Measuring contact charge transfer at interfaces: a new experimental technique

Journal of Electrostatics, 26 (1991) 291-308

291

Elsevier

Measuring contact charge transfer at interfaces: a new experimental technique Douglas T. Smith Ceramics Division, Bldg. 223, Rm A335, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA (Received July 1, 1990; accepted August 2, 1991)

Summary A new experimental technique has been developed for measuring *he amount of electric charge transferred when two dissimilar surfaces are brought into contact and separated without lateral motion in a controlled environment. The primary surfaces studied, silica and mica, are prepared with very low surface roughness (average roughness less than 0.5 nm); the macroscopic contact area, which can be measured directly, is therefore a better measure of microscopic contact area than is typically the case in contact electrification experiments, and the surface charge density can be more accurately determined. Observations reported include (1) the transfer of charge densities as high as 1 × 10 -2 C/m 2 after only a few contacts of a mica and silica surface in dry air or N2 gas at room temperature and (2) a reduction in transferred charge density in the silica-mica system as the relative humidity of the air in the environmental chamber is increased from 0% to 98%. The technique also permits the study of the rate at which the surface charge decays under various environmental conditions.

1. Introduction The study of the transfer of electronic charge to or from an insulating surface as the result of contact with another material has been a subject of great interest for many years [1,2 ]. Charge transfer has been observed in a wide variety of systems, including the contact of polymer surfaces with both solid [3-12 ] and liquid [ 13-15 ] metals, the contact of metals with other insulators including glass [4,16-18] and diamond [ 19,20], the contact of semiconductors with metals [1,21,22 ] and insulators [23 ], and the contact of two insulators [ 1,2426]. These phenomena are of interest because they provide a means to study the mechanisms by which charge is transferred from one su~'face to another [3,4,5,7,16,27-35], the nature of the states in which charge resides in the insulator [36-41], the depth of the surface charge layer [13,28,42], and the mechanisms by which charge is (or is not) able to migrate away from the contact region, either through the bulk, across the surface, or into the external environment, upon separation of the surfaces [ 19,20,43-46 ]. 0304-3886/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

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A number of techniques have been developed to measure surface charge density [47,48 ]. The simplest technique involves the use of a conducting proof plane which is held at ground potential through an electrometer circuit. The proof plane is brought close to the surface to be measured and the amount of charge flowing into or out of the proof plane in response to the field from the surface charge is measured by the electrometer. In the limit where the proof plane can be brought very close to the surface charge to be measured, such that the effects of edges and other ground surfaces are negligible, the amount of charge seen by the electrometer is equal to the surface charge. When studying thin dielectric films, the proof plane can be bulk metal or a metallic film in physical contact with the back side of the dielectric film [ 13 ]. Such electrometer ground plane measurements have two main drawbacks: first, an appropriate placement of the ground plane can be difficult or impossible in some experimental arrangements; and second, electrometer circuits always suffer some degree of drift as a result of small stray currents in the experimental leads and the integration of the input bias current of the electrometer amplifier. These drifts pose problems when measuring very small charge densities or when a stable absolute measurement of surface charge is required over an extended period of time, as for example when monitoring the decay of surface charge. The first problem can be mitigated by the use of a smaller, guarded electrometer probe [49 ] which, although it does not directly sense all of the charge on the surface, can be calibrated to measure surface charge density through the use of an experimentally-determined geometrical factor involving the probe diameter and distance from the surface. Such a probe technique has the additional advantage of providing lateral resolution in the measurement of surface charge which again depends on the geometry of the probe and its position relative to the surface. The problems of electrometer drift can be avoided by periodically checking the "zero" of the electrometer circuit. This is typically accomplished by alternating readings from charged and uncharged (grounded) regions, either by revolving samples under a probe [5] or by periodically interposing a ground plane between the charged plane and the electrometer lead, which could be either a small probe [5 ] or a larger plane [50 ]. Results have also been reported from a technique which uses an Atomic Force Microscope (AFM) [51 ] to image the charge on an insulating surface. Charge is deposited either by applying a pulsed bias voltage (100 V) between the AFM tip and a conducting plane beneath a variety of insulating samples [52 ], or by contacting a polymethyl methacrylate surface with an unbiased (0 V ) Ni tip or a silicon surface (with the native oxide) with 0.3/~m polystyrene spheres [53 ]. In addition to its use as a basic research tool to study electronic states in insulators, an understanding of the mechanisms of surface charging is important in a variety of fields as diverse as spacecraft charging, xerography and the prevention of spark discharge in hazardous environments. An area which has

