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Adhesion Enhancement through Micropatterning at Polydimethylsiloxane-Acrylic Adhesive Interfaces M. Lamblet,‡,⊥ E. Verneuil,†,§ T. Vilmin,‡,| A. Buguin,† P. Silberzan,† and L. Le´ger*,‡ Physico-Chimie Curie, UMR 168, CNRS - Institut Curie, 11 rue Pierre et Marie Curie, 75231 Paris, Cedex 05, France, and Laboratoire de Physique des Fluides Organise´ s, FRE 2844, CNRS - Colle` ge de France, 11 Place Marcelin Berthelot, 75231 Paris, Cedex 05, France ReceiVed October 23, 2006. In Final Form: February 16, 2007 Adhesion at polydimethylsiloxane (PDMS)-acrylic adhesive interfaces is shown to be enhanced through micropatterning of the PDMS substrate. By varying the geometry of the patterns (groves and hexagonal arrays of pillars of micrometer sizes, obtained through soft lithography techniques) and comparing rigid and deformable substrates, the respective roles of the geometry and the size and flexibility of the pattern features on the level of adhesion have been analyzed. For cylindrical pillars, two regimes are clearly identified: for a relatively low aspect ratio (h/r < 3, with h and r, respectively, the height and the radius of the pillars), soft patterned substrates are more efficient than rigid ones at increasing adhesion, pointing out the role of the elastic energy associated with the deformation of the pattern that is lost when the adhesive detaches from the substrate. Using scaling laws, the predominant contribution to that elastic energy can be further identified: deformation of the substrate underlying the pillars for h/r < 1.6 or bending of the pillars for h/r > 1.6.; for a high aspect ratio (h/r > 3), only rigid patterned substrates enhance adhesion, then the only possible contribution to energy dissipation comes from the enhanced viscoelastic losses associated with the pattern that induce modifications of the strain field within the adhesive layer. Soft, high aspect ratio patterns lose their efficiency even if still bent under the effect of the peel forces. This is because when bent, some of the pillars touch each other and remain stuck together, lying flat on the surface after the passage of the peel front. The bending elastic energy of the pillars (which is still lost) is then balanced by the corresponding gain in surface energy of the substrate in the peeled region. These systematic experiments demonstrate that the ability of the patterned surface to be deformed plays a crucial role in enhancing adhesion and allow us to propose a way to fine tune the level of adhesion at PDMS-acrylic adhesive interfaces, independently of the chemistry of the adhesive.
Introduction The precise tuning of the adhesion properties is of great interest for a number of applications, especially when one deals with weak adhesion. Substrates made of polydimethylsiloxane (PDMS) have many advantages as antiadhesive coatings because of their low surface energy and their weak chemical reactivity. Their adhesion with acrylic tapes, however, is too low for most usual applications. Therefore the design of PDMS surfaces with tailored adhesive properties remains a real technical challenge. A commonly followed path relies on chemical modification1-3 of the antiadhesive coating, which usually leads to an adhesion enhancement that is strongly dependent on the chemical nature of the adhesive. An alternative and potentially more universal solution based on microstructuration has been proposed and has started to be investigated recently by several authors.4-7 It is * Corresponding author. Current address: Laboratoire de Physique des Solides, UMR 8502 CNRS - Universite´ Paris Sud, Baˆtiment 510, 91405 Orsay, France. E-mail:
[email protected]. Tel: + (33) 1 69 15 56 45. Fax: + (33) 69 15 60 86. † CNRS - Institut Curie. § Current address: Fluides, Automatique et Syste ` mes Thermiques, Baˆtiment 502, 91405 Orsay, France. | Current address: Laboratoire de Physico-Chimie The ´ orique, ESPCI, 10 rue Vauquelin, 75231 Paris, Cedex 05, France. ‡ CNRS - Colle ` ge de France. ⊥ Current address: Physico-Chimie Curie, UMR 168, CNRS-Institut Curie, 11 rue Pierre et Marie Curie, 75231 Paris, Cedex 05, France. (1) Owen, M. J.; Jones, J. D. Silicone Release Coating. In The Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996. (2) Amouroux, N.; Petit, J.; Le´ger, L. Langmuir 2001, 17, 6510. (3) Gordon, G. V.; Schmidt, R. G. J. Adhes. 2000, 72, 133. (4) Gathak, A.; Mahadevan, L.; Chung, J. Y.; Chaudhury, M. K.; Shenoy, V. Proc. R. Soc. London, Ser. A 2004, 460, 2725.
