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Adhesion of elastomeric surfaces structured with micro-dimples
Applied Surface Science 326 (2015) 145–150
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Adhesion of elastomeric surfaces structured with micro-dimples Gabriele Nanni a,b,∗ , Despina Fragouli a , Luca Ceseracciu a , Athanassia Athanassiou a,∗∗ a b
Smart Materials, Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy Dipartimento di Fisica, Università degli Studi di Genova, Via Dodecaneso, 33, 16146 Genova, Italy
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 9 November 2014 Accepted 19 November 2014 Available online 27 November 2014 Keywords: Silicone elastomers Surface texturing Dry adhesion Crack propagation Suction effect
a b s t r a c t Topography has a dominant role in determining the adhesion properties of a surface. In this work we explore how arrays of micron-sized dimples can alter the adhesion performance of elastomeric surfaces. We study the effect of the dimple surface coverage, showing that the dimples act both as passive suction devices, allowing to exceed the adhesion performance of untextured surfaces, and crack-like defects, generating stress concentration at the edge of the contact area between the surface of the sample and a flat surface. Interestingly, our results reveal that the suction effect generated by the negative pressure produced by the dimples can be effectively tuned by adjusting their depth. These findings have significant relevance for the fabrication of adhesive systems in which selective adhesion to objects with small difference in weight is required. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Adhesion of solid objects is receiving great attention in the last years because of promising practical applications arising from research. New insight into the adhesion mechanism of biological systems [1–6] showed that, in addition to the chemistry, the morphology is also critical in determining the adhesion properties of a surface. For example, it has been demonstrated both theoretically and experimentally that by splitting a surface into smaller fibrillar micro-contacts can lead to an increase of adhesion [7,8]. The remarkable performance of such fibrillar surfaces derives from the mechanical independence of each topographic feature. In particular, the separation between a fibrillar surface from a substrate can be represented by a crack that has to be initiated for each fibril, whereas for a flat surface a single crack propagates continuously and uninterrupted after its initiation. Inspired by this principle, in recent years much progress has been made in the fabrication of biomimetic structures similar in design and performance to those found in biological systems [9]. Furthermore, following biomimetic designs, surfaces with switchable adherence have been fabricated using responsive materials that modify their topography under the
∗ Corresponding author at: Smart Materials-Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy. Tel.: +39 010 71781856. ∗∗ Corresponding author at: Smart Materials-Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy. Tel.: +39 010 71781528. E-mail addresses: [email protected] (G. Nanni), [email protected] (A. Athanassiou). http://dx.doi.org/10.1016/j.apsusc.2014.11.108 0169-4332/© 2014 Elsevier B.V. All rights reserved.
action of an external stimulus [10–12], resulting in the modification of the final adhesion response. However, such surfaces do not fit those applications where precise control of adhesion is needed, since the detachment always occurs at high loads. The ability to fine tune the adhesion between two different surfaces [13] is highly desirable in many fields, including micro and nanoelectronics, biotechnology and robotics. For this reason, alternative adhesive systems were developed. For instance, it has been shown how it is possible to regulate the adhesion with a relative easy real-time control by reversibly tuning the topography of polydimethylsiloxane (PDMS) wrinkles from a sinusoidal wavy shape to completely flattened [14]. In another work it has been demonstrated the transfer printing of solid objects by kinetically controlling the adhesion of elastomeric stamps [15]. The transfer printing strategy relies on relatively strong adhesion of rubber to solids at fast peel rates, and significantly weaker adhesion at slower rates. Despite such few remarkable examples, surfaces showing controllable adhesive forces remain poorly implemented. In this work we explore the adhesion properties of elastomeric surfaces textured with micron-sized dimples with the expectation of providing effective rules for fabricating surfaces with customtailored adhesive properties. We prove that specific pull-off forces can be obtained, higher or smaller than that of the respective untextured surface, as a result of the combined effect of crack formation at the edge of the contact area, and suction phenomena generated inside the dimples. In particular, we correlate the dimple surface coverage with the stress concentration generated at the edge of the contact area, and we demonstrate how the pull-off force can be effectively adjusted by varying the dimple depth. This study is
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relevant for the design of adhesive systems based on elastomeric surfaces for selective pick-and-place of objects where high weight sensitivity and selectivity is required. 2. Experimental 2.1. Materials and equipments Sylgard 184 was purchased by Dow Corning Corporation (Midland, MI, US). SU-8 type 25 and type 100, and SU-8 developer were purchased from Micro Chem Corporation (Newton, MA, US). S1813 resist was obtained by Shipley Europe Ltd. (Coventry, UK). According to the manufacturer, it is possible to fabricate structures in S1813 as high as 2 m. In order to fabricate structures higher than 2 m using this material, we let the solvent of S1813 to partially evaporate, obtaining a more viscous resist (v-S1813). A spin-speed versus thickness calibration-curve provided us the information required to select the appropriate spin conditions to achieve the desired film thickness in v-S1813. All the others chemicals were used as received. Lithography with SU-8 resists was performed using a mask aligner MA6 (SUSS MicroTec), while lithography with v-S1813 was performed using a laser writer DWL 66FS (Heidelberg Instruments Mikrotechnik GmbH). The patterned surfaces were characterized by Scanning Electron Microscopy (SEM) using a Helios NanoLab 650 Focused Ion Beam SEM (FEI Company). A dynamic mechanical analyzer Q800 (TA instruments) was used to perform adhesion measurements of surfaces textured with dimples arrays. An Ultra Nanoindentation Tester (CSM Instruments SA, Switzerland) was used to measure the pull-off force onto precisely positioned dimples. The height of the fabricated features was measured by a XP-2 Profiler (Ambios technology). Photos of the replicas were taken with an optical microscope DM2500 (Leica Microsystems).