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stimulated considerable debate over the years has been the importance of surface charge transfer on the adhesion between dissimilar materials, for example at a metal/glass interface or a grain boundary in a composite material. Strong electrostatic forces at such interfaces, if they exist, could play an important role in determining the fracture properties of the interface; this is particularly true in brittle materials, where most of the work required to propagate a crack goes into creating new surface area at the interface. The debate focuses on the relative importance of electrostatic forces versus van der Waals forces at the interface, with some workers (Derjaguin and co-workers in particular) claiming that electrostatic forces play a major role in adhesion [23,24,54-56 ], while others claim that electrostatic forces, although present, are significantly weaker than van der Waals forces in all but exceptional cases [57,58 ]. Indirect but compelling evidence for large charge separation during the fracture of a variety of interfaces comes from the fracto-emission experiments of Dickenson and co-workers [59-63 ]. In this report, we describe a new experimental technique which permits the simultaneous study of contact charging and force measurement between surfaces much smoother than those typically used in charge transfer experiments. The amount of charge transferred is seen to "saturate'quickly (typically in fewer than 10 contacts), and charge densities as high as 1×10 -2 C/m 2 are observed in a dry nitrogen environment for silica-mica and silica-silver contacts. The resultant attractive force measured between the surfaces under those conditions is observed to be significantly stronger than the van der Waa!s force, even at separations smaller than 100 nm, where van der Waals forces might be expected to dominate. The technique also permits the study of the dissipation of transferred charge in a variety of vapor environments; preliminary rev,llts are presented on the effects of humidity on both the amount of charge transferred and the rate of decay of that charge in the silica-mica system. The rate of decay is seen to increase by almost two orders of magnitude when the relative humidity (RH) is increased from 0% to 11%, although the amount of charge initially transferred does not change significantly over that range.

2. Experimental technique 2.1. The Surface Force Apparatus The experimental system to be described is based on the Surface Forces Apparatus (SFA), a device developed primarily be Israelachvili [64 ] to measure the forces between molecularly smooth solid surfaces in a variety of environments including dry air or nitrogen, partial pressures of other vapors, or bulk liquids (Fig. 1). The substrates to be studied are prepared as thin sheets (typically 2/~m to 10/~m thick) which are coated on the back surfaces with 50 nm of evaporated silver and then glued to cylindrical lenses of radius R ~ 20 mm which are mounted in the SFA with their axes at right angles (Fig. 2 (a) ).

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Two mechanical stages and a final piezo-electric stage are used to control the surface separation to better than 0.1 nm. The force between the surfaces is determined by measuring the deflection of the double cantilever spring supporting the lower surface. Optical interferometry is used to determine the separation profile of the two surfaces; white light is incident on the crossed-cylinder system from below, as shown in Fig. 2 (a), and light which passes through the interferometer formed by the silver layers is collected by a microscope objective and focused on the entrance slit of a spectrometer. At the output of the spectrometer, Fringes of Equal Chromatic Order (FECO) are observed and recorded; they are analyzed to yield the surface separation profile and the thicknesses of the dielectric films between the silver layers [65 ]. Resolution in surface separation of 0.1-0.2 nm is possible. Figure 2(b,~ shows typical surface separation profiles obtained by analyzing video images of two FECO fringes. One profile (A) shows surfaces which are not in contact (rounded) and another shows surfaces which are in

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contact and have formed a flattened contact region (B). Note the difference in length scale between the two axes. The flattening is the result of elastic deformation in the glue supporting the substrates; the glue is substantially softer than the mica and silica substrates. In the experiments described here, the flattening in contact is not the result of any external load; attractive forces between the surfaces create the deformation [66]. Contact areas in this work typically have diameters, ~, in the range 75~m ~<~ < 150 ~m.