inspired by the amazing aptitude of some insects and lizards, such as geckos, to adhere reversibly to many substrates using micro and nanoscale high aspect ratio structures located on the tips of their fingers, usually referred to as fibrillar structures or fibrils (or setae).8-10 For geckos, the dominant mechanism is still under debate. This lizard exhibits a dry adhesion system, and both van der Waals forces9 and capillary forces10 have been discussed. The ability of geckos to climb on various substrates is then supposed to be governed by a complex interplay between the mechanical properties of the fibrils, their hydrophilicity, and their geometry. More recently, the rapid switching between gecko foot attachment and detachment has been analyzed theoretically on the basis of a tape model taking into account the geometry of the fibrillar structure and the macroscopic action of the gecko toes.11 Thus, we have a lot to learn from nature, and fibrillar structures seem to be good candidates to tune adhesion. This assumption has started to be systematically studied, both theoretically and experimentally, in the past few years. (5) Geim, A. K.; Dubonov, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Y. Nat. Mater. 2003, 2, 461. (6) Peressadko, A.; Gorb, S. N. J. Adhes. 2004, 80, 247. (7) Hui, C. Y; Glassmaker, N. J.; Tang, T.; Jagota, A. J. R. Soc. Interface 2004, 10.1098. (8) Arzt, E.; Gorb, S.; Spolenak, R. Proc. Natl. Acad. Sci. U.S.A. 2003, 16, 10603. (9) Autumm, K.; Yiching, A.; Liang, S.; Hsieh, T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, 681. (10) Huber, G.; Mantz, H.; Spolenak, R.; Meckr, K.; Jacobs, K.; Gorb, S. N.; Arzt, E. Proc. Natl. Acad. Sci. U.S.A. 2005, 45, 16293. (11) Tian, Y.; Pesika, N.; Zeng, H.; Rosenberg, K.; Zhao, B.; McGuiggan, P.; Autumn, K.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 51, 19320. (12) Autumm, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R., Israelachvili, J. N.; Full, R. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12252.
10.1021/la063104h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
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From a theoretical point of view, Arzt et al.8 have shown, using the Johnson-Kendall-Roberts (JKR) theory of adhesive contacts, that splitting up one contact into many smaller subcontacts increases adhesion. Persson et al.13,14 have shown that the effective elastic modulus of a fibrillar structure is much smaller than that of the corresponding bulk material. As a result, a fibrillar structure is expected to be highly deformable, which should help in forming good contact. This is, of course, of fundamental importance for adhesion on both smooth and rough substrates. Later, Jagota et al.15 have shown that the work required to separate a fibrillar structure from a substrate is larger than that of the same smooth material because the elastic strain energy stored in the fibrils, when they deform, is lost during pull-off. Hui et al.,7 for their part, have shown that in the case of fibrillar structures the stress concentration at the crack tip is redistributed over a zone described by a characteristic length that is significantly greater than the cross-sectional dimensions of the fibrils. Within this zone, the fibrils are under equal load-sharing conditions. Consequently, the failure of the interface involves a simultaneous failure of all fibrils inside this zone, which is quite different from the usual crack propagation for which stress concentration favors a sequential failure of fibrils starting with the fibrils closest to the crack tip. Recently, a number of experiments have been performed to show how fibrillar structures could enhance adhesion. For example, Ghatak et al.4 have studied the adhesion properties between an incision-patterned PDMS elastomer layer and a flexible plate, pulling the plate to initiate a crack at the plateelastomer interface. The crack propagates in an intermittent manner. The authors mentioned that multiple crack arrest and initiation on such substrates should result in extra dissipation of the elastic energy as observed for the fracture of soft elastomers.18 If two-dimensional textured surfaces (similar to a chocolate bar) are used and if the length scale of these patterns is small enough (typically smaller than the decay length of the stress applied to the elastomer), then a large enhancement of the interfacial fracture toughness is observed. Geim et al.,5 using substrates made of polyimide hairs posts (micrometer size and interdistance to mimic Gecko finger tips) supported by double-stick viscoelastic tape, measured high pull-off forces. Similar experiments have been performed and analyzed by Hui et al.7 They established that without the double-stick viscoelastic tape the adhesion is not increased by the fibrillar structure, suggesting that the enhanced adhesion measured by Geim et al.5 was due to dissipation in the viscoelastic layer. Peressadko et al.6 have also shown, through tack experiments, that the tenacity (force per actual contact area) of a structured surface made of polyvinylsiloxane was higher than that of an unstructured one. Finally, Crosby et al.19 have demonstrated through JKR experiments that adhesion between glass and PDMS substrate patterned by low aspect ratio cylindrical posts could be altered from 20 to 400% of the value of conventional adhesion descriptors for nonpatterned interfaces. Different local separation processes at the interface were observed, and general relationships between material properties, pattern length scales, and adhesion were established, depending on the characteristic sizes of the array (typical value are a few micrometers for the height and 20 to 500 µm for the post radius and edge-to-edge spacing). (13) Persson, B. N. J.; Gorb, S. J. Chem. Phys. 2003, 119, 11437. (14) Persson, B. N. J. J. Chem. Phys. 2003, 118, 7614. (15) Jagota, A.; Bennison, S. J. Integr. Comp. Biol. 2002, 42, 1140. (16) Hui, C. Y.; Glassmaker, N. J.; Jagota, A. J. Adhes. 2005, 81, 699. (17) Glassmaker, N. J.; Jagota, A.; Hui, C. Y; Kim, J. J. R. Soc. Interface 2004, 10.1098 (18) Lake, G. J.; Thomas, A. G. Proc. R. Soc. London, Ser. A 1967, 300, 108. (19) Crosby, A. J.; Hageman, M.; Duncan, A. Langmuir 2005, 21, 11738.
