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Stickiness—Some Fundamentals of Adhesion1
1 Centre de Recherche Paul Pascal, CNRS, Av. Schweitzer, 33600 Pessac, France
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SYNOPSIS |
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We review some adhesion mechanisms that have been understood in the field of synthetic adhesives, and more precisely for adhesives that adhere instantaneously (a property named tackiness) and whose adhesive strength usually depends on the applied pressure (pressure-sensitive adhesives). The discussion includes effects of surface roughness, elasticity, cavitation, viscous and elastic fingering, substrate flexibility.
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INTRODUCTION |
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Two bodies are said to adhere when energy is needed to achieve separation. When a good adhesive is used, the required energy may range from 100 to 1,000 J/m2, much higher than typical surface energies which are in the 0.01–0.1 J/m2range. What is really an adhesive? It is usually a soft material that doesn't flow, made of polymers whose molecular architecture may vary (cross-linked polymers, block-copolymers). They accomodate large deformations in order to dissipate a large amount of energy, yet they are essentially incompressible. Their behavior in a particular instance depends crucially on the substrate properties. In order to understand these features which may at first seem contradictory, we will discuss the need for a strong interface between the adhesive material and the solid bodies, as well as the requirement for a confined geometry, we will describe the non-homogeneous deformations that allow for the mechanical contact to remain active at large separations, we will illustrate the role of the mechanical compliance of the substrate, we will finally discuss how energy and force compete, especially in the adhesion of layered systems. Our discussion is restricted to the concepts established in the field of synthetic adhesives. The reader is encouraged to consult other reviews (Creton and Fabre, 2002; Kinloch, 1996
; Gay and Leibler, 1999b).
Strong interface
On the molecular scale, the contact between the adhesive and the substrates generically results from van der Waals interactions (Israelachvili 1992). Surface chemical bonds (Gent and Schultz, 1972), macromolecular interdigitation (Raphaël and de Gennes, 1992) or elongation (Lake and Thomas, 1967), however, may enhance significantly the strength of the interface for specific substrate-adhesive pairs. Surface treatments and cleaning are thus essential in the industries that rely heavily on adhesive materials.
Strong interactions on the molecular scale do not ensure that the contact is good on larger length scales: solid substrates usually display some degree of surface roughness (Greenwood and Williamson, 1966) which may reduce the degree of intimacy of the contact with the adhesive (Fuller and Tabor, 1975; Creton and Leibler, 1996) (see Fig. 1). This explains the criterion introduced empirically by Dahlquist as an upper bound on the elastic modulus of a material (G < 105 Pa) for it to appear sticky (Dahlquist, 1966): if the material is highly deformable, a good contact may be achived despite the roughness through surface interactions on the molecular scale, with no need for applied pressure. Conversely, the contact may remain poor even under reasonable pressure if the adhesive material is too rigid. This is particularly well illustrated by temperature-switchable adhesives whose softness can be made to vary significantly over a narrow temperature range (Crevoisier et al., 1999). For soft adhesives, the surface roughness of the substrate may paradoxically enhance the strength of the interface. Indeed, tiny air bubbles can be trapped at the interface during the contact (Fig. 1) and generate suction effects upon traction (Gay and Leibler, 1999a): the energy dissipated in a bonding-debonding cycle originates in the fact that the amount of stored elastic energy in the material differs between bonding and debonding. But while usual suction effects on the macroscopic scale are achieved mechanically, here the intimate contact is due interactions on the molecular scale.
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THIN LAYERS |
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A stretched elastic band breaks when some threshold force is exceeded. The longer is the band, the more elastic energy is dissipated when fracture occurs. Conversely, as every kid has experienced while discovering the virtues of glue, adhesives are more efficient when used as thin layers. Indeed, since the adhesive material is virtually incompressible, volume conservation implies that pulling on the solid bodies causes a convergent deformation at an enhanced velocity (see Fig. 2A), which results in a large energy dissipation in the adhesive and thus a strong resistance to separation. This is measured in a controlled manner: in the probe-tack test (Zosel, 1985
), a flat, solid punch, called the probe, is brought into contact with an adhesive film deposited on a rigid substrate. The force is then recorded while the probe is being pulled away (Fig. 2B and insert). As can be seen from the force curve, a sudden drop in the measured force (shown by an arrow) indicates that the resistance of the adhesive is too strong and that instabilities develop in the adhesive material so as to relieve some of the stress. Such instabilities have later been observed directly during separation in a modified version of the probe-tack test (Lakrout et al., 1999) and shown to fall into two main categories: fingering instabilities and cavitation, shown on Fig. 2C and 2D. Both types of instabilities are driven by the need for relieving the stress: air fingers (similar to the Saffman-Taylor instability [Saffman and Taylor, 1958]) protrude from the edge of the sample towards the center, bringing atmospheric pressure well into the sample, while growing bubbles provide some of the extra volume required by the plate separation. Viscous fluids exhibit not only fingering, but also cavitation, depending on the separation rate (Poivet et al., 2002). Adhesives, which are viscoelastic solids, can also exhibit cavitation and fingering. Fingering is also typically observed at low separation rates while cavitation occurs at higher rates. Cavitation does not result in rapid bubble coalescence: bubbles usually remain distinct and form a two-dimensional foam as the plate separation proceeds (Lakrout et al., 1999). As for fingering in adhesives, although it may look like viscous fingering, it reflects an instability of the contact line and is thus interfacial (Shull et al., 2000; Ghatak et al., 2000).