2.2. Surface charge density measurement If charge were being transferred from one surface to the other during the contact of two dissimilar materials in the SFA, it would be desirable to measure that charge in situ, without having to remove the surfaces and use an external probe. The presence of the silver layer on the back side of each substrate used in the SFA presents the means for making that in situ measurement. The technique, shown schematically in Fig. 3 (a), relies on the fact that if a double layer of charge (~s) forms at the contact interface and is separated by pulling the surfaces out of contact, the potential of an electrically isolated silver layer will shift in response to the field from each isolated layer of surface charge. Con-

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(b) Fig, 3, (a) Schematic representation of the separation of two insulating surfaces with grounded metal backing layers, If a charge double layer ± Q. exists at the insulator interface, a current will llow into or .ut of each metal layer when the surfaces are separated as a result of the induced charge layer at the i.sulator°metal interface, (b) The intc~ator circuit used to measure the amount of cl~arge tlowiag to or from the metal layer, The circuit sensitivity can be varied by changing the value of capacitor C. Switch S~ re-zeros the integrator by discharging C through a small resistance R ( 100 ~), Switch S~ can be used to connect a silver layer to an external potential. Two identical circuits were built, one for each substrate, and were operated simultaneously.

versely, if the silver layer is held at ground during separation, through an electrometer circuit for example, a current will flow into or out of the silver layer; this is the proof plane technique described in the Introduction. If the surfaces are pulled quickly apart to large separation (relative to the substrate thickness ), the integral of the current into or out of each silver layer (charge Q^g in Fig. 3 (a)) will be, to a very good approximation, equal to the amount of charge on the surface of each substrate. Figure 3(a) shows schematically only the section of the surfaces which contact; their shape changes during separation, from parallel plates to cylindrical curvature, but because they are moved to what is effectively infinite separation, the small change in geometry is not a source of error. The surfaces are extremely smooth (their preparation is discussed in the following section) and the contact area can be directly measured from the interferometry fringes; therefore, the true surface charge density in the contact area can be determined to a relatively high degree of precision. In

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addition, the time required to meke the measurement is limited only by the time required to separate the surfaces, typicallyone second or less;electrometer driftistherefore not a problem unless the amount of charge transferred is very small. The sensitivityof the technique is described below. The integrating circuitused for these measurements is ghown in Fig. 3(b); two complete circuitswere built so that both silverlayers could be monito~-ed simultaneously. The outputs from the integrators were stored by a 2-channel Nicolet I digitaloscilloscope,Model 310. The operational amplifiers used are Burr-Brown O P A l l l - B M F E T op-amps, chosen for their very high input impedance (differentialinput impedance 1013£~,common-mode input impedance 1014f~) and low input bias current ( < 1 pA). Care was taken in the construction and operation of the circuitto avoid electrostaticdamage to the very sensitive F E T input stages of the op-amps. The value of the integrator capacitor C was typicallyeither 100 or 1000 pF, but values from 10 pF to 0.1/~F could be selected to vary the sensitivityof the circuit.Integrating capacitors were monitored and found to be stable to better than one part in 104.Capacitors not in use were disconnected from the circuitto minimize the effectsof stray capacitance. Switch $2 was used to re-zero the amplifierby discharging C through R. In addition, switch $I permitted the connection of the silver layer to an external circuitvia a B N C connector, typicallyfor the application of an external voltage. All circuitry was mounted in a small cast aluminium box which was mounted directlyto the front of the SFA, fornfinga continuous shield with the heavy stainless steel walls of the SFA. Leads were brought from the electronics box into the S F A chamber via glass-sealedfeedthroughs. All leads were kept as short and rigidas possible to minimize the effectsof stray capacitance; the mutual capacitance of the leads and feedtbrough assembly was less than 0.2 pF '~,and leakage resistances between silver layers under dry conditions were in the range 101 ~-10~2D, regardless of whether the surfaces were in contact or not. Typical values of the capacitor Ibrmed by the silverlayers when the surfaces were close to or in contact were 5 pF to 7 pF, depending on sub. strate thickness and total silvered area, both of which varied somewhat from sample to sample. The capacitance of each silver layer circuit to the SFA/ electronicschassis was typically15 pF to 20 pF, but since both the silverlayers and chassis are kept at ground potential,this capacitance did not constitute a significant source of error. The integrator circuitscould detect the movement of as few as 10 4 electrons ( ~ 1 charge//~m 2 for typical contact areas) if they moved in approximately one second or less (constituting a current of 2 fA or ~Certain ti'ade names and products of companies are identified in this paper to adequately specify the materials and equipment used in this research. In no case does such identification imply that the products are necessarily the best available for the purpose or that they are recommended by NIST. '-'Capacitance measurements were with an Andeen Hagerling capacitance bridge, model 2500A. Capacitance resolution was better than I aF ( 10- TMF).