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All of these findings are indeed along the lines of early experiments from Fuller and Tabor,2020 who measured the rolling resistance of unvulcanized rubber on a rigid substrate. They observed that adhesion was enhanced on rough substrates, a result interpreted using two major assumptions: - The viscoelastic properties of the rubber are essential to forming an intimate contact with a rough surface (as a result of stress relaxation). - The substrate roughness leads to the formation of isolated contact regions during peeling. The final stages of separation involve only isolated still-adhering zones whose associated elastic energy, built up during peeling, is lost when the contact is broken. It is thus now well known that fibrillar structures are involved in the formation of intimate contact with a rough substrates because they are more deformable than bulk smooth material. Moreover, different mechanisms of energy dissipation can lead to enhanced adhesion on fibrillar substrates either because of the stretching and deformation of the fibrils themselves or because of an overall modification of the stress field both inside the fibrillar zone and in the underlying material. These arguments, however, still remain qualitative, and the relative balance between the different contributions still has to be elucidated. In this article, we investigate the adhesion properties between micropatterned PDMS elastomer surfaces and a commercial acrylic adhesive tape. We have varied the shape and the geometrical characteristics of the pattern in order to gain deeper fundamental insight into the role of patterning on adhesion by identifying the relevant parameters of the pattern that control the modulation of adhesion. To uncouple the effects on adhesion of the deformation of the patterned substrate from the modification of the dissipation within the acrylic adhesive associated with the modification of the shape of the interface due to patterning, comparative experiments on rigid polyurethane patterned substrates with the same geometrical characteristics as the soft PDMS elastomer patterned surfaces have been developed. Experimental Section Fabrication of the Patterned Substrates. Patterned PDMS substrates, either lamellae or micropillars, were produced by classical molding techniques21 using an etched silicon wafer as a mold. This mold was obtained with standard photolithographic techniques: a thin layer of positive photoresist (AZ 5206, Clariant) spin coated onto a silicon wafer was exposed to UV light through a quartz/ chrome photomask (Compugraphics) decorated with the desired pattern. After development, the bare parts of the wafer (corresponding to dots or lines) were etched by deep reactive ion etching (DRIE). The remaining photoresist was then removed to give a bare silicon wafer carved with an array of patterns with various depths and shapes (cylindrical holes or grooves). This silicon wafer was silanized (after plasma activation) with tridecafluoro-trichlorosilane in the vapor phase to ease the next demolding step. PDMS replicas were obtained by pouring a 1 mm thick layer of liquid silicone PDMS prepolymer (Sylgard 184, Dow Corning), curing at 65 °C for 24 h, and finally peeling off the crosslinked PDMS elastomer from the mold. Rigid patterned substrates were made with a UV photocurable polyurethanebased adhesive (type J-91, Summer Optical). They were prepared by pouring this optical cement on silanized, patterned PDMS elastomers replicas and by peeling off the PDMS mold after exposure to UV light (10 min) to cross link the cement. The patterned PDMS elastomer replicas of the initial wafer were obtained by first making a negative replica of the wafer using the method previously described. This process was repeated a second time to yield an exact PDMS copy of the initial wafer. To adjust the surface properties of the cement, in particular, to target adhesive properties versus the acrylic (20) Fuller, K. N. G.; Tabor, D. Proc. R. Soc. London, Ser. A 1975, 345, 327. (21) Xiaa, Y.; Whitdesides, G. M. Angew. Chem., Int. Ed. 1998, 550.