Table 1 Topographical features of the different textured surfaces investigated in this work: inter-dimples spacing (s), spherical dimples radius (a), spherical dimple depth (h), square-shaped dimple side (l), square-shaped dimple depth (d). Spherical micro-cups
Square-shaped dimples
s (m)
a (m)
h (m)
s (m)
l (m)
d (m)
28 50 63 77 90 120
24 24 24 24 24 24
13 13 13 13 13 13
77 77 77 77 50 50 50 50 28 28 28 28
42 42 42 42 42 42 42 42 42 42 42 42
4 6.5 10 110 4 6.5 10 110 4 6.5 10 110
obtained by washing the sample for 10 min in SU-8 developer, followed by rinsing in 2-propanol. (2) Pillars in SU-8 type 25 with d = 10 m and l = 42 m were obtained following the same procedure described in (1) using the following process parameters: spin coating at 3000 rpm, soft bake at 65 ◦ C for 2 min and at 95 ◦ C for 5 min on a hotplate, exposure dose 200 mJ cm−2 , post exposure bake at 65 ◦ C for 1 min and at 95 ◦ C for 2 min, developing for 3 min in SU-8 developer followed by rinsing in 2-propanol. (3) Pillars having d = 6.5 m and l = 42 m were fabricated in v-S1813 by laser writing. First, v-S1813 was dispensed on a silicon wafer. The wafer was spin-coated at 2000 rpm. Squares with the desired side were imaged by shining a laser beam on the resist film. The pillar array was obtained by washing the sample in MF-319 for 315 s, followed by rinsing in distilled water. (4) Squares-shaped pillars having d = 4 m and l = 42 m were obtained in v-S1813 by laser writing as described in (3), using a spin-rate of 6000 rpm and developing in MF-319 for 150 s.
2.2. Fabrication of spherical micro-dome arrays
2.4. Replica molding for the fabrication of dimple arrays
Arrays of micro-domes were fabricated in v-S1813 by thermal reflow technique. In a first step, a silicon wafer was chemically cleaned with acetone and afterwards dried with nitrogen. A liquid solution of v-S1813 was dispensed on the wafer, and a uniform thin layer of resist was formed by spinning the wafer at 2000 rpm. The thickness of the film obtained was 9 m. Circular features having a diameter 2a = 48 m were imaged on the resist by laser writing ( = 405 nm) and cylindrical pillars were obtained after development, followed by washing the sample in distilled water. Micro-domes were produced by thermal softening and roundening of the pillars at 150 ◦ C for 30 min on a hotplate. Due to the reflow, the final height of the domes was 13 m. SEM images of selected arrays are shown in Fig. S1 in Supporting Information.
A PDMS solution was prepared by mixing prepolymer and crosslinker with a weight ratio of 10:1. After degassing to remove air bubbles, the solution was poured on the master and negative replicas were obtained by carefully peeling the elastomeric film after thermal curing. Curing was performed on a hotplate at 70 ◦ C for 1 h. Fig. 1a and b shows representative optical micrographs of arrays of spherical and square-shaped dimples, respectively. The key geometrical features of the fabricated surfaces are depicted in Fig. 1c, while the dimensions of the dimples used in this study are specified in Table 1.
2.3. Fabrication of square-shaped micro-pillar arrays Arrays of square-shaped micro-pillars having four different heights were fabricated as follow. (1) SU-8 type 100 was dispensed on a silicon wafer. The wafer was spin-coated at 2500 rpm. Next, the sample was soft-baked at 65 ◦ C for 10 min and at 95 ◦ C for 30 min on a hotplate. The thickness of the film obtained was 110 m. A sodalime masks (Deltamask, The Netherlands) of squareshaped patterns (42 m side) with various spacing were used for the exposure of the resist. Patterning was performed by exposing the samples to UV radiation ( = 365 nm) using an exposure dose of 300 mJ cm−2 . Exposure was followed by a bake on a hotplate at 65 ◦ C for 1 min and at 95 ◦ C for 10 min. The samples were allowed to cool and, finally, pillars of d = 110 m and l = 42 m were
2.5. Adhesion measurements on dimple arrays Pull-off forces were obtained by measuring quasi-static force versus displacement curves. In Fig. 2 schematics of the key steps involved in a typical adhesion test are shown. A square-shaped flat glass having a surface area of 13.5 mm2 glued to a metallic shaft by means of ∼1 mm thick wax layer was used as probe. The shaft can be moved up and down in displacement or force control. During a typical adhesion test, the probe was put in contact with the surface of the sample applying a force of 0.1 N. The nature of the probe makes the measured pull-off forces very sensitive even to slight misalignments. To align the glass punch to the surface of the samples, the wax was first softened with an air flow at 50 ◦ C, and then it was left to solidify for a few minutes being in contact with the surface of the sample under the preload of 0.1 N. At this point, after an additional compressive displacement of 20 m, the probe was retracted at a constant rate of 50 m min−1 . The magnitude of
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Fig. 1. Surfaces textured with dimple arrays fabricated for this study. (a) Representative optical micrograph of spherical dimples (scale bar 100 m). Inset: single spherical dimple with a diameter of 48 m. (b) Representative optical micrograph of square-shaped dimples (scale bar 100 m). Inset: single dimple with a square side of 42 m. (c) Description of the key geometrical parameters of the arrays.
the pull-off force was considered equal to the force required to separate the probe surface from the sample. Experiments were always performed 24 h after the preparation of the samples and under controlled environmental conditions (temperature 22 ◦ C and relative humidity 35% ± 5%).
in load control, (3) pause at a constant load, (4) unloading in load control, (5) and retraction of the tip in depth control. To guarantee a good depth or load control during the indentation measurement we used the following parameters: contact load of 50 N, pause time of 10 s, loading and unloading rate of 200 N min−1 . The evaluation of the error bars is done by performing 4 measurements on exactly the same area.