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more). The overall accuracy of the circuitswas checked by replacing an experimental substrate with one side of another reference capacitor and pulling wellknown amounts of charge in or out of the electrometer circuitby varying the potential of the other plate. Total error in the circuitsis at the level of 1% or

less, neglecting electrometer drift. The circuits do, of course, integrate their own input bias currents; this results in electrometer drift, and is responsible for the largest errors in the measurements, perhaps as high as 5% to 10% when little charge is being transferred (high humidity or mica-mica contact in Fig. 4), but the error is typically 2% or less for the dissimilar-substrate data at low humidity. Although quickly pulling the surfaces to large separation is the most convenient way to measure the surface charge density, it is not the only way to detect the presence of surface charge. A straightforward analysis of the electric field strength resulting from the separation of the transferred double layer at the interface gives the relationship between the charge measured on the silver layer, QAg,to the actual charge on the surface Q,, as a function of surface separation D to be QA~_ Q8 -

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% Relotive Humidity Fig. 4. Surface charge density o, transferred to a silicasurface after contact with a mica surface (circles,tri.Anglcs,diamonds) or silversurface ( × ) as a function of relativehumidity. In the case of a mica-mica contact ( + ),a, is the charge transferred from one mica surface to the other. The three symbols used for the silica-mica data d~note the results of three separate experiments with three separate pairs of substrates. Error in individual measurements isestimated to be not significantly larger (and in most cases smaller) than that represented by the size of the symbols used. One possible explanation for the scatter in the low-humidity data is discussPd in the text.

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where Y1 (Y2) and K1 (K2) are respectively the thickness and dielectric constant of the first (second) substrate. This analysis assumes that the system behaves as an infinite parallel-plate capacitor within the contact area, a good approximation here because substrate thicknesses are on the order of 5/~m while contact diameters are typically 75 to 150/Ira. in the limit of D large compared to Yn/Kn, this reduces to the expected result that the full charge Qs appears on each silver layer; a separation of only 200/tin results in QAg=0.99Qs for 5/~m-thick silica against mica. In the limit of D small, however, the charge seen on the silver layer depends linearly on D for constant Qs if the surfaces retain their parallel-plate geometry. In practice, the surfaces return to having an essentially cylindrical curvature upon separation; this effect, although not important when measuring the total charge transferred by separation to infinity, would need to be included in any quantitative analysis of charge transfer that relied only on measurements of changes in QAg for small changes in D, near contact for example. It is possible, however, to monitor relative changes in surface charge density accurately using small changes in D without a detailed analysis of the surface geometry out of contact; such a technique was used in this work to monitor the decay of surface charge, and is described in Section 3.2.