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Figure 1. Two examples of PDMS substrates patterned with micropillars (SEM images). The pillar diameter is about 2 µm, the spatial period is 4 µm, and the heights are 0.5 µm (left) and 3.8 µm (right). Both scale bars are 2 µm. Table 2. Sizes of the PDMS Lamellaea sample
width (µm)
period (µm)
height (µm)
1 2 3 4
4.5 4.5 20 20
15 25 25 30
7.5 7.5 7.5 7.5
a
Figure 2. AFM image of a PDMS surface patterned with micropillars. The measured diameter is about 2 µm, the period is 4 µm, and the height is 0.3 µm. Table 1. Height of the Pillars on Patterned Substrates Made of PDMS or Optical Cement height (µm) of optical cement
height (µm) of PDMS
0.5 0.8 1.6 3.3 3.8
0,3 0.7 1.8 3.3 3.8
tape similar to those of the PDMS elastomer, the rigid patterned substrate was further oxidized in an oxygen plasma, immersed in a PDMS polymer (20 000 g/mol, I ) 1.3), held at 80 °C for 12 h, and finally rinsed with toluene to remove all PDMS chains not attached to the surface. The thickness of the adsorbed PDMS layer was then about 10 nm thick (as estimated to be similar to what is measured by ellipsometry for the same PDMS chains adsorbed on a silicon wafer). All analyzed substrates (PDMS or polyurethane) consisted of two or three microtextured squares (1 cm × 1 cm) separated by a smooth nonpatterned region of the same material (width 0.5 cm) and supported by a microscope slide. Characterization of the Patterned Elastomers. The pillar patterns have been characterized either by scanning electronic microscopy, SEM (Figure 1), or atomic force microscopy, AFM (Figure 2). The characteristics of the patterns, pillar radius r, spatial period i (hexagonal pattern), and height h are given in Table 1. In all experiments presented below, r and i were kept constant and equal, respectively, to 1 and 4 µm, and the aspect ratio h/r was varied from 0.3 to 3.8 by varying the pillar height.
The exact height was measured by optical microscopy.
The PDMS elastomer patterns with lamellae have been characterized by optical microscopy, imaging a section of the sample normal to the lamellae. Their dimensions (height and width) and their spatial period are given in Table 2. Acrylic Adhesive. The acrylic adhesive was a commercial tape (3M 600). Its width b is 19 mm. The thickness of the adhesive layer is 17 µm, and that of the backing is 40 µm. Material Properties. The storage modulus E′ and the loss modulus E′′ of all materials have been measured using dynamic shear experiments at 24 °C and at a frequency of 0.1 Hz. For the PDMS elastomers, E′ ) 2 MPa and E′′ ) 0.07 MPa. The adhesive tape is much softer with E′ ) 0.02 MPa and E′′) 0.005 MPa. The optical cement is much harder, with a Young’s modulus of 1.6 GPa (Summer Optical information). Peel Test. The peel force F has been measured with a homemade apparatus (Figure 1 in Supporting Information), which allows us to peel a sample in 90° geometry at a chosen velocity V (ranging from 500 nm/s to 1 mm/s) and to visualize the peel front through an inverted optical microscope. The sample was illuminated perpendicularly to its plane with an optical fiber. The peel energy G was deduced from the peel force F by the well-known relation G ) F/b, with b being the width of the tape. Prior to each experiment, the adhesive tape was put into contact with the substrate, and a load corresponding to a pressure of 5 × 104 Pa was applied to the assembly for 24 h to ensure that the adhesive fills the whole space between the micropatterns (which can be easily checked optically because of the important difference in the refractive index between air and adhesive). To obtain a direct estimate of the effect of patterning on the adhesion energy, whatever the possible fluctuations of the peel energy on the corresponding nonpatterned substrate (which may result from small differences in the curing or demolding process), the peel forces on both the patterned part Fp and on the smooth part Fb of the substrate have been systematically measured on each sample. The error bar on the G values presented here is about 15%.
Results PDMS Lamellae. The simplest geometry tested consists of a regular array of equally spaced linear rectangular PDMS
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Figure 3. Image of a PDMS substrate patterned by lamellae obtained by optical microscopy (300 µm × 230 µm).
Figure 5. Variation of the logarithm of peel energy G with the logarithm of peel velocity V for an acrylic adhesive peeled off a PDMS elastomer patterned with lamellae of different geometries (Table 1). Values for the smooth, bare elastomer are also presented (9).
Figure 4. Peel front (observed from the setup described in Figure 1 in Supporting Information) advancing from bottom to top as the adhesive is peeled off a PDMS elastomer patterned with regularly spaced lamellae at a peel velocity of 1 µm/s. The lamellae width is about 4.5 µm, the height is 7.5 µm, and the period is 15 µm. The scale bar is 50 µm. As the peel front passes, the lines deform, and bubbles (shown by the arrow) are nucleated in the middle of the fingerlike instabilities.