2.6. Adhesion measurements on specific sample zones Pull-off forces were obtained with an Ultra nanoindentation tester (CSM Instruments SA, Switzerland). As a punch, we used a flat-ended diamond cone with a tip diameter of 204 m. A typical measurement cycle consisted of five steps (Fig. 3): (1) approach of the tip to the sample surface in depth control, (2) indentation
3. Results and discussion 3.1. Effect of the dimple surface coverage Initially, spherical dimples were used to study the effect of the dimple surface coverage in the adhesion between the elastomeric
Fig. 2. Key steps of an adhesion test performed with a flat probe. (a) A square-shaped flat glass substrate glued to a metallic shaft by means of ∼1 mm thick wax layer was used as probe. (b) The probe was brought in contact with the surface of the sample. (c) The wax was softened with an air flow at 50 ◦ C, and it was left to solidify for a few minutes in contact with the surface of the sample under a preload of 0.1 N. (d) After an additional compressive displacement of 20 m, the probe was retracted at a constant rate of 50 m min−1 until detachment occurred.
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400 (1)
(2)
(3)
(4)
100
150
(5)
300 200
force (μ N)
100 0 0
50
200
250
300
350
400
-100 -200 -300 -400 -500
me (s) Fig. 3. Typical adhesion measurement cycle performed with the nanoindenter: (1) approach of the tip to the surface of the sample in depth control, (2) indentation in load control, (3) pause at a constant load, (4) unloading in load control, (5) and retraction of the tip in depth control.
samples and the flat surface of a glass probe, being the surface covered by dimples in a range between 0 (untextured surface) and 31%. Analyzing the data of the pull-off forces obtained for the various surfaces, we can clearly distinguish a general trend: the greatest pull-off force is obtained for surfaces at dimple surface coverage between 5 and 10%, after which the pull-off force decreases monotonously, as shown in Fig. 4. In order to isolate the effect of the texturing in the adhesion mechanism from other factors that may affect the results (such as the preparation, handling and storage of the samples), the pull-off force is normalized by the force necessary to detach the punch from an untextured control sample of the same material (∼278 mN). As it can be seen, the pull-off force shows a maximum increase of 2.4% at a value of ∼10% of surface covered by dimples. This increase of pull-off force can be explained considering that when a remote pulling force acts in a direction normal to the surface, the punch retracts and the PDMS deforms,
1.04 1.02
normalized pull-off force
1 0.98 0.96 0.94 0.92 0.9 0.88 0.86 0.84 0.82 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
dimple surface coverage (%) Fig. 4. Pull-off force measured between a flat punch and samples of PDMS as a function of the dimple surface coverage. Each data point is the mean value of six measurements on two different samples. The variation of pull-off force for the adhesion tests was very small.
producing a change of volume of the dimples. If we assume that the two surfaces are perfectly aligned, air is trapped into the dimples. Since the pressure has an inverse relationship with the volume [16], the expansion of the air volume generated by the retraction of the punch is balanced by the formation of negative pressure inside the dimples, producing a suction effect. From Fig. 4 it can be noticed that the decrease of the pull-off force becomes more pronounced at higher values of dimple surface coverage. This behavior can be explained by the linear elastic theory [17], that predicts intensified magnitude of the stress at a crack tip compared to the nominal stress level. We suppose that the dimples present at the edge of the contact area between the probe and the sample may act as crack-like defects, intensifying the stress caused by the applied remote load, and eventually being responsible for the initiation of cracks along the interface between the elastomer and the punch. As the dimple surface coverage increases, a higher probability to have dimples along the edge of the contact area is expected and, as a consequence, the critical stress at which the crack propagation initiates will be reached at smaller values of remote load. In other words, above the value of 10% of dimple surface coverage the expansion of the air volume inside the dimples generated by the retraction of the punch produces a negligible suction effect, as the pull-off occurs at low loads due to the crack formation and propagation at the edge of the contact area. A similar hypothesis was invoked by Spuskanyuk et al. to explain the observed increased pull-off force of a mushroom-shaped fibril compared with that of a punch-shaped fibril [18]. They proposed that a possible defect at the edge of the contact area is more damaging for the adhesion of a punch-shaped fibril, as the highest stress during pull-off is applied at the edges of this structure, compared to a mushroom-shaped fibril, where the highest stress is expected to be at the center of the contact. Since the shape of the dimples does not affect the measured pull-off forces as long as their volume remains the same (see Fig. S2 in Supporting Information), for simplicity from now on all the results will be presented using square-shaped dimples. To verify the importance of the presence of defects at the edge of the contact area between the textured elastomer sample and the punch for the detachment mechanism, we performed dedicated experiments, carefully controlling the position of the dimples with respect to the edge of the contact area. For the tests we used an Ultra Nanoindentation Tester, equipped with a flat-ended diamond cone having a diameter of 204 m. A single line of dimples was used for this experiment. The dimples were square-shaped with side of 42 m, depth of 10 m, and separation distance of 50 m. The flat end of the cone was initially aligned with respect to the patterned line so that only the first dimple was located inside the contact area between the punch and the elastomer. Then the punch was moved horizontally by subsequent steps of 15 m toward the adjacent dimples. Fig. 5 shows the results of the measured pull-off forces for each location of the punch. Also in this case, the pull-off force is normalized by the force necessary to detach the punch from an untextured control sample of the same material (∼1300 N). The maximum pull-off force was obtained at the initial position, when the single dimple was totally located inside the area generated by the projection of the punch on the elastomer sample, exceeding the pull-off force obtained for the untextured surface (see inset drawing in Fig. 5). At that position there were no defects present at the edge of the contact area between the elastomer samples and the punch, from which a crack could initiate, and therefore the pull-off force exceeded the adhesion performance of the untextured surface. By moving the punch toward the adjacent dimples, the edge of the contact area always coincides at various degrees with the position of the dimples, offering different crack initiation points. Thus, the pull-off force remains always below the value of the untextured control sample. The minimum pull-off force occurs
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and the samples. As demonstrated, the pull-off force increases by reducing the depth, and thus the volume of the dimples. The maximum pull-off force was obtained with ∼12% dimple surface coverage and dimple depth of 4 m, exceeding the pull-off force of the untextured control sample by 10%. The increase of the pull-off force for decreasing volume of the dimples can be explained considering Boyle’s law [16], as previously introduced by Jiang et al. to describe the high adhesion of water droplets to superhydrophobic porous nanostructures [19–21]. Assuming the air as an ideal gas, the suction force of a single dimple is: Fsuction = pin A
Fig. 5. Pull-off force as a function of the punch position with respect to underlying dimples. The presence of dimples along the edge of the contact area controls the adhesion mechanism. The cycles of the schematics represent the actual punch position with respect to the dimples.
