What is going on? For the first minute or so after you stuck the tape to the glass, electrons moved across the interface of the two surfaces, drawn to either the glass or the adhesive by electrical forces. The migration, which is called contact charging, left electrically charged patches along the interface. Some patches acquired an abundance of electrons and became negatively charged, whereas others lost electrons and became positively charged.

When you peel away the tape, you stretch, distort and rupture the adhesive material along the line of separation. As the adhesive detaches from the glass and air fills the rupture zone, the patches of charge on the two surfaces separate. When their separation reaches a certain critical value, sparks jump through the air, exchanging charged particles between oppositely charged patches to bring about a degree of electrical neutrality.

Along the way the particles collide with the air's gas molecules, leaving the molecules in excited energy states. The molecules quickly return to the unexcited state by emitting light, some of which is visible. The air's nitrogen molecules, which happen to emit light at the blue end of the visible spectrum, are responsible for most of the glow you see. Additional light may be emitted by the surface of the glass or the tape when they are struck by the particle streams, and there are still other processes that may also generate some light. All the emissions are brief, but the glow appears to be continuous if you keep peeling the tape, thereby creating fresh patches of charge and more discharges.

The separation glow is so faint that it is difficult to photograph by conventional means, but Tom Dickinson and Ed Donaldson of Washington State University have devised a clever way to record the glow on film. Working in a dark room, they peel the tape directly from the film itself or from a thin transparent plate laid on the film. When the film intercepts some of the light emitted by a spark, h an image is recorded in the film's emulsion. When the film is developed, it holds a record of the light emitted during the peeling process. Dickinson and Donaldson call such a record an "autograph" of the peeling. The experiments are part of their work on fracture problems involving adhesives, composites and ceramics.

Sometimes the peeling mechanism creates a peculiar pattern of bright and dark stripes. In other cases it produces images

of bright, isolated sparks. How the peeling proceeds depends partly on the type of surface to h which the tape is attached. A variety of surfaces can be tested, but only k; transparent ones serve to expose film. A microscope slide works but produces blurred images because it is thick enough to allow the light from a spark to spread before it reaches the film.

Dickinson and Donaldson studied the glow characteristics of 3M Scotch Brand Magic Transparent Tape (No. 810) and 3M Scotch Brand Filament Tape (No. 893). The adhesive on the transparent tape is alkyl acrylate and the backing is cellulose acetate. The adhesive on the filament tape is natural rubber combined with a "tackifying" agent. The sticky face of the tape has more of the agent than the interior does, where glass filaments are bound to the polyester backing.

The film types, all Polaroid brands, were chosen for their variety of surface coatings and sensitivity to light. Type 146 has an ISO rating of 200 and is uncoated. Types 47 and 107C are rated at 3,000 and have a gelatin coating. Type 612 is rated at 20,000; the nature of its coating is proprietary information.

The films were loaded into the opened rear section of Polaroid cameras that accommodate them, with the emulsion side of the film exposed h for experimentation. When the room lights were on, the loaded film was protected by an opaque black card. With the lights off and the card removed, tape was pressed onto the film with a finger. After a few seconds it was peeled off and another tape was applied to a different area of the film. After two or three tapes had been peeled, the film was developed in the usual way by bringing it in contact with the developing material. Dickinson and Donaldson wondered if any residue left on the film by a peeled tape might create artifacts in the final pictures. They found no evidence that any residue interfered with the film's development or its sensitivity to light.

Tape peeled directly off the film leaves images, but are they really due to visible light from sparks? Or might they instead be created when the emulsion is struck by particles, acted on by chemical species from the tape or illuminated by ultraviolet light from the sparks? To check for these other possibilities, tape was first peeled off a glass slide placed on the film. The slide was chosen because it blocked particles and chemical species and absorbed ultraviolet light. The images left under the slide were then compared with those produced when the tape was peeled directly off the film. The two sets of images differed only in sharpness (because the slide spread the light), indicating that visible light from the sparks must be the only significant source of the images on the film.