lamellae. The adhesive is peeled off of the sample with the peel front parallel to the lamellae (Figure 4). One can observe two main features: First, as the peel front passes, the lamellae deform and bend locally to follow the peel front, Second, the detachment of the adhesive proceeds through the nucleation of bubbles in the middle of the fingerlike instabilities of the peel front. Once the peel experiment is over, the whole pattern recovers its original shape and remains undamaged. As shown in Figure 5, the peel energy G is slightly increased on the patterned substrates as compared to the smooth elastomer substrates (solid squares). Moreover, its variation with peel velocity V is different for both types of substrates. G appears to be independent of V for the smooth elastomer, whereas G increases with V on the patterned substrates. As one can clearly see in Figure 6, the enhancement in G, as estimated through ∆G ) Fp/b - Fb/b, is larger for smaller periods or smaller widths of the lamellae (patterns 1-3). When the adhesive is peeled off with the peel front perpendicular to the lines, then the increase in peel energy appears to be much smaller and the lamellae are barely deformed close to the peel front (not shown). Of course, because of the patterning, the true surface of the interface is increased in the patterned region compared to that in the smooth one, automatically leading to an apparent increase in adhesion if G is deduced through G ) F/b. Knowing the geometry of the pattern, one can easily correct for that increased area of contact. When this is performed for lamellar patterns, the remaining increase in adhesion hardly overcomes the error bar in the measurement of the peel energy. Visual observation of the lamellar patterns is thus useful to give trends (the more deformable the lamellae, the higher the peel force) but is not efficient enough to allow for a detailed quantitative investigation.
Figure 6. Enhancement of peel energy ∆G ) Gp - Gb with peel velocity V for different patterns of lamellae (Table 1).
PDMS Micropillars. Data Analysis. Figure 7 shows the peel force as a function of the peel front displacement while peeling the adhesive on a PDMS elastomer with two regions: one is patterned with micropillars (right side), and the other is smooth (left side). The peel force is significantly increased on the patterned part of the substrate (0.12 N) compared to that on the smooth one (0.06 N). Because the adhesive has invaded the channels between the pillars, the real area of contact is larger than the apparent one. Knowing the dimensions of the pillars, this surface enhancement can easily be estimated. We assume that the pillars are perfect cylinders and that the pattern remains nondeformed inside the contact
S - S0 ∆S 4πrh ) ) S0 S x3i2 where S is the contact area on the patterned elastomer, S0 is the contact area on the smooth elastomer, r is the radius of the pillars, h is their height, and i is the period of the pillar lattice. Assuming
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Figure 7. Peel force versus peel front displacement when peeling the adhesive tape on a PDMS sample at 6 µm/s. One can distinguish two parts in the plot: the left side with a low value of the peel force corresponding to the smooth elastomer and the right side with a higher value of the force corresponding to the elastomer patterned with pillars (height 0.7 µm).
Figure 8. Variation of the logarithm of peel energy G with the logarithm of peel velocity V while peeling elastomer samples patterned with micropillars of different heights. Values for the bare elastomer are also represented.
a peel energy that is similar on both patterned and smooth elastomers, the peel force Fex expected on the patterned elastomer (and enhanced compared to that on the smooth substrate as a result of the increased contact area) can be estimated from the value of Fb measured on a smooth elastomer:
(
Fex ) Fb
∆S +1 S0
)
However, this estimated Fex value (0.08 N) is much lower than the peel force actually measured on the patterned elastomer (0.12 N). Thus, the observed enhancement in peel force reported in Figure 7 cannot be accounted for by a simple renormalization of the surface of contact, and we can conclude that patterning results in an enhanced adhesive energy, Gp, that is greater than Gb, the adhesive energy on the smooth substrate. To quantify this enhancement, we chose to calculate Gp from the peel force Fp measured on the patterned part of the substrate using the following equation (which takes into account the surface enhancement due to patterning):
Fp Gp ) ∆S b +1 S0
(
)
The enhancement in adhesion energy is then defined by ∆Gp ) Gp - Gb. Gp(V) CurVes. The peel energy Gp versus the peel velocity V is shown in Figure 8 for a series of PDMS substrates patterned with micropillars of various heights. We selected substrates of similar Gb. As observed with lamellar patterns, the variation of Gp with V differs between patterned and smooth elastomer substrates: for smooth substrates, Gp is independent of V, whereas for patterned substrates, Gp increases with V and eventually saturates. Again, this suggests different origins for energy dissipation. ∆Gp(h) CurVes. As shown in Figure 9, the enhancement in the peel energy due to the patterning of the elastomer, ∆Gp, first increases with the height of the pillars. As the peel front passes, each pillar appears to be strongly deformed and stretched and
Figure 9. Enhancement of the peel energy measured on a PDMS elastomer patterned with micropillars versus the pillar height h for different peel velocities.
