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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39227456
70693
10.1038/s41598-024-70693-z
Article
Paradoxical peeling patterns
Reiter Mary Pat mpr97@scarletmail.rutgers.edu

1
Shinbrot Troy 12
1 https://ror.org/05vt9qd57 grid.430387.b 0000 0004 1936 8796 Department of Biomedical Engineering, Rutgers University, 599 Taylor Road, Piscataway, NJ 08854 USA
2 https://ror.org/05vt9qd57 grid.430387.b 0000 0004 1936 8796 Department of Physics, Rutgers University, Piscataway, NJ USA
4 9 2024
4 9 2024
2024
14 2052431 1 2024
20 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Processes ranging from fracture of crystals to peeling of tape have been known for many decades to emit light through a mechanism believed to be associated with electrical charging of separating surfaces. This topic, broadly termed fractoluminescence, has been proposed to be involved in several remarkable phenomena, including medical diagnostics and the generation of X-rays in the lab and earthquake lightning in nature. Here we add the paradoxical finding that two separating surfaces produce entirely different charge patterns, despite originating from the same interface. Further, we report the discovery of a rich variety of new and unexplained patterns, and we examine the hypothesis that the patterns are produced by migration of either polar or non-polar discharge ions onto contact-charged surfaces. This hypothesis may first explain prior findings that charge patterns can extend far beyond points of contact, and second suggests that the ultimate charge imparted on surfaces depends both on well-characterized mechanisms of surface potential and on highly variable discharge ions in the surrounding environment.

Subject terms

Surfaces, interfaces and thin films
Applied physics
Condensed-matter physics
http://dx.doi.org/10.13039/100000146 Division of Chemical, Bioengineering, Environmental, and Transport Systems 1804286 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The simple act of peeling tape represents a gift that keeps giving for scientists and engineers. It is easily confirmed that peeling tape from a roll in a darkened room produces light at the point of separation between tape and roll1. Light is also emitted during fracture of many crystals, commonly mica, calcite, or diamond, as well as in amorphous materials including polymers2 and silica glass3. Light emission from separating or fracturing materials is collectively termed mechano-luminescence and was reported over 400 years ago by Francis Bacon4, who noted that sugar will “sparkle when broken or scraped”. Mechano-luminescence may even date from ancient aboriginal rites, in which shamans rattled quartz rocks to summon5 “light from heaven”.

Modern experiments reveal that light emitted by peeling tape can range from terahertz6 to X-ray frequencies1,7,8 through an unknown mechanism that amplifies energies by up to two orders of magnitude8,9. Most recently, microscopic analysis has demonstrated that heterogeneous “mosaics” of electric charge appear on tape and other plastics when peeled10–12. Beyond being a source of unexplained curiosities, mechano-luminescence has led to proposed applications ranging from medical diagnostics13 and monitoring wear and friction3,14 to generation of terahertz6 radiation and charging of nanodevices15,16.

In the present work, we report another scientific gift from this system: as shown in Fig. 1, charge patterns are produced by peeling tape from a substrate, here Scotch® Magic™ tape from a PTFE sheet. We focus in the present work on the unexpected and paradoxical finding that patterns on the two formerly contacting surfaces are starkly different.Fig. 1 Patterns from peeling tape. (a) Schematic of peeling of tape (vertical) from rigid substrate (horizontal) that is translated at the same speed as peeling. Immediately after tape is peeled, negative (red) and positive (black) toners are sprayed toward the separating surfaces, yielding the patterns shown in (b) for Scotch™ tape peeled from a PTFE substrate, and (c) when the toners are sprayed during the peeling process using the same type of tape peeled from an acrylic substrate. Evidently (confirmed by separate voltage measurements), PTFE (acrylic) charges negatively (positively) following tape peeling. In these, and other, materials, the positive surface develops spots, while the negative surface displays branched patterns. Enlarged section in (b) highlights alignment of spots and branched patterns (outlined in broken lines). The spots and branched patterns invariably emanate from originating points that were in contact (asterisks) and extend toward the direction of separation of the surfaces. All experimental images presented in this, and subsequent, figures are contrast enhanced. Originals and additional details are included in Supplementary Information.