1.2
dimple depth 4 um 6.5 um 10 um 110 um
normalized pull-off force
1.1
1
0.9
V Vin + V
(1)
where pin and Vin are the pressure and the volume of the air in the dimple, A is the area of the base of the dimple, V is the increased volume of the sealed air under the action of the pulling force. V can be assumed to be constant and independent from the depth of the dimples. Therefore, according to Eq. (1), shallow dimples characterized by lower Vin than deep dimples are expected to provide higher suction forces. The results above indicate that the pull-off force can be easily controlled by changing the volume of air trapped inside the dimples, resulting in the tuning of the suction effect. Moreover, the trend suggests that higher pull-off forces can be expected by further reducing the volume of the dimples. Although the aim of the specific work is to finely tune the adhesion between surfaces by dimple texturing rather than to enhance the adhesion of the respective untextured surfaces, it is worth mentioning that also in the case of mushroom-shaped fibrillar structures the suction may contribute up to 10% of the overall measured pull-off forces [22,23], a value consistent with our results.
4. Conclusions
0.8
0.7
0.6 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38
dimple surface coverage (%) Fig. 6. Effect of the dimples depth in the adhesion mechanism. At any value of dimple surface coverage, the pull-off force increases by reducing the dimple depth.
at a punch position of 120 m that corresponds to the presence of two defects along the perimeter of the contact area. As expected, the minimum pull-off force is repeated with a periodicity of 90 m, equal to the periodicity of the texture. These results demonstrate that the position of the dimples with respect to the edge of contact area significantly impacts the adhesion behavior of the system, as the dimples can act both as passive suction devices and defects for the initiation of a crack. 3.2. Effect of the dimple depth A second geometric parameter of the studied structures that influences the pull-off force values is the depth of the dimples. Fig. 6 shows the pull-off forces obtained for surfaces textured with square-shaped dimples having a depth ranging from 4 to 110 m. The volume of the dimples was tuned by varying their depth d, while keeping constant the surface of contact between the punch
In this study surface texturing of elastomers having diverse dimple surface coverage and depth is proposed as a mechanism to fabricate custom-tailored elastomeric adhesive systems. The tests performed showed that the presence of the dimples is beneficial for adhesion due to suction effect if the dimple surface coverage stays around 10%, whereas above this value the adhesion decreases. This effect was attributed to the presence of dimples at the edge of the contact area that act as crack-like defects. At the same time, it is shown that changing the dimple depth is an effective way to tune the adhesion response of the surfaces, since the suction effect is more intense as the dimple depth decreases. The appropriate combination of these two factors results in surface textures with desired fine-tuned adhesion properties. In future works, mechanisms of crack arresting [24,25] at the edge of the contact area will be exploited to significantly increase the adhesion performance. By contrast, the dimple surface coverage and depth can be deliberately increased to reduce the adhesive response of the surfaces, or surfaces with dimple surface coverage and depth gradient can be fabricated to induce directional adhesion behavior. These finding may be useful in several fields such as medicine, for the development of biocompatible patches less irritating than acrylic-based ones, or in electronics, for the manipulation and assembly of microscopic components.
Acknowledgments The authors thank Dr. E. Mele for helpful discussion and Ms. E. Rondanina for technical assistance in the laser-writing processes.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2014.11.108. References [1] K. Autumn, Y.A. Liang, S.T. Hsieh, W. Zesch, W.P. Chan, T.W. Kenny, Adhesive force of a single gecko foot-hair, Nature 405 (2000) 681–685. [2] K. Autumn, M. Sitti, Y.A. Liang, A.M. Peattie, W.R. Hansen, S. Sponberg, Evidence for van der Waals adhesion in gecko setae, Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 12252–12256. [3] K. Autumn, A.M. Peattie, Mechanisms of adhesion in geckos, Integr. Comp. Biol. 42 (2002) 1081–1090. [4] I. Scholz, W.J.P. Barnes, J.M. Smith, W. Baumgartner, Ultrastructure and physical properties of an adhesive surface, the toe pad epithelium of the tree frog, Litoria caerulea White, J. Exp. Biol. 212 (2009) 155–162. [5] F. Tramacere, L. Beccai, M. Kuba, A. Gozzi, A. Bifone, B. Mazzolai, The morphology and adhesion mechanism of Octopus vulgaris suckers, PLOS ONE 8 (2013) e65074. [6] F. Tramacere, A. Kovalev, T. Kleinteich, S.N. Gorb, B. Mazzolai, Structure and mechanical properties of Octopus vulgaris suckers, J. R. Soc. Interface 11 (2014) 20130816. [7] E. Arzt, S. Gorb, R. Spolenak, From micro to nano contacts in biological attachment devices, Proc. Natl. Acad. Sci. U. S. A. 100 (2003) 10603–10606. [8] M. Kamperman, E. Kroner, A. del Campo, R.M. McMeeking, E. Arzt, Functional adhesive surfaces with gecko effect: the concept of contact splitting, Adv. Eng. Mater. 12 (2010) 335–348. [9] L.F. Boesel, C. Greiner, E. Arzt, A. del Campo, Gecko-inspired surfaces: a path to strong and reversible dry adhesives, Adv. Mater. 22 (2010) 2125–2137. [10] S. Reddy, E. Arzt, A. del Campo, Bioinspired surfaces with switchable adhesion, Adv. Mater. 19 (2007) 3833–3837.