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SUBSTRATE COMPLIANCE |
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We have examined so far the role of the interface between the adhesive and the substrates, as well as the importance of the adhesive layer being thin and the resulting non-homogeneous deformations arising through instabilities. The mechanical response of an adhesive in fact does not only depend on its own characteristics or those of its interface with the substrates: it also depends on the substrates being rigid or compliant. If the substrates are rigid (Fig. 3A), pulling the adhesive is hard because it is thin, as we discussed above. Because the resistance of the adhesive is so important, the compliance of the machine itself (denoted by the spring on Fig. 3A), or that of your finger if you perform the test manually, may play a role: elastic energy is stored for some time, until the instabilities described above cause a sudden separation, as discussed in a specific geometry in (Francis and Horn, 2001
). Conversely, if at least one of the substrates is flexible (a geometry named peeling, see Fig. 3B), the separation is considerably easier, as only a restricted region of the adhesive is under tension at any given time.
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In order to illustrate this fact, let us consider an adhesive band of width W = 1 cm with adhesion energy G = 100 J/m2 under typical peeling conditions. The corresponding force applied on the band is essentially the product F = G.W (although it may be altered if the peeling angle differs from 90° or if the backing of the adhesive band is extensible), hence the applied force is F = 1N. By contrast, the typical force peak in a probe-tack experiment corresponds to a pressure of a few times the atmospheric pressure. Hence, for a solid punch with diameter 2R = 1 cm, the force that must be applied to reach the peak and achieve separation is a few times 10N, considerably higher than in the peeling experiment.
In fact, peeling is so much easier that it is avoided as much as possible in practical situations. For instance, adhesive hooks are designed as depicted on Figure 3C, where the adhesive layer is compressed when a weight is suspended to the hook. The design of Figure 3D would induce a tendency to peeling from the top of the adhesive film and soon result in the ruin of the hook.
The ability of tropical lizards called gekkos to climb walls at amazing velocities can be related to flexibility issues. Described in detail elsewhere in this symposium (Autumn et al., 2000), their adhesive feet consist in a hierarchy of structures. The smallest structure is the spatula (0.2 micron in width). Spatulae can achieve contact with the wall somewhat independently of one another: gekkos can accomodate rough surfaces and achieve a good adhesion. Although adherence to the wall is important, clearly feet removal should also be achievable without too much effort. Again, the mechanical independance of spatulae is essential: because they are independent, they can be tilted simultaneously and peeled away from the wall, and due to their small dimension, this can be done with little effort.
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ENERGY VERSUS FORCE IN LAYERED SYSTEMS |
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One important issue for synthetic adhesives is whether there remains some adhesive material on the substrate after the separation is complete (so-called cohesive rupture) or whether the ahdesive film recovers its integrity and is ready for further use with almost unchanged performance (adhesive rupture). In other words, does the final fracture occur within the adhesive film (i.e., is its cohesion affected), or does it take place at the interface with one of the substrates? More generally, consider a stack of several layers from different materials: where will the system break upon traction or peeling? This question involves force and energy. In the probe-tack geometry, some force has to be exceeded (roughly, the value of the force peak on Fig. 2B) for separation to occur. The full separation also requires some amount of energy (integral of the force curve) which reflects the non-homogeneous deformations described earlier. The system may choose between different fracture mechanisms (interfacial or cohesive, with further choice in a layered system). At first, it usually chooses the weakest mechanism in terms of force since it triggers separation first and thereby relieves the stress on the other possible mechanisms. On the long run, however, the fracture propagation is driven by the elastic energy stored in the system under tension which must exceed the energy required by the fracture. The system thus tends to choose the separation mechanism that dissipates the smallest amount of energy per unit surface area since the fracture will be able to propagate faster. In some cases, however, the fracture may remain at a location where the energy dissipation is not minimal: a mismatch between the elastic properties of the various layers may prevent the fracture from migrating towards its energetically optimal location. A detailed study is thus necessary to understand the various scenarii which may even depend on the applied traction force.
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CONCLUSION |
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Synthetic adhesives have been much studied and as we have seen, several mechanisms have been identified. Yet adhesion phenomena are far from being fully understood: the chemical architecture of polymeric materials can be much varied and usual rheological characterizations involve only shear deformations. Thus, the mechanical response in the adhesion geometry can be only partially estimated since adhesion induces strong elongations in the adhesive material. Bioadhesion is even richer for a number of reasons. Specific interactions may have arisen in the course of evolution. The mechanical properties of the bodies may vary in the course of separation due for instance to muscular activity. Fine mechanical design can substantially enhance the performance of adhesive structures, as we have briefly discussed for gecko feet. Such structures are currently well beyond operating or economically usable technologies and can be a source of inspiration. Also, the study of bioadhesion may well uncover qualitatively new adhesion mechanisms.
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ACKNOWLEDGMENTS |
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I gratefully thank the organizers for the invitation to attend the Symposium on the Mechanics of Adhesion. I acknowledge fruitful discussions and collaborations on adhesion phenomena over the past few years, with Arnaud Chiche, Ioulia Chikina, Costantino Creton, Guillaume de Crevoisier, Pascale Fabre, Gwendal Josse, Ludwik Leibler, Frédéric Nallet, Sylwia Poivet, Elie Raphaël, Didier Roux.
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FOOTNOTES |
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1 From the Symposium Biomechanics of Adhesion presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 2–6 January 2002, at Anaheim, California.
2 E-mail: cgay{at}crpp.u-bordeaux.fr
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References |
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