In the experiment depicted in Fig. 1a, peeling is performed at a controlled rate using an Instron™ tensile tester, and charges are visualized by simultaneously spraying both surfaces with oppositely charged xerographic toners17 immediately after separation. Detailed methods and materials are included in Methods, but in short, the tensile tester is mechanically linked to both a vertical tape peeling clamp and a horizontal substrate carriage as indicated by green arrows in the figure: in this way, peeling occurs at a constant angle. Virgin tape is adhered to a surface that has been cleaned with alcohol and then treated with a static eliminator (ExAir 7193). The tape is pressed to the surface using a rubber roller and is then re-cleaned and eliminated as before.

The magenta toner is negatively charged, and the black toner is positive, and so the magenta (black) regions are positive (negative). The patterns shown are examples from a rich variety of distinct morphologies that we will overview (Fig. 5); we stress that in all trials the positive and negative surfaces exhibit qualitatively different patterns, despite originating from the same interface.

Two observations are consistent in all trials, using multiple tapes, substrates, peel angles and rates, and preparation techniques. First, each spot and branching arbor emanates from a common origin, identified by stars in Fig. 1b,c. Second, both the spots and the arbors project from their origins toward the separation direction. Thus, the tape is peeled from left to right in Fig. 1a, while the arbors project from right to left. While not strictly impossible, it seems non-causal for multiple branches in an arbor to travel “upstream” (rightward in Fig. 1a) as if with advance knowledge of their common origin. Additionally, we note these observations persist whether peeling is performed using a tensile tester, as shown in Fig. 1a, or a simpler capstan connected to a DC motor. Thus Fig. 1b uses the tensile tester, Fig. 1c uses a capstan, but either device—or indeed peeling by hand—consistently produce branches on PTFE and spots on PMMA. For comparison with Fig. 1c, we display tape peeled instead with the tensile tester in Fig. 2.Fig. 2 Voltage plots aligned above respective snapshots of tape peeled from PMMA (contrast enhanced). (a) Voltage plot for (b) tape peeled from PMMA. (c) Schematic of voltage probe facing tape-substrate junction during peel (dotted line to aid the eye). (d) Voltage plot for (e) tape peeled from PMMA. Note from scale bar that discharges extending over several cm are common. Experiments here use tape peeled with tensile tester.

We propose here that these two observations provide compelling clues to mechanisms of separation charging. Specifically, we examine the hypothesis that the patterns observed result from electrical discharges between tape and substrate that spawn arbors and spots “downstream,” from a common discharge origin toward the free ends of the separating surfaces. In this scenario, as surfaces separate, each surface would charge oppositely18 and uniformly, but as the free surfaces lengthen, they would produce an increasing electric field where tape and substrate meet, ultimately provoking a Paschen discharge (reported previously for tape peeling17,19).

The differences between branched and spotted patterns can thus be viewed as signatures of differences in polarization between discharge ions. The proposition that positive (negative) discharges generate branched (rounded) surface charges is consistent with research dating to the nineteenth century, in which intentionally generated corona discharges were reported to leave Kreisen (stars) and Stellen (circles) on insulating surfaces17,20,21.

Here, we propose that positive discharge ions are polarized and so align into chains, while negative ions are non-polar and so produce (nearly) isotropic ovals. Indeed, a similar distinction is seen in biochemical systems: both actin molecules22 and liquid condensates23, transition from globular to filamentary structures as molecular polarization grows.

To test this hypothesis, we first confirm that voltage differences grow as tape is peeled, leading to an electrical discharge that corresponds spatially to the pattern origins (stars in Fig. 1). Second, we provide a simple heuristic that reproduces both spots and branched arbors on oppositely charged surfaces, and finally we present a sampling of results seen in peeling in different materials, revealing new and unexplained patterns.

Charging and discharging

We assess charging dynamics in the experiment depicted in Fig. 1a, by instrumenting the peeling apparatus with a shielded non-contact voltage probe, sketched in the inset to Fig. 2a to monitor voltages as tape is peeled. We use the tensile tester for these trials so that the rate and duration of peeling are accurately known. Since the rate of voltage data sampling is also known, it is straightforward to scale the voltage records to match respective toner images. We synchronize the initial time of tape peeling with voltage measurements by tapping the suspended tape with a charged rod: this produces a simultaneous spike in the voltage series and in the recorded load on the tensile tester. The result is shown in Fig. 2 for two trials of Scotch™ tape peeled from a 1/8″ thick PMMA sheet.