[11] J. Cui, D.-M. Drotlef, I. Larraza, J.P. Fernández-Blázquez, L.F. Boesel, C. Ohm, Bioinspired actuated adhesive patterns of liquid crystalline elastomers, Adv. Mater. 24 (2012) 4601–4604. [12] D.-M. Drotlef, P. Blümler, A. del Campo, Magnetically actuated patterns for bioinspired reversible adhesion (dry and wet), Adv. Mater. 26 (2014) 775–779. [13] C. Yu, J. Ma, J. Zhang, J. Lou, D. Wen, Q. Li, Modulating particle adhesion with micro-patterned surfaces, ACS Appl. Mater. Interfaces 6 (2014) 8199–8207. [14] S. Yang, K. Khare, P.-C. Lin, Harnessing surface wrinkle patterns in soft matter, Adv. Funct. Mater. 20 (2010) 2550–2564. [15] M.A. Meitl, Z.-T. Zhu, V. Kumar, K.J. Lee, X. Feng, Y.Y. Huang, Transfer printing by kinetic control of adhesion to an elastomeric stamp, Nat. Mater. 5 (2005) 33–38. [16] J.B. West, The original presentation of Boyle’s law, J. Appl. Physiol. 87 (1999) 1543–1545. [17] J.R. Barber, Elasticity, 3rd ed., Springer, Dordrecht/Heidelberg/London/New York, 2010. [18] A.V. Spuskanyuk, R.M. McMeeking, V.S. Deshpande, E. Arzt, The effect of shape on the adhesion of fibrillar surfaces, Acta Biomater. 4 (2008) 1669–1676. [19] M. Jin, X. Feng, L. Feng, T. Sun, J. Zhai, T. Li, L. Jiang, Superhydrophobic aligned polystyrene nanotube films with high adhesive force, Adv. Mater. 17 (2005) 1977–1981. [20] Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin, L. Jiang, Designing superhydrophobic porous nanostructures with tunable water adhesion, Adv. Mater. 21 (2009) 3799–3803. [21] L. Heng, X. Meng, B. Wang, L. Jiang, Bioinspired design of honeycomb structure interfaces with controllable water adhesion, Langmuir 29 (2013) 9491–9498. [22] L. Heepe, M. Varenberg, Y. Itovich, S.N. Gorb, Suction component in adhesion of mushroom-shaped microstructure, J. R. Soc. Interface 8 (2011) 585–589. [23] G. Carbone, E. Pierro, Effect of interfacial air entrapment on the adhesion of bio-inspired mushroom-shaped micro-pillars, Soft Matter 8 (2012) 7904. [24] J.Y. Chung, M.K. Chaudhury, Roles of discontinuities in bio-inspired adhesive pads, J. R. Soc. Interface 2 (2005) 55–61. [25] S.A. Pendergraph, M.D. Bartlett, K.R. Carter, A.J. Crosby, Enhancing adhesion of elastomeric composites through facile patterning of surface discontinuities, ACS Appl. Mater. Interfaces 6 (2014) 6845–6850.
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Adhesion of elastomeric surfaces structured with micro-dimples Gabriele Nanni a,b,∗ , Despina Fragouli a , Luca Ceseracciu a , Athanassia Athanassiou a,∗∗ a b
Smart Materials, Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy Dipartimento di Fisica, Università degli Studi di Genova, Via Dodecaneso, 33, 16146 Genova, Italy
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 9 November 2014 Accepted 19 November 2014 Available online 27 November 2014 Keywords: Silicone elastomers Surface texturing Dry adhesion Crack propagation Suction effect
a b s t r a c t Topography has a dominant role in determining the adhesion properties of a surface. In this work we explore how arrays of micron-sized dimples can alter the adhesion performance of elastomeric surfaces. We study the effect of the dimple surface coverage, showing that the dimples act both as passive suction devices, allowing to exceed the adhesion performance of untextured surfaces, and crack-like defects, generating stress concentration at the edge of the contact area between the surface of the sample and a flat surface. Interestingly, our results reveal that the suction effect generated by the negative pressure produced by the dimples can be effectively tuned by adjusting their depth. These findings have significant relevance for the fabrication of adhesive systems in which selective adhesion to objects with small difference in weight is required. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Adhesion of solid objects is receiving great attention in the last years because of promising practical applications arising from research. New insight into the adhesion mechanism of biological systems [1–6] showed that, in addition to the chemistry, the morphology is also critical in determining the adhesion properties of a surface. For example, it has been demonstrated both theoretically and experimentally that by splitting a surface into smaller fibrillar micro-contacts can lead to an increase of adhesion [7,8]. The remarkable performance of such fibrillar surfaces derives from the mechanical independence of each topographic feature. In particular, the separation between a fibrillar surface from a substrate can be represented by a crack that has to be initiated for each fibril, whereas for a flat surface a single crack propagates continuously and uninterrupted after its initiation. Inspired by this principle, in recent years much progress has been made in the fabrication of biomimetic structures similar in design and performance to those found in biological systems [9]. Furthermore, following biomimetic designs, surfaces with switchable adherence have been fabricated using responsive materials that modify their topography under the
∗ Corresponding author at: Smart Materials-Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy. Tel.: +39 010 71781856. ∗∗ Corresponding author at: Smart Materials-Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy. Tel.: +39 010 71781528. E-mail addresses: [email protected] (G. Nanni), [email protected] (A. Athanassiou). http://dx.doi.org/10.1016/j.apsusc.2014.11.108 0169-4332/© 2014 Elsevier B.V. All rights reserved.