After these early trials, tape was peeled directly off the film, normally a few seconds after being applied. To produce the autographs in Figure 1, the transparent and filament tapes were peeled from left to right off Type 146 film, with the rate of separation accelerating from about one millimeter per second to about five millimeters per second. In both cases the pull on the tape was perpendicular to the plane of the film. Part of the autograph is striped. The rest of the display has many individual images that apparently were produced by rapid, intense bursts of light, making the autograph resemble a gathering of fireflies. Under magnification the structure of some of the individual images suggests that they were produced by sparks flashing parallel to the film surface.

The faster rate of separation produces more pronounced stripes for the transparent tape, whereas for the filament tape a slower separation works best. The stripes result from periodic variations in the speed at which the tape separates from the film. The variations are similar to the "stick and slip" sequence observed when one surface slides over another surface. During the stick phase for the tape the separation is slow, yielding a few feeble sparks that barely expose the film. The sparks leave a dim (but not totally dark) line in the developed autograph. During the slip phase the separation is rapid, generating vigorous, bright sparks that trace a bright line across the film. Both bright and dark regions are lines running across the width of the tape-film interface, because the full width of the tape uniformly underwent either stick or slip.

The sharp features in the pattern must be due to sparks near or on the emulsion, because the light from such sparks had no chance to spread. The more diffuse features probably derived from sparks farther from the emulsion, perhaps along the tape surface after the surface was lifted from the film. Dark circles left in an autograph are caused by air bubbles trapped under the tape when it was applied to the film: charged patches are not created in a bubble area, because the tape is not in contact with the film. Defects in the tacky side of the tape also produce regions that do not spark, thereby leaving unexposed regions on the film.

During a stick-and-slip separation the distance between the stripes depends on the properties of the adhesive, the backing and the film surface. For example, the stiffness of the tape partially determines the mechanics of how the adhesive layer on the tape breaks free of the film. Dickinson and Donaldson ran trials with transparent or filament tape stiffened by one or two layers of electrical tape attached to the backing. When the stiff composite was peeled at about the same rate as in previous trials with unstiffened tape, the distance between the stripes doubled to about two millimeters and the intensity of the sparks weakened.

The distance between stripes also depends on the direction in which the tape is pulled. When it is pulled directly upward from the plane of the film, the stripes are closely spaced. When it is pulled along the plane of the film, the stripes are farther apart. There are also more high-intensity sparks then, presumably indicating that the distortion and rupture of the adhesive must be severe at times, so that highly charged patches are created. To make the autograph in Figure 3 the tape was first pulled along the plane of the film and then pulled directly upward.

The pressure with which a tape is attached to the film influences the extent of sparking. When Dickinson and Donaldson attached one strip of Magic Transparent Tape to a Type 107C film, they pressed particularly firmly on the bottom half of the strip. When the tape was peeled, more and brighter images were produced in the bottom half. The influence of pressure can sometimes be seen without extra pressing because of the way the transparent tape lifts off a surface: the edge of the tape flexes just before it lifts off, pressing down on its tacky side. The firmer attachment of the edge prior to separation increases the sparking that takes place in the course of separation, and so the autograph displays a bright outline of the tape's edge.

The rupture of a tape's adhesive material at an interface with a surface is called adhesive failure. In contrast, the rupture of a single type of material is called cohesive failure. It is normally considerably weaker than adhesive failure in producing charged patches and the consequent sparking. To test this idea, Dickinson and Donaldson found a way to have both types of failure take place during the peeling of a strip of Magic Transparent Tape. They first stuck a layer of Scotch Brand double-sided tape (both sides of which are sticky) to the film. On the tape they attached three letters, W, S, and U, that they had cut from the transparent tape. The sticky side of the letters faced down. Then they stuck a strip of Magic Transparent Tape over the entire length of the double-sided tape.

When the strip of Magic tape was h peeled away from the double-sided tape, both types of failure took place. Where the strip was pulled directly from the double-sided tape, the Magic tape underwent cohesive failure, because the separating surfaces were identical in composition. Where the Magic tape was pulled from the backing of the tape forming the letters, it underwent adhesive failure, because the separating surfaces were unlike in composition.

In order to avoid any additional production of light, the double-sided tape was then lifted off the film gently while being wetted with methanol applied with a cotton swab. As the methanol seeped into the crack between the tape and the film, either it eliminated the sparking by providing a conducting path for the electrons or it destroyed the tape's adhesive bond with the film. When the film was developed, the letters showed up as white on a dark background [see illustration]. Apparently the adhesive failure had produced sparking, whereas the cohesive failure had produced little or no sparking. The result was somewhat counterintuitive, because the tape had been more difficult to pull during cohesive failure, incorrectly suggesting that cohesive-failure sparking should be more vigorous.