Tape charges negatively when peeled from PMMA (cf. Fig.1c), and as before, Fig. 2b,e exhibit branches on the tape. Figure 2a,d show corresponding voltages that grow monotonically during tape peeling, and that reverse direction when the origins of arbors appear on the separating tape. This is consistent with the hypothesis that voltages build during peeling until a discharge spawns patterns.

Though the correspondence between voltages and patterns is gratifying, electrostatic measurements are notoriously variable24,25, and several remarks are in order. First, to limit spurious effects we use a non-contact voltage probe (Trek Model 347) with drift-free electronics, and we shield the probe by enclosing it in a cylindrical grounded sheath with a 4 cm diameter opening facing the peeling region. Shielding is necessary because distant tape and substrate are highly charged and so strongly influence voltage readings, whereas we seek to confine measurements to the peeling location. We position the probe 4 cm from the separation location, which is itself kept nearly stationary during peeling by the mechanical linkage between vertical peel- and horizontal carriage- speeds mentioned earlier (cf. Fig. 1a).

Second, because voltages, and not electric fields, are measured, measurements contain no directional information—meaning that although the sheath opening faces the peeling location, the probe responds to voltages omnidirectionally. We have mentioned that Paschen breakdown is observed26 in this problem: in air, breakdown arises at several kV/mm, so the voltage measured must vary substantially with small changes in probe position. The entire experiment is isolated from ground, hence a perfectly symmetric probe ought to detect no net voltage27—which implies that the voltages recorded must reflect an asymmetry in probe location. Therefore, the growing negative voltage shown in Fig. 2a indicates that the probe in that trial was slightly closer to the charging tape surface, while the growing positive voltage of Fig. 2b indicates that the probe was closer to the PMMA surface. We have confirmed in separate experiments that a change in probe angle of 12° results in an 800 V change in voltage, spanning positive and negative values, the details of which can be found in Methods.

Finally, although voltage growth followed by an abrupt reversal does coincide spatially with pattern origins, this is not apparent for every spot and arbor. In some trials, voltages wander faster than the trends shown in Fig. 2 due to effects24 that we have been unable to identify or exclude; in other trials, patterns are too dense (as in Fig. 1b,c) to distinguish individual voltage signals. Nevertheless, as demonstrated in Fig. 2, coincidence between voltage changes and pattern formation is seen and, with care, does recur.

Heuristic

To rationalize the patterns shown in Fig. 1, we note that arbors (spots) arise on negative (positive) surfaces, irrespective of material—for example in Fig. 1b, arbors appear on negatively charged PTFE, and in Fig. 1c, they appear on negatively charged tape, while in both cases spots appear on the mating surface. This suggests that the patterns depend on something other than the materials being separated, and we propose that differences in ions produced by electrical discharge may account for the distinct branched and spotted patterns. To prevent misunderstanding, we emphasize that arbors are not simply Lichtenberg patterns caused by a plasma discharge itself—again, spots, not branches, appear on the substrate, whereas a simple plasma discharge would be expected to generate similar patterns on both surfaces20,21.

We therefore propose that when a discharge occurs between separating surfaces, as sketched in Fig. 3a, an ionized volume containing both polar cations and non-polar anions is produced. In this scenario, the cations (being positive) would be attracted to a negative surface, and being polar would align head to tail, while the anions (being negative) would be repelled from the same surface and would repel to form ovals on a nearby positive surface.Fig. 3 Sketch of heuristic patterning mechanism. (a) As tape is peeled from substrate, electric field near separation point grows until discharge occurs within volume indicated in gray. (b) Negative (positive) discharge ions within the discharge volume are attracted to the positive (negative) surface by a normal field, Ez, as well as by a transverse field, Ex, produced by the greater length of charged surface the free surface direction (leftward as sketched). (c) As described in text, positive dipoles are modeled by permanently conjoined pairs of positive and negative spheres, with larger charge on the positive sphere. Model dipoles released periodically at random locations within a small volume above the surface spontaneously assemble in a discrete element simulation into branched pattern. (d) Identical simulations of negative but non-polar spheres released above a uniformly charged positive surface produce oval patterns. (e) Multiple discharge points compress upstream (rightward) patterns, (f) compared with experimental example on tape peeled from PMMA. (g) Line of discharge points (green) produces tree-like arbor, compared with experiment showing linear cluster of spots on tape displayed above PTFE substrate. (h) Curve of discharge points (green) produces transverse (nearly vertical here) stripes, compared with experiment using ceramic substrate displayed above tape. Experimental images here obtained using tape peeled with capstan; tensile tester produces similar results (see “Methods”).