2.3. Surfacepreparation The substrate material most often chosen for use in the SFA is mica [64,67 ], because it can easily be cleaved into smooth, thin transparent sheets with no atomic steps over areas as large as several square centimeters. Additional work has been done with mica surfaces which have been modified by the deposition of other materials [68-72 ]. One set of measurements has been made using thin single crystals of sappbire [73 ], but these crystals are difficult to prepare and in limited supply. More recently, we have developed a technique for preparing silica surfaces (Suprasil, Heraeus) of sufficient smoothness and cleanliness for use in the SFA [74 ]. The availability of silica for use in the SFA represents a significant new opportunity in the measurement of surface forces, first because the silica surface is more chemically reactive than mica, allowing for a greater variety of chemical surface modifications, and second because it is now possible to study the forces between two dissimilar surfaces. It is this second possibility which is of interest for the study of contact charging, because it means that two dissimilar insulators (mica and silica) can be brought into contact and separated in well-controlled environments and the forces between them studied. Several surface preparation techniques were used for the substrates studied. Mica surfaces were prepared by cleaving thin step-free specimens from thicker samples of Muscovite mica. The mica sheets used in the mica-mica contact experiments were each 4.7/~m thick. The mica used for the majority of the silica-mica measurements was 5.8 ~m thick. Other mica samples used were of

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comparable thickness. The technique for preparing smooth silica surfaces is described elsewhere [74 ], but involves blowing thin bubbles of molten silica which cool rapidly and solidify under tension. Talystep profilometry measureme,~ ~ made at NIST show an average surface roughness of less than 0.5 nm over 30/lm traces. The silica samples used in the majority of the silica-mica measurements was 6.6 !~m thick; the sample used in the silica-silver contacts was 12.6/~m thick. The silver film used in the silica-silver experiment was prepared by evaporating 50 nm of silver onto a smooth mica substrate, gluing the substrate silver-side down onto a cylindrical support, and then peeling off the mica, leaving the smooth side of the silver exposed. Initial checks that all layers of the mica had been removed were made by gently touching the surface with electrical probes (well away from the contact region). After several days, the silver had visibly tarnished, confirming that the metal surface had been exposed. Electrical contact to the silver layers was accomplished in the same fashion regardless of whether the silver was the exposed layer used in the silica-silver experiment or one of the backing layers used with the other samples. A # 40 copper wire was attached to each silver layer using indium just above its melting point as solder; no flux was used. Contact resistances were typically less than 1 ~; silver layer resistance was on the order of I D/square.

3. Results

3,1, Measuremeats of charge transfer The experiment~! technique described above was used to study contact charging in three different systems: silica against mica, silica against silver and mica against mica. In the silica-mica system, the measurements were made both in dry nitrogen gas and in air with relative humidi*,ies from 11% to 98%; in the mica-mica system, measurements were made in dry nitrogen only. The conditions for the silica-silver experiment are described below. Relative humidity was controlled by placing approximately 5 ml of different saturated salt solutions [75 ] in a beaker inside the SFA, which has an internal volume of 0.4 liters; room temperature was regulated at 21°C __0.5°C. The primary measurement made in each experiment involved moving the surfaces in and out of contact several times with the piezo-electric tube until subsequent contacts transferred no further charge (typically 5 to 10 contacts, discussed below), then pulling the surfaces from contact to large separation (greater than 500/~m) in approximately I second. This separation resulted in sharp ~teps in the integrator output voltages which were proportional to the charge remaining on the surfaces, as described in Section 2.2. Voltage-vs-time data during this process were digitized and stored for later analysis, and a videotape recording of the FECO fringes was made so that the contact diameter could be accurately measured.