eventually relaxes back abruptly to its initial position when the adhesive is detached from the top of the pillars (Figure 10 and movie 1 in Supporting Information). The PDMS substrates remain undamaged and can even be reused. However, for h > 3.3 µm (about 3 times the pillar radius), ∆Gp drops. One can notice that long pillars tend to bend, overlap, and remain bent flat on the surface after peeling, especially for the pillars located at the trough of the fingerlike patterns of the peel front (Figure 11). ObserVations of the Peel Front. On both Figures 10 and 11, the fingerlike deformations of the peel front have a regular shape with a well-defined wavelength. This wavelength is independent of the pillar height. By comparing the shape of the peel front on bare and patterned zones of the PDMS substrates (one example is given in Figure 12), one finds that, first, the fingerlike oscillations of the peel front appear more periodic on the patterned part than on the bare one and second, the length of the finger-like pattern decreases as the peel velocity increases (see Figure 13 for details) until the instability disappears and the peel front becomes a straight line above a given velocity. The length of the fingers is larger on the patterned part than that on the bare part
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Figure 10. Picture of the peel front advancing from bottom to top while the adhesive tape is peeled off at 6 µm/s from a PDMS elastomer sample patterned with micropillars (height 0.8 µm). The size of the image is 100 µm × 130 µm.
Figure 12. Pictures of the peel front progressing from bottom to top while the adhesive tape is peeled off (at 6, 24, and 60 µm/s) a PDMS elastomer sample patterned with micropillars (height 1.8 µm). (Left) bare part. (Right) patterned part. The size of the image is 380 µm × 500 µm. The length of the fingerlike pattern decreases as the peel velocity increases. On the patterned part, this length is larger, and the shape of the instability is much more regular. Figure 11. Picture of the peel front progressing from bottom to top while the adhesive tape is peeled off at 6 µm/s from a PDMS elastomer sample patterned with micropillars (height 3.8 µm). The size of the image is 380 µm × 500 µm. At the edge of the fingerlike patterns of the peel front, the pillars (appearing dark) remain flat on the surface after the adhesive has been detached from the substrate.
at a given velocity. Also, as shown in Figure 14, this length increases with the pillar height for h < 3.3 µm and drops above h ) 3.3 µm. One can notice that the length of the fingers follows the same evolution with the pillar height as does ∆Gp, and third, the velocity, above which the peel front remains a straight line, is higher for the patterned part of the substrate than for the smooth one. Taking these observations all together, the presence of the pillars induces a better-defined fingering deformation of the peel front, and the length of the fingers evolves with pillar height as the enhancement of the peel energy does. Different mechanisms could be responsible for the observed enhancement in the peel energy: energy dissipation in the adhesive due to the more complicated shape of the interface leading to a modified field of deformation inside this acrylic adhesive (compared to the situation with a smooth elastomer), dissipation due to the elastic deformation of PDMS elastomer (either in the underlying PDMS film or in the micropillars themselves), and dissipation in the adhesive due to the final very rapid detachment of the adhesive from the top of the deformed pillars. To discriminate between these contributions, similar experiments have been performed on substrates patterned with rigid micropillars (polyurethane) that do not deform under peeling. Polyurethane Micropillars. Gp(V) CurVes. As shown in Figure 15, the slope of the G(V) curves is affected only for h > 3 µm.
∆Gp(h) CurVes. In Figure 16, the enhancement in the peel energy due to the patterning of rigid substrates is reported as a function of the pillar height. One can easily see that the rigid pillars are efficient only in enhancing the adhesive strength for pillars that are long enough (h > 3 µm). This is quite different from the flexible pillars that were losing their efficiency in that range of h values.
Discussion The first important factor that affects the aptitude of a patterned surface to enhance adhesion seems to be the deformability of the features formed on the surface of the substrate. Indeed, the adhesion enhancement observed with the lamellae depended on their ability to be deformed under the effect of peel forces: the thinner the lamellae, the higher the adhesion, and the trend was clear even if the adhesion enhancement remained quite weak with the lamellar geometry. With the more deformable PDMS micropillars, a significant adhesion enhancement, ∆Gp, was clearly measured, in addition to trivial effects of enhanced surface contact, and was increasing with pillar height. Accordingly, at the peel front, the long micropillars were observed to be strongly deformed before they detached from the adhesive to recover their initial state. It appears clear that the elastic energy paid to deform the micropillars was lost in the process, in a way quite similar to the mechanism proposed by Lake and Thomas18 to describe the rupture of cross-linked elastomers or the mechanism proposed by Jagota et al.15 to describe adhesion through fibrillar microstructures. To understand how the micropillars, in the case of PDMS substrates, was affecting the adhesion energy, we have modeled the elastic energy of deformation of the patterned substrates as
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Figure 13. (Left) definition of lp, the length of the fingerlike pattern, on an image of the peel front (380 µm × 500 µm). (Right) variation of lp with the peel velocity for patterned substrates.
