractions[46],
tential with the cathode, and the particles acquire thus
however to a much smaller extent than with the other
morenegativecharges(cf. Fig.E1c). Oncethenetcharge
two mechanisms.
of the insulator becomes negative, the particle separates
from the cathode (cf. Fig. E1d).
E. Possible mechanism for contact electrification of
hydrophobic insulators
ACKNOWLEDGEMENTS
Even though contact electrification was strongly re-
sisted by the hydrophobic insulators made of PE and We acknowledge support from grants NSFC No.
PS,itstilloccurred. Herewehypothesizehowthecharge 51275529, NSFC No. 41350110226, Science and Tech-
transfer mechanism might have looked like in case of an nology Development Plan of Qingdao City No. 12-1-4-
insulator-electrode contact. First, we note that the size 7-(2)-jch, and Taishan Scholar Construction Project of
of the water islands on the surfaces of hydrophobic in- Shandong Province No. TS20110823. We thank Fer-
sulators is much smaller than on hydrophilic insulators, nando Galembeck for discussions.
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