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The results of these measurements are shown in Fig. 4. For the silica-mica and silica-silver contacts, the value of #s shown is the charge density remaining on the silica surface after separation. In the mica-mica contacts ( + ), done only in dry nitrogen, small amounts of charge were observed to transfer from one surface tc the other, but the magvitude and sign of as for a given mica surface varied, as the contact position was moved, presumably as a result of local inhomogeneities. In all cases, the charges measured on opposing surface.s upon separation were equal in magnitude and of opposite sign, as expected. The most extensive set of data was taken for the silica-mica interface; three silica-mica pairs were studied in dry nitrogen, with one of those pairs also being studied in air with relative humidities from 11% to 98%. Data for that pair were taken for both increasing and decreasing humidity, with good agreement between the two, indicating that the adsorbed water present at higher humidities did not permanently alter the charge transfer properties of the surfaces. Scatter in the data, particularly at low humidity, is substantially larger than the experimental error inherent in the individual measurements; one possible explanation for this behavior is presented below. In the silica-silver system, the first measurements were made in dry nitrogen with a freshly-prepared silver surface; a high charge transfer was observed ( - 9 × 10 -'~ C/m 2 on the silica surface). The environment was then changed to ai=' at 33% RH, and the observed charge transfer was substantially less. To determine whether there had bean an irreversible change in the surface properties, measurements were then made at 11% and finally at 0% RH. The amount of charge transferred was still small, but of the opposite sign; i.e. the silica charged positivelywith respect f~ ihe silver.Upon removal of the substrates from the apparatus later in the experiment, the silver was seen to have tarnished visibly. T w o aspects of these charge density measurements require further discussion: the accumulation of charge with repeated contacts and the magnitude of the charge density observed afterrepeated contacts. In previous studiesof contact between solid metals and dielectrics,it was often observed (see, for example, Refs. [1] and [7]) that repeated contacts, often as many as several hundred, were required to approach asymptotically a m a x i m u m value of transferred charge; in some systems the amount of charge transferred continued to increase even after several thousand contacts [6 ]. In some cases, this effect could clearlybe linked to the electricalconductivity of the dielectric [19,20]; charge was able to move into the bulk of the dielectric,effectively"making room" for more at the surface.However, even insulatorswith very high resistivity can exhibit this charge accumulation behavior; in those systems it can be argued [30 ] that the effectis largely due to changes in contact area, either because contact is at slightlydifferent points each time or because repeated contacts physically change the nature of the contact region by, for example, causing plasticdeformation of one or both surfaces.This view is supported by

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the fact that in contact charge measurements where liquid metals are used in place of sclid metals and where particular attention is paid to insuring that precisely ~.he same macroscopic area comes into contact each time, charge accumulation is not observed; repeated contacts do not add significantly to the amount of charge transferred on the first contact [ 14,15 ]. In both the silicamica and silica-silver experiments, we observe a small but reproducible amount of chsrge accumulation; typically 5 to 10 contacts are required to bring the measured charge within 5% of its maximum value, with approximately 25% to 50% of the total being transferred on the first contact. These observations are consistent with contact area explanations of charge accumulation, since our surfaces are much smoother than most solid surfaces used in contact charging experiments, but are not as smooth, and certainly not as deformable, as a liquid m~, al surface. The ~ther aspect of the results shown in Fig. 4 which should be discussed is the magnitude of the charge densities observed. Except in the case of micamica contact or high humidity, transferred charge densities were in the range 10 -'~ to 10-2 C/' rn2, substantially higher than is usually seen in metal-insulator or insulator-insulator contact charging (see, for example, Ref. [2] and references therein). Suct~ high densities are typically not stable on surfaces in contact with gases; it is therefore useful to discuss how it is that high densities can persist in our experir,~ental system. There are several charge recombination mechanisms which should be considered: a recombination of charge through a substrate at large surface separation, where the field in a substrate reaches a maximum and may exceed the dielectric strength of the material; and a recombination across the gap as the surfaces are separated, either in the form of a discharge triggered by An existing ion in the gap, or from field emission of an electron from a surface at very high field if no stray ions are present. We cannot say with absolute certainty that breakdown through a substrate does not occur in our experiment, but there is strong evidence that, if it does occur, it must involve only a very small fraction of the total charged surface area and will not significantly affect the measurement. To picture why this is so, imagine for example mica initially in contact with silica. When the interfacial doui)le layer is essentially in contact, the field in each substrate is of course very close to zero. As the surfaces are separated, the field increases, reaching its maximum for large surface separation. If, as the field increases, it reaches a value which causes a breakdown of the mica, then surface charge will combine with the image charge which has accumulated in the silver layer on the opposite side of the mica. This combination will no~ be seen by the electrometer, however, because it causes no current to flow iI~ or out of the silver layer. This argument obviously applies only to the first contect and separation; if breakdown is occurring, conductive paths will most likely be developed through the substrate, making it increasingly difficult to hold charge on the surface, even under dry conditions. We observe no increase in the charge decay