We simulate the geometry sketched in Fig. 3b where, as indicated, ions released within a small volume are exposed to a vertical field, Ez, produced by a uniformly charged horizontal surface, and a second, transverse field, Ex, produced by an extended free surface to the left as sketched, ending at a neutral surface to the right. The fields are produced in our simulation by imposing a constant downward acceleration and by fixing a single charge to the left of the simulated volume.

We release ions at a fixed rate in time but randomized location within a small volume centered a fixed distance above the surface shown, and use the discrete element method28,29 to simulate ions as isolated spheres (for non-polar anions) and permanently conjoined pairs of positive and negative spheres (for polar cations). Ions move freely subject to Newton’s laws of motion and Coulomb’s law in the presence of the imposed electric fields as well as the electric fields of all neighboring ions, and particles collide visco-elastically with one another and with the surface boundary.

This simulation approach has been used previously to model collective behaviors of net and dipole charged particles30–32 and is further described in Supplementary Information. The simulation is used here to demonstrate that a cloud of either net or dipole charged ions will assemble under identical conditions into spots or branches respectively on a uniformly charged surface. In Fig. 3c we show a typical arbor produced using a dipole with negative charge twice that of the positive, while in Fig. 3d, we show a spot produced using singly charged ions. Both simulations are performed for exactly the same length of time using exactly the same conditions, excepting ion charge and polarization.

As expected, in identical geometry and simulation, charged dipoles and monopoles produce very different patterns. The same is true for more complicated discharge dynamics. For example, if two sequential discharges are produced by peeling tape, spots formed by anions merely overlap unremarkably, but the cations form two arbors, where the later arbor (labeled ② in Fig. 3e) compresses behind the earlier one (labeled ①). Similar behaviors are seen in experiments, for example on tape peeled from PMMA shown in Fig. 3f. Three-body behaviors are predictably more complex: a couple of simpler patterns are shown in Fig. 3g, where we display ten simultaneous discharges in green above overlapping spots and a tree-like pattern seen in peeling tape from PTFE, and in Fig. 3h, where we show a nearly continuous curve of discharges above similar patterns seen on ceramic-tape peeling.

A final remark from is that this heuristic model can only produce patterns consistent with experiment—i.e. arbors (spots) on negative (positive) surfaces—provided the negative ions are non-polar, and the positive ions are polar. This seems to differ from the longstanding proposition that OH− and H+ are essential carriers to contact charging10,33–37.

Recurrent motifs

Based on the heuristic that patterns are produced by discharges, which as the adage goes never strike twice, one might expect that no pattern would repeat. Moreover, it has previously been established that tape discharges are associated with stick–slip events1,38, and since stick–slip occurs through multi-scale detachment fronts39,40, one would expect to see a range in pattern scales. The same is true of natural discharges, which range in the atmosphere from localized phenomena like St. Elmo’s Fire41 to jets, sprites and elves that extend over tens of kilometers42.

Indeed, we do see fine-scale (mm or smaller) patterns co-existing with larger (cm and above) ones. We also observe several distinguishable types of recurrent patterns. These pattern types recur in multiple experiments using different materials, angles and speeds of tape peeling, pretreatment and attachment approaches, and relative humidities (RH: providing RH < 40%).

A first pair of examples showing both multi-scale and recurrent patterns is shown in Fig. 4. Figure 4a shows a portion of a polystyrene Petri dish after a PDMS layer has been poured, allowed to set, and peeled from left to right. Patterns here span nearly two orders of magnitude in arbor lengths and appear both in isolation and in clusters. It is not clear what sets the length scales here, but we speculate that higher fields and ion densities may generate larger arbors, while a wider detachment front (as in the upper left quadrant of the image) may produce extended clusters of arbors. We note also that the presence of long arbors may be relevant to prior findings that surfaces can transfer charge far from points of contact43.Fig. 4 Examples of multiscale charge patterns. (a) PDMS peeled by hand from polystyrene petri dish: length scales are not associated with stick–slip dynamics in any obvious way. (b) Tape peeled from PMMA using capstan. Note that patterns appear to bridge consecutive stick events (arrows). (c) Enlargement of highlighted region from panel (b) showing a sample recurring pattern.