action of an external stimulus [10–12], resulting in the modification of the final adhesion response. However, such surfaces do not fit those applications where precise control of adhesion is needed, since the detachment always occurs at high loads. The ability to fine tune the adhesion between two different surfaces [13] is highly desirable in many fields, including micro and nanoelectronics, biotechnology and robotics. For this reason, alternative adhesive systems were developed. For instance, it has been shown how it is possible to regulate the adhesion with a relative easy real-time control by reversibly tuning the topography of polydimethylsiloxane (PDMS) wrinkles from a sinusoidal wavy shape to completely flattened [14]. In another work it has been demonstrated the transfer printing of solid objects by kinetically controlling the adhesion of elastomeric stamps [15]. The transfer printing strategy relies on relatively strong adhesion of rubber to solids at fast peel rates, and significantly weaker adhesion at slower rates. Despite such few remarkable examples, surfaces showing controllable adhesive forces remain poorly implemented. In this work we explore the adhesion properties of elastomeric surfaces textured with micron-sized dimples with the expectation of providing effective rules for fabricating surfaces with customtailored adhesive properties. We prove that specific pull-off forces can be obtained, higher or smaller than that of the respective untextured surface, as a result of the combined effect of crack formation at the edge of the contact area, and suction phenomena generated inside the dimples. In particular, we correlate the dimple surface coverage with the stress concentration generated at the edge of the contact area, and we demonstrate how the pull-off force can be effectively adjusted by varying the dimple depth. This study is
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relevant for the design of adhesive systems based on elastomeric surfaces for selective pick-and-place of objects where high weight sensitivity and selectivity is required. 2. Experimental 2.1. Materials and equipments Sylgard 184 was purchased by Dow Corning Corporation (Midland, MI, US). SU-8 type 25 and type 100, and SU-8 developer were purchased from Micro Chem Corporation (Newton, MA, US). S1813 resist was obtained by Shipley Europe Ltd. (Coventry, UK). According to the manufacturer, it is possible to fabricate structures in S1813 as high as 2 m. In order to fabricate structures higher than 2 m using this material, we let the solvent of S1813 to partially evaporate, obtaining a more viscous resist (v-S1813). A spin-speed versus thickness calibration-curve provided us the information required to select the appropriate spin conditions to achieve the desired film thickness in v-S1813. All the others chemicals were used as received. Lithography with SU-8 resists was performed using a mask aligner MA6 (SUSS MicroTec), while lithography with v-S1813 was performed using a laser writer DWL 66FS (Heidelberg Instruments Mikrotechnik GmbH). The patterned surfaces were characterized by Scanning Electron Microscopy (SEM) using a Helios NanoLab 650 Focused Ion Beam SEM (FEI Company). A dynamic mechanical analyzer Q800 (TA instruments) was used to perform adhesion measurements of surfaces textured with dimples arrays. An Ultra Nanoindentation Tester (CSM Instruments SA, Switzerland) was used to measure the pull-off force onto precisely positioned dimples. The height of the fabricated features was measured by a XP-2 Profiler (Ambios technology). Photos of the replicas were taken with an optical microscope DM2500 (Leica Microsystems).
Table 1 Topographical features of the different textured surfaces investigated in this work: inter-dimples spacing (s), spherical dimples radius (a), spherical dimple depth (h), square-shaped dimple side (l), square-shaped dimple depth (d). Spherical micro-cups
Square-shaped dimples
s (m)
a (m)
h (m)
s (m)
l (m)
d (m)
28 50 63 77 90 120
24 24 24 24 24 24
13 13 13 13 13 13
77 77 77 77 50 50 50 50 28 28 28 28
42 42 42 42 42 42 42 42 42 42 42 42
4 6.5 10 110 4 6.5 10 110 4 6.5 10 110
obtained by washing the sample for 10 min in SU-8 developer, followed by rinsing in 2-propanol. (2) Pillars in SU-8 type 25 with d = 10 m and l = 42 m were obtained following the same procedure described in (1) using the following process parameters: spin coating at 3000 rpm, soft bake at 65 ◦ C for 2 min and at 95 ◦ C for 5 min on a hotplate, exposure dose 200 mJ cm−2 , post exposure bake at 65 ◦ C for 1 min and at 95 ◦ C for 2 min, developing for 3 min in SU-8 developer followed by rinsing in 2-propanol. (3) Pillars having d = 6.5 m and l = 42 m were fabricated in v-S1813 by laser writing. First, v-S1813 was dispensed on a silicon wafer. The wafer was spin-coated at 2000 rpm. Squares with the desired side were imaged by shining a laser beam on the resist film. The pillar array was obtained by washing the sample in MF-319 for 315 s, followed by rinsing in distilled water. (4) Squares-shaped pillars having d = 4 m and l = 42 m were obtained in v-S1813 by laser writing as described in (3), using a spin-rate of 6000 rpm and developing in MF-319 for 150 s.
2.2. Fabrication of spherical micro-dome arrays
2.4. Replica molding for the fabrication of dimple arrays
Arrays of micro-domes were fabricated in v-S1813 by thermal reflow technique. In a first step, a silicon wafer was chemically cleaned with acetone and afterwards dried with nitrogen. A liquid solution of v-S1813 was dispensed on the wafer, and a uniform thin layer of resist was formed by spinning the wafer at 2000 rpm. The thickness of the film obtained was 9 m. Circular features having a diameter 2a = 48 m were imaged on the resist by laser writing ( = 405 nm) and cylindrical pillars were obtained after development, followed by washing the sample in distilled water. Micro-domes were produced by thermal softening and roundening of the pillars at 150 ◦ C for 30 min on a hotplate. Due to the reflow, the final height of the domes was 13 m. SEM images of selected arrays are shown in Fig. S1 in Supporting Information.
A PDMS solution was prepared by mixing prepolymer and crosslinker with a weight ratio of 10:1. After degassing to remove air bubbles, the solution was poured on the master and negative replicas were obtained by carefully peeling the elastomeric film after thermal curing. Curing was performed on a hotplate at 70 ◦ C for 1 h. Fig. 1a and b shows representative optical micrographs of arrays of spherical and square-shaped dimples, respectively. The key geometrical features of the fabricated surfaces are depicted in Fig. 1c, while the dimensions of the dimples used in this study are specified in Table 1.