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rate as a function of the numbcr of contacts, indicating that conducting "holes" are not accumulating in the mica. In addition, after a separation the surfaces can be brought back together while integrating the reverse of current measured on the way out, and under dry conditions the same charge is observed on the way back in within an uncertainty of about 5 or 10%, a limitation of interpolating electrometer drift (which is slight.y non-linear) while the surfaces are out of contact. This would not be the case if a substantial amount of surface charge had combined with image charge. Charge ~ecombination across the gap, if it is occurring, could either be the result of a breakdown event initiated by the presence of a stray ion or, in the absence of an ion, could occur at higher field if an electron is field-emitted from a surface and accelerated across the gap. Unlike breakdown through a substrate, recombination across the gap will reduce the charge density observed with our technique. It is worth noting that if a discharge does occur, x,ucleated either by an existing ion or a field-emitted electron, it will not necessarily discharge a large fraction of the total charge because of the confined geometry and low charge mobility on the surfaces at low humidity. To better illustrate what is going on in the gap on separation, we have calculated the electric field strength in the gap as a function of surface separation for the maximum observed charge density of 0.01 C/m 2 in a "typical" experiment using a silica and a mica sheet each of 5/~m thickness. The calculation assumes a non-diverging field, a reasonable approximation for gaps small compared to the contact diameter, and an overestimate of the field strength for larger gaps. Immediately upon separation, the field is indeed very high--just over 1 × 10t3V/ram, comparable to that required for field emission from the surface. It drops rapidly as the separation becomes comparable to the substrate thicknesses, however, because of the image charge developing in the silver layers. By the time the gap is 20/~m the field has dropped by a factor of 10, and by 200/~m it is down by a factor of 100; for further separation, it continues to drop as (separation)- ~.

Once the surfaces have reached the final, large (several millimeter ) separation where the total charge is measured, the field in the gap is well below breakdown

strength. We believe the.t a very high field can exist for small gaps because the probability of an ion being present in the gap is very low. For a separation of 10 pm and a charged area resulting from a typical experimental contact over a circle of diameter 100 ~m, the volume of gas subjected to high electric field is only approximately 8 × 10-ecru s. Given a concentration of free ions in the gas of the order of 10/cm 3 available to initiate a discharge (from gamma rays--the passing of an alpha particle near the gap is taken to be a rare occurrence), the probability of an ion initiating a discharge is quite low for ve~'ysmall gaps, but may increase for somewhat larger gaps if the field strength remains high enough. In the absence of an ion to nucleate a discharge, charge densities will be limited by the field strength required for field emission. It is quite possible that small

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discharges occur during separation, particularly at intermediate distances; this effectcould well be responsible for the large scatter in the finalcharge densities observed at low humidity. It is important to note, however, that the charge densities shown in Fig. 4 are the densities remaining after any discharges have occurred, and represent a lower limit to the amount of charge transferred at contact.