A second and distinct set of patterned states is shown in Fig. 4b, where tape peeled from PMMA similarly exhibits scales that extend over more than an order of magnitude. Here, though, patterns are dense, and as discussed previously, branches from one detachment front (yellow arrows) collide with the previous front (cyan: recall peeling is left to right). As in panel (a), it is unclear what determines distances between detachment fronts, however unlike panel (a), patterning length scales here appear to involve both discharge events—producing tendrils that extend along the peeling direction (left–right here)—and detachment events—producing transverse lines (bottom-top).

Beyond demonstrating that two mechanisms (discharges and detachment events) are involved in electrostatic patterning, Fig. 4b shows an example of complex recurrent patterns. As highlighted in the green box and enlarged in Fig. 4c, Ψ- or “hands-up” shapes, with florets within the arms, occur repeatedly here and for other conditions and materials. It is intriguing and unexplained why complex patterns such as this recur, but they do so for multiple conditions and materials. It is known; however, that both electrostatic and stick–slip phenomena are nonlinear, and from that perspective, the appearance of recurrent (in other contexts termed coherent44 or non-equilibrium45) patterns may not be exceptional.

We have observed other recurrent patterns as well, and we collect several in Fig. 5. In these photos, tape is peeled from top to bottom, the substrates are described in the figure caption, and original images are included in Supplementary Information. We have attempted to control experimental and material parameters, but in our hands the patterns appear repeatedly but we cannot produce any one at will. Moreover, under the same conditions and even in the same trial, multiple different patterns are seen. We attribute this nonreproducibility24 to the fundamental nonlinearity46 of the underlying mechanisms and hope that future investigations may unveil what the selection of patterns depends on.Fig. 5 Recurrent patterns seen in tape peeling experiments; peeling is from top to bottom. (a) Transverse patterns coincide with a vertebra-like column of spot discharges on the complementary surface (from Fig. 3h). (b) Longitudinal patterns, aligned with peel direction, here on tape peeled from PMMA. (c) Cascade emanating from a detachment line at bottom of image. (d) Frilly patterns on tape peeled from glazed ceramic tile. (e) Tree pattern from Fig. 3g. (f) Squid-like patterns on PTFE from Fig. 1b. (g) Hands-up pattern from Fig. 4c. (h) Disordered, glassy pattern on PTFE. Panels (a–e) and (g) are obtained using a capstan to peel the tape; panels (f) & (h) use the tensile tester. Details of experimental materials and conditions are included in Supplementary Information.

For those investigations, we note that some aspects of these patterns seem to be correlated with surface microstructure—for example, Fig. 5b,h both use commercial PTFE with barely detectable extrusion lines, but the “Longitudinal” patterns are aligned with those lines, while the “Glassy” patterns are largely perpendicular to the lines. We have attempted to pretreat the surfaces by sanding and by coating them with substances ranging from fine powders to lubricants, but have found that provided the surface allows adhesion, largely unpredictable charge patterns result.

Open questions

As we have shown, some of the recurrent patterns shown in Fig. 5 can be rationalized using a simple heuristic model, while other patterns involve additional physics yet to be determined. For example, using the heuristic simulation we can reproduce “Transverse” stripes by initiating a longitudinal sequence of discharges that repel assembling dendrites, and we can produce “Tree”-like states using the heuristic by applying a large field in the peeling direction (Ex in Fig. 3b) to a cluster of discharges (shown in Fig. 3g).

Other patterns shown in Fig. 5 remain to be explained. The longitudinal pattern of Fig. 5b shows indications that prior discharges influence the location and occurrence of future discharges—hence they occur along lines crossed by multiple detachment fronts. The mechanism for this influence is not known, nor is it understood why it sometimes does not act—as in the “Frilly”, “Squid”, or “Glassy” states that show no apparent correlation between discharges. Manufacturing processes (such extrusion of polymer sheets) affect the substrate surface; thus, we cannot dismiss the possibility that patterns like those in Fig. 5b,h are due to small defects affecting the formation of the Lichtenberg figures (both those identified on the tape or those on the substrate). The “Hands-up” pattern is particularly intriguing, as it seems to be highly stylized, arises in multiple experiments, and shows some, but not persistent, correlation along the peeling direction. Of practical importance, possible feedback between discharges and stick–slip also remains an open question.