Figure 14. Variation of lp, the length of the fingerlike pattern, with pillar height.
a function of the aspect ratio of the micropillars using scaling arguments and assuming that the pillars were acting independently of each other. Several contributions can be distinguished: each micropillar is bent and stretched, and the substrate itself can be deformed under the effect of the local peel force transmitted by the micropillar. The elastic bending energy Eb stored by one pillar submitted to a force F can be evaluated22 as Eb ) 2F2h3/ 3πEr4 (E is the Young’s modulus of the material), whereas the elastic contribution Es associated with its stretching is Es ) F2h/ 2πEr2. To determine the contribution of the elastic deformations in the bulk PDMS elastomer underneath one pillar, one can consider the classical Boussinesq problem where a cylindrical punch indents an elastic medium. The force exerted on the punch is then related to the deformation by δ ) 2F/3rE, and the resulting energy is Ed ) 8πF2/27Er. Variations of these three contributions with respect to the pillar height are shown in Figure 17. This approach clearly predicts that the higher the pillars, the larger the enhancement of adhesion. Moreover, two regimes can be (22) Landau, L.; Lifshitz, E. In The´ orie de l’EÄ lasticite´ , 2nd ed.; M.I.R.: Moscow, 1967; p 107.
Figure 15. Variation of the logarithm of the peel energy G with the logarithm of the peel velocity V while peeling the adhesive tape on rigid samples (made of optical polyurethane cement) patterned with micropillars of different heights. Values for the bare sample are also represented.
distinguished depending on the value of h. For h < 1.6r, the elastic energy stored in the system is dominated by the substrate deformation. This is in good agreement with our observations: for small h ) 0.3 or 0.7 µm (we recall again that all of the above presented experiments have been performed with a fixed micropillar radius of r ) 1 µm), the micropillars are only slightly deformed, but they transmit stresses to the bulk elastomer underlying the pillars. The resulting deformation of the substrate relaxes back after the peel front has gone, and the corresponding elastic energy is lost. For h > 1.6r, we predict that the dominant contribution to the elastic energy is related to the bending of the pillars. This bending contribution increases quickly with pillar height, as observed experimentally. For our system, the bending energy of the pillars overcomes the two others contributions by 1 order of magnitude for h g 3.5 µm. For long micropillars, a buckling instability of the pillars is expected under the pressure exerted on the system to form the contact between the adhesive and the substrate. This buckling instability has not been observed
Adhesion Enhancement through Micropatterning
Figure 16. Enhancement of the peel energy measured on rigid patterned substrates (made of optical polyurethane cement) versus the pillars height h for different peel velocities.
for our samples even for the largest height investigated. However, in the range of height above 3.5 µm, another effect comes into play: above h ) 3.8 µm, the micropillars were observed to lose their efficiency in enhancing adhesion. For this same height, they started to no longer be able to relax to their original state after the peel front had gone. Some micropillars remained irreversibly bent down, sticking to the surface, especially along the fingers formed in the adhesive by the periodic distortion of the peel front, as shown in Figure 11. When such is the case, the system can no longer be described in terms of independent pillars. The above-developed picture (stretching of the micropillars followed by their detachment one after the other, thus acting independently of each other with respect to energy losses) no longer holds. The gain in surface energy resulting from a final state in which some pillars stick all together then needs to be taken into account. This additional term (opposite in sign to the elastic energy of deformation of the pillars that is lost when the peel front passes) is not easy to estimate because it depends on the exact way in which the micropillars remain connected to each other. However, it clearly contributes to decrease the adhesion enhancement. There is thus an optimum height for the
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deformable pillars to enhance adhesion (between 3.3 and 3.8 µm for the geometry that we have investigated). The simple description in terms of elastic deformable pillars independent of each other should hold below a limiting height hc. This limiting height hc should depend on the spacing between pillars (another parameter of the pattern that we have just started to explore). When it holds, the description we have developed shows that the important parameter controlling which contribution (bending of the pillars or deformation of the underlying substrate) dominates the elastic energy of the patterned substrate lost during peeling is not the height of the pillars but their aspect ratio. Increasing the pillar height improves the enhancement of adhesion, but this is limited: for very long pillars (or very small spacing between pillars), interactions between neighboring pillars induce a partial collapse of the pillars after peeling, which corresponds to a surface energy that is lower than that of the substrate in contact with air and in the initial configuration of the pattern. The corresponding gain in surface energy partially compensates for the loss of energy associated with the bending of the micropillars, and a noticeable decrease in adhesion enhancement is measured. For the non-deformable pillars, no elastic contribution due to the deformation of the pillars is expected. Indeed, experiments show that the rigid pillars with h < 3 µm did not produce noticeable adhesion enhancement. We then face a new question: what is the origin of the observed enhanced adhesion in the case of long rigid pillars? The only possibility comes from the acrylic adhesive side of the contact. The adhesive layer is a viscoelastic material. Patterning the substrate should affect the viscoelastic losses within the adhesive layer because the whole strain field in the vicinity of the interface should be modified. Obviously, the depth of the layer in the adhesive inside which the deformations are affected by the patterning depends on the geometry of the pattern and, in the case of micropillars, on their height. It is then reasonable to assume that the enhancement of adhesion energy that we have observed for rigid micropillars with h > 3 µm is due to a change in the deformation field inside the adhesive layer associated with the patterning of the substrate. The next open question is then to evaluate whether such a contribution to the enhancement of adhesion due to the adhesive layer was not already present in the case of deformable substrates and superimposed on the contribution of the elastic deformations in the PDMS substrate. One way to try to answer this question is to analyze the velocity effects because the viscoelastic losses
Figure 17. Contributions to the peel energy versus the pillar height h as the adhesive is peeled off the PDMS elastomer substrate patterned with micropillars. Only dissipation inside the elastomer is taken into account and evaluated by scaling laws.