2.3. Fabrication of square-shaped micro-pillar arrays Arrays of square-shaped micro-pillars having four different heights were fabricated as follow. (1) SU-8 type 100 was dispensed on a silicon wafer. The wafer was spin-coated at 2500 rpm. Next, the sample was soft-baked at 65 ◦ C for 10 min and at 95 ◦ C for 30 min on a hotplate. The thickness of the film obtained was 110 m. A sodalime masks (Deltamask, The Netherlands) of squareshaped patterns (42 m side) with various spacing were used for the exposure of the resist. Patterning was performed by exposing the samples to UV radiation ( = 365 nm) using an exposure dose of 300 mJ cm−2 . Exposure was followed by a bake on a hotplate at 65 ◦ C for 1 min and at 95 ◦ C for 10 min. The samples were allowed to cool and, finally, pillars of d = 110 m and l = 42 m were
2.5. Adhesion measurements on dimple arrays Pull-off forces were obtained by measuring quasi-static force versus displacement curves. In Fig. 2 schematics of the key steps involved in a typical adhesion test are shown. A square-shaped flat glass having a surface area of 13.5 mm2 glued to a metallic shaft by means of ∼1 mm thick wax layer was used as probe. The shaft can be moved up and down in displacement or force control. During a typical adhesion test, the probe was put in contact with the surface of the sample applying a force of 0.1 N. The nature of the probe makes the measured pull-off forces very sensitive even to slight misalignments. To align the glass punch to the surface of the samples, the wax was first softened with an air flow at 50 ◦ C, and then it was left to solidify for a few minutes being in contact with the surface of the sample under the preload of 0.1 N. At this point, after an additional compressive displacement of 20 m, the probe was retracted at a constant rate of 50 m min−1 . The magnitude of
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Fig. 1. Surfaces textured with dimple arrays fabricated for this study. (a) Representative optical micrograph of spherical dimples (scale bar 100 m). Inset: single spherical dimple with a diameter of 48 m. (b) Representative optical micrograph of square-shaped dimples (scale bar 100 m). Inset: single dimple with a square side of 42 m. (c) Description of the key geometrical parameters of the arrays.
the pull-off force was considered equal to the force required to separate the probe surface from the sample. Experiments were always performed 24 h after the preparation of the samples and under controlled environmental conditions (temperature 22 ◦ C and relative humidity 35% ± 5%).
in load control, (3) pause at a constant load, (4) unloading in load control, (5) and retraction of the tip in depth control. To guarantee a good depth or load control during the indentation measurement we used the following parameters: contact load of 50 N, pause time of 10 s, loading and unloading rate of 200 N min−1 . The evaluation of the error bars is done by performing 4 measurements on exactly the same area.
2.6. Adhesion measurements on specific sample zones Pull-off forces were obtained with an Ultra nanoindentation tester (CSM Instruments SA, Switzerland). As a punch, we used a flat-ended diamond cone with a tip diameter of 204 m. A typical measurement cycle consisted of five steps (Fig. 3): (1) approach of the tip to the sample surface in depth control, (2) indentation
3. Results and discussion 3.1. Effect of the dimple surface coverage Initially, spherical dimples were used to study the effect of the dimple surface coverage in the adhesion between the elastomeric
Fig. 2. Key steps of an adhesion test performed with a flat probe. (a) A square-shaped flat glass substrate glued to a metallic shaft by means of ∼1 mm thick wax layer was used as probe. (b) The probe was brought in contact with the surface of the sample. (c) The wax was softened with an air flow at 50 ◦ C, and it was left to solidify for a few minutes in contact with the surface of the sample under a preload of 0.1 N. (d) After an additional compressive displacement of 20 m, the probe was retracted at a constant rate of 50 m min−1 until detachment occurred.
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400 (1)
(2)
(3)
(4)
100
150
(5)
300 200
force (μ N)
100 0 0
50
200
250
300
350
400
-100 -200 -300 -400 -500
me (s) Fig. 3. Typical adhesion measurement cycle performed with the nanoindenter: (1) approach of the tip to the surface of the sample in depth control, (2) indentation in load control, (3) pause at a constant load, (4) unloading in load control, (5) and retraction of the tip in depth control.
samples and the flat surface of a glass probe, being the surface covered by dimples in a range between 0 (untextured surface) and 31%. Analyzing the data of the pull-off forces obtained for the various surfaces, we can clearly distinguish a general trend: the greatest pull-off force is obtained for surfaces at dimple surface coverage between 5 and 10%, after which the pull-off force decreases monotonously, as shown in Fig. 4. In order to isolate the effect of the texturing in the adhesion mechanism from other factors that may affect the results (such as the preparation, handling and storage of the samples), the pull-off force is normalized by the force necessary to detach the punch from an untextured control sample of the same material (∼278 mN). As it can be seen, the pull-off force shows a maximum increase of 2.4% at a value of ∼10% of surface covered by dimples. This increase of pull-off force can be explained considering that when a remote pulling force acts in a direction normal to the surface, the punch retracts and the PDMS deforms,
1.04 1.02
normalized pull-off force
1 0.98 0.96 0.94 0.92 0.9 0.88 0.86 0.84 0.82 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
dimple surface coverage (%) Fig. 4. Pull-off force measured between a flat punch and samples of PDMS as a function of the dimple surface coverage. Each data point is the mean value of six measurements on two different samples. The variation of pull-off force for the adhesion tests was very small.