Dickinson and Donaldson noticed that fingerprints on the sticky side of a tape reduced the sparking when the tape was peeled off film. The oil from the finger reduces the adhesion of the tape, and it may also provide a conducting path, reducing the chance that there will be a discharge through the gas that fills the crack during the peeling. (Uncontaminated regions underwent normal sparking.) Ink deposited by a felt-tip pen on the sticky side of the tape or on the film's emulsion similarly diminished the sparking.

When tape is peeled off film, the distribution of charged patches creates strong electric fields in the crack region. Suppose a metal conductor such as aluminum foil is sandwiched between the tape and the film, so that when the tape is peeled, the foil is pulled up along with it. Does the presence of the foil alter the distribution of charge and subsequent sparking? Dickinson and Donaldson sandwiched several small pieces of foil between Magic tape and film [see Figure 5]. One would expect that when the tape was peeled, sparking would be normal except for where the foil lay- that there the absence of adhesive failure would prevent sparking, leaving a dark replication of the foil's shape on the developed print. Indeed, the print did have such replications, but each had curious bright bands along the right-hand edge (remember that the peeling is from left to right) and sometimes along the top and bottom edges; there was no band along the left-hand edge. The bands were separated from the rest of the autograph by a dark zone where the thickness of the foil had kept the tape from adhering to the film.

What accounts for these bands? When the peeling reaches the left edge of a piece of foil, some of the charges created there are conducted throughout the foil. For example, if the left edge of the foil receives an abundance of electrons, the entire foil becomes negatively charged as the electrons spread over it. Their concentration is greatest along the edges, particularly at sharp irregularities left by the scissors when the foil was cut. Regions of film just below the edges of the foil become positively charged as some of their electrons are driven away by the electrons on the foil. An electric field then exists between the edges and the underlying film. When the field is strong enough, a discharge through the air trapped under the foil brings electrons to the film to neutralize both foil and film. As the peeling continues, more sparks are produced.

If the foil's right edge has been cut to form a point, the electric field is stronger there, because charges congregate more at points than they do along straight edges. The bright sparking that ensues creates a bright band in the replication of the point in the developed film. The peeling need not reach the point to create the band. To show this, Dickinson and Donaldson stopped the peeling seven millimeters short of the point on a triangular section of foil. To prevent further sparking, they then removed the rest of the tape by wetting it with methanol. When the film was developed, a bright band marked the point of the triangle's replication.

The tendency of adhesive tape to emit light when it is peeled off a surface can be judged by peeling it near the antenna of a transistor radio that is tuned to an unused portion of the AM spectrum. (The antenna is probably near the top of the radio.) The discharge brought about by the peeling emits radio noise with an intensity roughly proportional to the brightness of the sparks. Peel the tape a second or so after it has been applied and then try longer delays. Does the noise level vary? Try different surfaces and peeling speeds. Does the radio pick up the noise if it is tuned to the FM spectrum? What happens to the noise if the tape is surrounded by air of high humidity?

You might investigate how tape peels under other circumstances. For example, sprinkle metallic dust, lycopodium powder or fine flour lightly over the sticky side of the tape before applying it to the film. Are the autographs altered? What happens if you sandwich a fine metallic mesh between the tape and the film? You might also like to investigate the peeling characteristics of some nonadhesive materials, such as plastic food wrap. I should be pleased to hear about your findings.

Bibliography

FRACTO EMISSION FROM POLYMERS, 1: CRYSTALS, AND INTERFACES. J. T. Dickinson and L. C. Jensen in Proceedings Of the Society of Photo-Optical Instrumentation Engineers, Vol. 743, pages 68-75; January, 1987.

AUTOGRAPHS FROM PEELING PRESSURE SENSITIVE ADHESIVES: DIRECT RECORDING OF FRACTURE-INDUCED PHOTON EMISSION. J. T. Dickinson and E. E. Donaldson in The Journal Of Adhesion, Vol. 24, Nos. 2-4, pages 199-220; December, 1987.

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