Three additional issues are also open. First, findings presented in the experiments and heuristic model identify a behavioral difference between the interactions of positive polar and negative non-polar ions within the same system subjected to the same conditions and parameters. This model supposes an initially, uniformly charged surface, (produced here through tape peeling). We reiterate that similar (if less varied) branched and spotted patterns have been reported historically following corona charging17,20,21, indicating that the ultimate patterning process on a charged surface may depend more on discharge effects than on the original mechanism of charging.

This is a useful distinction, since contact charging is notoriously variable and controversial24,25, and distinguishing charging, which involves multiple interacting phenomena40,47, from subsequent discharging, which may involve fewer, may contribute to improving our understanding of ultimate charging that results from contact and separation.

Second, also relating to pattern scales, micron-scale charge “mosaics” have been identified10–12 following separation of contacting surfaces. Likewise, previous reports of charge transfer several centimeters distance from a contact point43 suggest that long-, as well as short-, range patterning of surface charges may be more general than previously understood. How, or whether, mosaics interact with the millimeter-scale patterns that we report is unclear, as is whether mosaics arise due to discharges and so may depend on the surrounding environment.

Third, since ultimate charge patterns appear to depend both on initial contact charge and on deposition following discharges, the ultimate charge imparted during contact electrification must also depend on discharge—as well as on electron, ion and mass transfer between surfaces as is usually assumed47. From this perspective, attempts to calculate contact potentials or to generate reliable triboelectric series without the ability to control the underlying nonlinear mechanisms associated with charge patterning are bound to fail.

On a more encouraging note, the dependence of ultimate contact charge on discharge events may account in part for the well-known variability of contact charging24,25. Moreover, the charge patterns are observed (e.g. Fig. 2) to extend several centimeters from the discharge point, which may explain prior experiments documenting that charges are produced as much as 10 cm from contacting surfaces43.

Methods

Materials and equipment

3.175 mm-thick Polytetrafluoroethylene sheet (United States Plastic Corporation), 3.175 mm-thick Polymethylmethacrylate sheet (United States Plastic Corporation), 3.175 mm-thick Nylon-6 sheet (United States Plastic Corporation), Scotch© Magic™ Tape, Packing tape, 91% isopropyl alcohol, Metal developer beads, Samsung Magenta (CLP-M300A) copier toner, Xerox Black (6R881) copier toner, Instron 5800 with 10N load cell, Ionizing gun (ExAir Model 7193), Trek Model 347 Electrostatic Voltmeter, Airbrush powder paint guns, Percussive device (HoMedics® Percussion Action Plus Handheld Massager).

Peeling methods

Substrates were cleaned using 91% isopropyl alcohol and task wipes and allowed to dry thoroughly. An ionizing gun (ExAir 7193) was used to neutralize the substrate surface before peeling, both sides of the tape, and again after tape application. Tape was applied to the substrates using a rubber roller starting from the right and pressing down toward the left based on the notation in the manuscript. The experiments were carried out using one of two motorized methods: a rotating capstan motor or a vertical testing machine (Instron 5800). For the capstan experiments, shown in Fig. 6a, the substrates were weighed down, and the tape was peeled from the substrate with an alligator clip attached to string connected to the motor. For the Instron tests, substrates were clamped to a carriage mounted to a guide rail. A pulley system was used to maintain an equal speed of the carriage on the rail with the speed of the peeling tape, thus maintaining a consistent angle of the peel shown in Fig. 6b. Multiple Instron peeling speeds were tested and no significant difference was identified between them.Fig. 6 Schematic for separating tape from substrates with two methods. Black arrows indicate direction of tape peeling. Thickness of tape and substrate are not to scale. (a) Sketch of capstan motor experiment: as the motor rotates, the tape is separated from the substrate. (b) Sketch of experiment using a tensile testing machine (Instron 5800). Substrate material is mounted to a cart on a guiderail. Cart slides horizontally as load cell rises vertically at equal speeds, maintaining a constant 90° angle between the separated tape and the substrate. For peeling experiments where voltage data was collected, a voltage probe encased in a grounded cylindrical aluminum shield was pointed at the intersection between the tape and substrate.