producing a change of volume of the dimples. If we assume that the two surfaces are perfectly aligned, air is trapped into the dimples. Since the pressure has an inverse relationship with the volume [16], the expansion of the air volume generated by the retraction of the punch is balanced by the formation of negative pressure inside the dimples, producing a suction effect. From Fig. 4 it can be noticed that the decrease of the pull-off force becomes more pronounced at higher values of dimple surface coverage. This behavior can be explained by the linear elastic theory [17], that predicts intensified magnitude of the stress at a crack tip compared to the nominal stress level. We suppose that the dimples present at the edge of the contact area between the probe and the sample may act as crack-like defects, intensifying the stress caused by the applied remote load, and eventually being responsible for the initiation of cracks along the interface between the elastomer and the punch. As the dimple surface coverage increases, a higher probability to have dimples along the edge of the contact area is expected and, as a consequence, the critical stress at which the crack propagation initiates will be reached at smaller values of remote load. In other words, above the value of 10% of dimple surface coverage the expansion of the air volume inside the dimples generated by the retraction of the punch produces a negligible suction effect, as the pull-off occurs at low loads due to the crack formation and propagation at the edge of the contact area. A similar hypothesis was invoked by Spuskanyuk et al. to explain the observed increased pull-off force of a mushroom-shaped fibril compared with that of a punch-shaped fibril [18]. They proposed that a possible defect at the edge of the contact area is more damaging for the adhesion of a punch-shaped fibril, as the highest stress during pull-off is applied at the edges of this structure, compared to a mushroom-shaped fibril, where the highest stress is expected to be at the center of the contact. Since the shape of the dimples does not affect the measured pull-off forces as long as their volume remains the same (see Fig. S2 in Supporting Information), for simplicity from now on all the results will be presented using square-shaped dimples. To verify the importance of the presence of defects at the edge of the contact area between the textured elastomer sample and the punch for the detachment mechanism, we performed dedicated experiments, carefully controlling the position of the dimples with respect to the edge of the contact area. For the tests we used an Ultra Nanoindentation Tester, equipped with a flat-ended diamond cone having a diameter of 204 m. A single line of dimples was used for this experiment. The dimples were square-shaped with side of 42 m, depth of 10 m, and separation distance of 50 m. The flat end of the cone was initially aligned with respect to the patterned line so that only the first dimple was located inside the contact area between the punch and the elastomer. Then the punch was moved horizontally by subsequent steps of 15 m toward the adjacent dimples. Fig. 5 shows the results of the measured pull-off forces for each location of the punch. Also in this case, the pull-off force is normalized by the force necessary to detach the punch from an untextured control sample of the same material (∼1300 N). The maximum pull-off force was obtained at the initial position, when the single dimple was totally located inside the area generated by the projection of the punch on the elastomer sample, exceeding the pull-off force obtained for the untextured surface (see inset drawing in Fig. 5). At that position there were no defects present at the edge of the contact area between the elastomer samples and the punch, from which a crack could initiate, and therefore the pull-off force exceeded the adhesion performance of the untextured surface. By moving the punch toward the adjacent dimples, the edge of the contact area always coincides at various degrees with the position of the dimples, offering different crack initiation points. Thus, the pull-off force remains always below the value of the untextured control sample. The minimum pull-off force occurs
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and the samples. As demonstrated, the pull-off force increases by reducing the depth, and thus the volume of the dimples. The maximum pull-off force was obtained with ∼12% dimple surface coverage and dimple depth of 4 m, exceeding the pull-off force of the untextured control sample by 10%. The increase of the pull-off force for decreasing volume of the dimples can be explained considering Boyle’s law [16], as previously introduced by Jiang et al. to describe the high adhesion of water droplets to superhydrophobic porous nanostructures [19–21]. Assuming the air as an ideal gas, the suction force of a single dimple is: Fsuction = pin A
Fig. 5. Pull-off force as a function of the punch position with respect to underlying dimples. The presence of dimples along the edge of the contact area controls the adhesion mechanism. The cycles of the schematics represent the actual punch position with respect to the dimples.
1.2
dimple depth 4 um 6.5 um 10 um 110 um
normalized pull-off force
1.1
1
0.9
V Vin + V
(1)
where pin and Vin are the pressure and the volume of the air in the dimple, A is the area of the base of the dimple, V is the increased volume of the sealed air under the action of the pulling force. V can be assumed to be constant and independent from the depth of the dimples. Therefore, according to Eq. (1), shallow dimples characterized by lower Vin than deep dimples are expected to provide higher suction forces. The results above indicate that the pull-off force can be easily controlled by changing the volume of air trapped inside the dimples, resulting in the tuning of the suction effect. Moreover, the trend suggests that higher pull-off forces can be expected by further reducing the volume of the dimples. Although the aim of the specific work is to finely tune the adhesion between surfaces by dimple texturing rather than to enhance the adhesion of the respective untextured surfaces, it is worth mentioning that also in the case of mushroom-shaped fibrillar structures the suction may contribute up to 10% of the overall measured pull-off forces [22,23], a value consistent with our results.
4. Conclusions
0.8
0.7
0.6 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38
dimple surface coverage (%) Fig. 6. Effect of the dimples depth in the adhesion mechanism. At any value of dimple surface coverage, the pull-off force increases by reducing the dimple depth.
at a punch position of 120 m that corresponds to the presence of two defects along the perimeter of the contact area. As expected, the minimum pull-off force is repeated with a periodicity of 90 m, equal to the periodicity of the texture. These results demonstrate that the position of the dimples with respect to the edge of contact area significantly impacts the adhesion behavior of the system, as the dimples can act both as passive suction devices and defects for the initiation of a crack. 3.2. Effect of the dimple depth A second geometric parameter of the studied structures that influences the pull-off force values is the depth of the dimples. Fig. 6 shows the pull-off forces obtained for surfaces textured with square-shaped dimples having a depth ranging from 4 to 110 m. The volume of the dimples was tuned by varying their depth d, while keeping constant the surface of contact between the punch
In this study surface texturing of elastomers having diverse dimple surface coverage and depth is proposed as a mechanism to fabricate custom-tailored elastomeric adhesive systems. The tests performed showed that the presence of the dimples is beneficial for adhesion due to suction effect if the dimple surface coverage stays around 10%, whereas above this value the adhesion decreases. This effect was attributed to the presence of dimples at the edge of the contact area that act as crack-like defects. At the same time, it is shown that changing the dimple depth is an effective way to tune the adhesion response of the surfaces, since the suction effect is more intense as the dimple depth decreases. The appropriate combination of these two factors results in surface textures with desired fine-tuned adhesion properties. In future works, mechanisms of crack arresting [24,25] at the edge of the contact area will be exploited to significantly increase the adhesion performance. By contrast, the dimple surface coverage and depth can be deliberately increased to reduce the adhesive response of the surfaces, or surfaces with dimple surface coverage and depth gradient can be fabricated to induce directional adhesion behavior. These finding may be useful in several fields such as medicine, for the development of biocompatible patches less irritating than acrylic-based ones, or in electronics, for the manipulation and assembly of microscopic components.
Acknowledgments The authors thank Dr. E. Mele for helpful discussion and Ms. E. Rondanina for technical assistance in the laser-writing processes.
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