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Lamblet et al.
Figure 18. (Right) comparison of pictures of the peel front (advancing from bottom to top at V ) 6 µm/s) while the adhesive tape is peeled off a PDMS elastomer sample patterned with soft (top picture) and hard (bottom picture) micropillars (h ) 1.6 µm). The size of the images is 380 µm × 500 µm. (Left) schematic side view of peeling and the definition of length L.
inside the adhesive are expected to be highly velocity-dependent, whereas elastic contributions are not. We have established that the velocity dependences were indeed quite different for smooth or patterned substrates with moderately long micropillars, either deformable or rigid, at a fixed adhesive layer. Through precise observations of the peel front, we have some additional experimental indications that kinetics effects show up for deformable moderately long micropillars. This is shown in Figure 18. The length L that defines the highly deformed region of the adhesive near the peel front is much larger for soft than for hard patterned substrates under the same peel conditions, suggesting different dissipation inside the adhesive. In the case of deformable substrates, the rapid detachment of the adhesive from the top of the pillars can lead to dissipation processes that are quite different from what happens in the case of rigid substrates where the adhesive detaches more progressively. The complexity of the situation would require a finite element numerical analysis, but this is beyond the scope of the present article. We think, however, that the systematic comparison between deformable and nondeformable patterned substrates having the same geometry is a way to guide such modeling. This should help in understanding how both sides of the interface are coupled in their deformations and dissipations, which is a difficult and unsolved question in most adhesive assemblies. Also, very simple experiments could be performed with a series of adhesives of various viscoelastic properties, which would certainly allow one to test and identify the underlying mechanisms.
Conclusions By investigating in a systematic manner the efficiency of surface patterning in enhancing the adhesive strength at PDMSacrylic adhesive interfaces and by comparing, at fixed geometry, rigid and deformable patterned surfaces (having similar adhesion energies on the smooth part of the substrate), we have shown that soft deformable micrometric patterns are quite efficient at enhancing adhesion. For soft arrays of micropillars, using simple scaling arguments, several regimes have been identified, de-
pending on the aspect ratio of the pillars, with either a dominant role of the deformations in the underlying bulk elastomer film through the peel force locally transmitted by the pillars (h/r < 1.6) or of the bending energy of the micropillar themselves, which is lost when the peel front passes (h/r > 1.6). For higher structures (h/r > 3), soft patterns are altered when the contact is broken. The corresponding gain in surface energy largely compensates for the energy loss associated with elastic deformation. Rigid patterns, for which there is no elastic contribution to the adhesion, then become more efficient than soft ones, probably because of enhanced dissipation inside the adhesive itself. The present set of experiments backs up the idea that the ability of the patterned surface to be deformed plays a crucial role in enhancing adhesion, similarly to what is seen, for example, in the case of geckos. This is quite similar in spirit to what has been reported by Crosby et al.,19 even if the present investigation strongly differs in the range of pattern sizes. (Our pillars have a 1 µm radius and the contact is 1 cm wide, whereas Crosby et al.19 were using 50 times wider cylinders and the contact covered only a few cylinders.) Our investigations show that, by varying the size of the pattern and/or the mechanical properties of the substrate, it is possible to fine tune the level of adhesion at PDMSacrylic adhesive interfaces, independently of the details of the chemistry of the adhesive. Acknowledgment. This work was financially supported by Rhodia Silicone. We thank Elie Raphael and Robert H. Austin for fruitful discussions and suggestions as well as Huges Bodiguel and Christian Fretigny for providing access to their inverted AFM. The DRIE process has been performed at the Cornell NanoScale Facility. Supporting Information Available: Setup of peel test experiment (homemade apparatus). Movie of the PDMS elastomer sample patterned by pillars (h ) 1.8 µm) peeled off at 6 µm/s. This material is available free of charge via the Internet at http://pubs.acs.org. LA063104H