3.2. Measur,~ments of charge decay In addition to measuring the total amount of charge transferred, the same experimental arrangement can also be used to monitor the decay of surface charge. We have made preliminary decay measurements for the silica-mica system in dry nitrogen and in air at 11% RH using the following procedure. The surfaces are brought in and out of contact several times until no further charge is transferred; they are then separated to about 1/~m and are held at that separation. At intervals of approximately 30 to 60 seconds, they are stepped quickly in to 500 nm separation and back out again to 1 ~m by applying voltage steps to the piezo-electric tube. The change in separation causes a change in integrator output voltage which is proportional to the surface charge, as described in Section 2.2. We observe that the charge decay in the silica-mica system is exponential over the times studied, from !.0 seconds to 30 minutes after contact, with time constants of 4.8×104 seconds in dry nitrogen and 6.5 × 102 seconds in air at 11% RH. This large change in decay rate is striking, given the low humidity, and is particularly interesting because the total amount of charge transferred does not ch~nge dramatically between 0% and 11% RH. 3.3. Observations of the effects of c ~arge transfer on adhesion In the normal use of the Surf~~ce Forces Apparatus, the deflection of the double cantilever spring which supports the lower surface (Fig. 1 ) provides a simple and effective means of measuring forces between surfaces. With the sliding clamp set all the way to the weakest position (full right in Fig. 1 ), the spring constant of the force-measuring spring is approximately 102 N/m, and the SFA can measure forces as small as 10 -7 N. There are, however, several problems with using this sensitive configuration. First, when the forces being measured (either attractive or repulsive) are large, the motion of the lower surface is not purely parallel to the surface normal; large spring deflections give rise to small lateral motions which can cause surface damage if the surfaces are in contact, particularly with mica against silica. Second, if surfaces with a strong attractive interaction are brought together or separated, there is a region of the force-separation curve where the system is unstable. On approach, t~e surfaces will jump together when the gradient of the force exceeds the spring constant of the force-measuring spring. On separation, a large external force must be applied to pull the surfaces apart; when the surfaces do finally jump apart, they move to a separation comparable to the spring deflec-

305

tion just before the jump, often as far as 100 to 200/~m. To avoid these two problems, the experiments described above were performed with the spring in the fully-stiff configuration (sliding clamp in Fig. 1 full left). The effective spring constant for the system was then the compliance of the entire SFA assembly, including the stiff spring connecting A and B in Fig. 1; this total effective spring constant was determined experimentally to be 1.0 × 105 N/re. The use of the stiff support for the lower surface allowed precise control over the surface separation, even at small separations where the weak spring would have been unstable. In the contact of silica with mica under dry conditions, spring deflections were typically in the range 250 to 450 nm inward (attractive) for surfaces which were contacted and then held 100 nm out of contact. These deflections, which correspond to a range of force of 0.025 to 0.045 N, are consistent with the values one obtains by calculating the electrostatic force between parallel plates using the observed charge densities and contact areas and the appropriate dielectric constants for the backing layers. By comparison, the van der Waals force between two crossed cylinders of mica whose point of closest approach is of atomic dimensions (0.2 nm) is on the order of 0.006 N [ 76 ] and falls off quickly with increasing separation, first as D - 2, then as D -:~ as electromagnetic retardation effects become significant. Clearly, when charge separation of the magnitude observed in this work occurs, electrostatic forces can play a dominant role in adhesion. When humidity is present, the situation is more complicated both because the surface charge dissipates in the course of a measurement and because adsorbed water layers form a meniscus at the perimeter of the contact area which adds a surface tension component to the adhesion [77,78]. We are currently studying in more d,tail the relative effects of the electrostatic, van der Waals and surface tension forces in the adhesion of a mica and silica surface [79 ]. 4. S u m m a r y

W e have developed a new technique for measuring contact charge transfer in the Surface Force Apparatus, and have used the technique to measure charge transfer between a silicaand a mica surface,both in dry nitrogen and in air at several relativehumidities. Charge densitiesas high as 1 × 10 -2 C / m 2 in dry r ~ i t r ~ e r e observed; the high denslti~s are attributed both to the exceptional smoothness of the substratesand to the factthat the samples were thin (typically4 to 6/Im ) and backed with a grounded proof plane. Charge transfer was seen to diminish at higher relativehumidity, but was stillsubstantial at humidities as high as 60%. Preliminary measurements were also made of the rate of decay of the transferred charge double layer at 0% and 11% RH; the decay was observed to be exponential at both humidities,with time constants of 4.8 × 104 seconds at 0% and 6.5 × 102 seconds at 11% RH.

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Acknowledgments The author would liketo thank Roger Horn, with w h o m the phenomenon of charge transfer in the silica-mica SFA system was firstobserved, for many valuable discussions,and J.F. Song and T. Vorburger for their help in characterizingthe silicasurface roughness. Programming by Kevin Schantz greatly facilitatedthe acquisitionand analysis of video F E C O information. This work was supported by the U S Officeof Naval Research under Contract No. N001488-F-0034.

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