In both peeling methods, samples were exposed to a cloud of copier toner sprayed from two powder paint spray guns, each containing a different toner, as in Fig. 7. Each powder paint spray gun jar contained one photocopier toner (one positive, one negative) and metal developer beads. The sprayers were mounted to a ”thumper” shown in Fig. 7 that rapidly tapped the jars to charge the toners through contact with the metal developer beads and to reduce clumping of the powders. In the capstan and Instron experiments, the samples were exposed to the toners during and after peeling, respectively. This difference is due to the requirement that the toner be contained within a filtered enclosure. Due to the size and location of the Instron tensile testing device (nearly 4 feet tall), this was not feasible, so the samples were rapidly transported within 30 s to a hood before spraying. No significant difference was observed between spraying during peeling versus spraying after peeling; trends reported were consistent for either methodology.Fig. 7 Schematic of powder paint sprayer. (a) Glass jar contains photocopier toner and metal developer beads. Thumper pad from percussive device rapidly taps side of jar to shake toner and beads. Air enters from compressor (not shown), toner is pulled up through straw, fine mist of toner particulates is released from sprayer nozzle. (b) Close-up sketch of charging of toner particulates. Beads are shaken in jar and charge the toner through repeated contact.

Voltmeter experiment details and confirmation

Main text Fig. 2 presents voltage plots with respective images of the tape peeled from PMMA. Despite having been pulled from the same material, the initial relative voltage slopes were opposite: (a) decreases while (d) increases. During the peel, the voltmeter probe was approximately pointed at the junction of the tape. However, since the results were different, we aimed to investigate the effect of probe angle, θ in Fig. 8b, on the sign of the voltage readings described in the body of this manuscript.Fig. 8 Voltage with respect to probe angle. (a) MATLAB plot of recorded voltage (y-axis) over time (x-axis) from varying the angle of the voltmeter probe arm. Angle change (∆θ) is denoted above the x-axis as indicated in Fig. 6. Raw data are black; smoothed with a moving average of 70 data points shown in red. (b) Mean voltage for each angle obtained by averaging the data points for each plateau in the raw data. Note that a change of 800 V, spanning positive and negative values, is generated by a 12° change in probe angle. (c) Schematic of probe position and direction with respect to the charged surfaces for the first and last angles recorded.

To do this, tape was peeled from PMMA starting with the Trek probe pointed slightly below the junction of the tape and the substrate. The angle of the probe arm, θ, was slowly increased (thus pointing more toward the tape) during which time the voltage was recorded as shown in Fig. 8. The probe arm length from rotation point to the voltmeter probe end was 43.18 cm. Note that the voltage is initially positive as the probe captures a voltage dominated by the positive substrate. As the angle increases at intervals, (panel (b)) the voltage decreases and switches to negative as the field becomes more influenced by the tape’s charge. In principle, at θ = 45° and directed at a 90-degree peel angle, the voltage would be approximately zero. However, since the voltmeter was used to compare relative, rather than absolute, voltages, quantitative calibration was not required. These data confirm that small variations in the probe placement and direction can cause considerable differences in the sign and slope of the voltages recorded. This suggests that the sign of the slope of the voltage data during a peel is determined by the angle of the probe arm determining whether the probe records more of the tape or the substrate fields.

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-70693-z.

Acknowledgements

We acknowledge funding from the National Science Foundation (NSF-CBET Award no. 1804286) and from the Society of Tribologists and Lubrication Engineers (STLE) through the Elmer E. Klaus Fellowship. We also thank Jonah Botvinick-Greenhouse and Matthew VanDusen-Gross for technical help. We thank Dr. Joseph W. Freeman of Rutgers University for the use of the Instron® mechanical testing equipment.

Author contributions

MPR and TS wrote the main manuscript and analyzed data. MPR prepared all figures for and wrote the Supplementary Information, built experimental equipment, performed experiments, and prepared Figs. 1, 2, 6, 7 and 8 of the main manuscript. TS devised experiments, wrote the abstract, and prepared Figs. 3, 4 and 5. All authors reviewed the manuscript and Supplementary Information.

Data availability

Originals of images presented in this manuscript can be found in Supplementary Information. Parameters for simulation data can also be found in Supplementary Information.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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