Discretely Supported Dry Adhesive Film Inspired by Biological

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Discretely-Supported Dry Adhesive Film Inspired by Biological Bending Behavior for Enhanced Performance on Rough Surface Hong Hu, Hongmiao Tian, Jinyou Shao, Xiangming Li, Yue Wang, Yan Wang, Yu Tian, and Bingheng Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14951 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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ACS Applied Materials & Interfaces

Discretely-Supported Dry Adhesive Film Inspired by Biological Bending Behavior for Enhanced Performance on Rough Surface Hong Hu,†,§ Hongmiao Tian,†,§ Jinyou Shao,*,† Xiangming Li,† Yue Wang,† Yan Wang,† Yu Tian,‡ Bingheng Lu† †

Micro-/Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China.

‡

State Key Laboratory of Tribology, Tsinghua University, Beijing 10084, China.

*Corresponding author: [email protected], §

These authors contributed equally to this work

Keywords: dry adhesive film, biological foundation element, mushroom-shaped micropillar, bending behavior, energy penalty, rough surface

Abstract Biologically inspired dry adhesion is recently a research hot topic because of its practical significance in scientific research and instrumental technology. Yet, most of the current studies merely focus on borrowing the concept from some finer biological contact elements, but lose sight of the foundation ones that play an equally important role in the adhesion functionality. Inspired by bending behavior of the flexible foundation element of gecko (lamellar skin) in attachment motion, in this study, a new type of dry adhesive structure was proposed, wherein a mushroom-shaped micropillar array behaving as a strongly adhesive layer was engineered on a discretely-supported thin film. We experimentally observed and analytically modeled the structural deformation, and found that the energy penalty could be largely reduced because of the partial shifting of the pillar bending to the film bending. Such behavior is very analogous in functionality to the lamellar skin in gecko’s pads and is helpful to limit effectively the damage of contact interface, thus generating an enhanced adhesion even on a rough surface. 1 ACS Paragon Plus Environment


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Introduction Biologically inspired dry adhesives, which artificially mimic the complex multi-scale architectures on the feet of some insects and lizards such as spiders, beetles, and geckos,1 have recently attracted significant attention. This is attributed to their repeatable and controllable adhesion performance without any contamination.2–6 Many research groups have realized various wonderful applications in industry and our daily life by utilizing dry adhesives, such as gripping and releasing fragile objects,7–9 special-purpose robots for climbing or transportation,10–12 and the skin-adaptive adhesive layer for biomedical signal monitors.13–15 However, development of future applications of dry adhesives is still limited due to the dependence of the actual adhesion properties on the structural design16 as well as surface quality of target.4 Considering the fact that rough surfaces are more common in nature, it is more practically significant to pour efforts into developing artificial design that is capable of performing well on rough surface. However, earlier theoretical researches by Persson et al.17, 18 and Hui et al.19, 20 showed that even a relatively small roughness on the surface can cause a dramatic drop in adhesive force, thus a careful structure design is definitely required to acquire an expected adhesion performance on rough surface. Very recently, Barreau et al.21 made a systematic investigation that quantified the influence of roughness parameters and structure geometries on the adhesive force, to experimentally demonstrate that both the classical contact splitting principle and the highly compliant microstructure are essential for strong adhesion on a rough surface. Unfortunately, artificial polymeric microstructures with high compliance (e.g. low material elastic modulus or high structure aspect ratio) get easily destabilized to collapse22, 23 or buckling24–26 no matter in fabrication process or in application, which is undesirable for adhesion due to the contact loss.4, 13, 27 An effective solution is the preparation of the hierarchical structures that allow for exponential enhancement in the robust adhesion on rough surface, as modeled theoretically by 2 ACS Paragon Plus Environment

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Chen et al.28 and Yao et al.29, 30 Such design of structural hierarchy is also an inspiration from some complex biological attachment systems, such as those of gecko,31 and has been confirmed to show stronger adhesion than the single-level structure, as experimentally reported by Jeong et al.32 and Murphy et al.33 However, owing to the configuration interplay amongst different structure levels, i.e., the upper-level features have to be engineered on the discrete top faces of lower-level ones,34 dimension optimization become harsh, and thus the structural stability problem is frequently observed on the top-level, probably leading to a poor adhesion performance in some special circumstances, as shown experimentally by Bauer et al.35 On the other hand, adhesion of the artificial design can also be largely enhanced by introducing some finer bio-inspired structural elements. Examples include engineering a flange plate to be bulged laterally at the free end of a vertical or tilted micropillar. Such design, as suggested theoretically by Carbone et al.36, 37 and Spuskanyuk et al.,38 is effective in homogenizing the interfacial contact stress, and thus shows excellent adhesion performance when in use.39–45 By expanding the flange plate area to an extreme case, Glassmaker et al.46 and Shen et al.47 developed another type of adhesive structure, i.e., a film-terminated fibrillar array, and expounded the high adaptability of the film portion to the contact regime because of its effect in trapping interfacial cracks. However, notably, these studies are mainly based on the contact element at the end of biological attachment system and ignore the rest. In fact, animals have also evolved some other specialized structures, such as the lamellar skin of the gecko, which behave as a soft transition part between the micro-scale attachment and macroscale dynamical systems, allowing them to locomote on any unknown surface, as shown in Figure 1a. By sliding this gecko-inspired lamellar structure, Tian et al.48 revealed its key role in sustaining most of the normal deformation as a result of the bending behavior (Figure 1b) during attachment motion, which leading the whole attachment system to be in a wide range of adhesive states. From a consideration of its location and function, this structure can be 3 ACS Paragon Plus Environment


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defined as a foundation element compared to the contact element. However, as a tilted structure in macro-scale, lamellar foundation is, in general, applied to the situation associated with friction-controlled adhesion (namely, lateral adhesion), and may not work well on a rough surface, because the structure dimension does not match the micro-scale asperities. Inspired by the biologically-evolved foundation elements, in this study, we proposed a new type of dry adhesive structures with enhanced performance on rough surface. Figure 1c shows the scanning electron microscopy (SEM) image of the proposed design, wherein a mushroomshaped micropillar array as a contact geometry is engineered on a thin film foundation, supported discretely by another bottom-level of micropillars. The proposed design based on gecko-inspired lamellar structure exhibits following two major advantages, making it superior in terms of adhesion on rough surface compared to the widely-investigated single-level adhesive or hierarchical structures. First, the continuous film foundation maximizes the area available for the top-level structures, leading to the functioning of the more adhesive features. Moreover, in this case, each of them experiences smaller compression and tension pressures in the attachment and detachment processes, which is beneficial for minimizing the possibility of structure buckling during its application. Second, we analytically modeled the deformation behavior in the cases involving both the absence and presence of thin film portion, and found that the elastically stored energy penalty that negatively influences the fibril adhesion21 can be largely reduced because of the partial shifting of the pillar bending to the film bending. Such behavior is very analogous in functionality to the lamellar skin as a foundation element in gecko’s pads, which bends as a result of the attachment motion, thus helping to limit effectively the damage of contact interface and generating a strong adhesion even on a rough surface.

Results and Discussions 4 ACS Paragon Plus Environment

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On account of the difference in the explicit functions at the structural levels, i.e., the toplevel mushroom-shaped micropillar array provides a strong adhesion and the bottom-level patterned foundation with a film terminal adapts to the surface asperity, the proposed design was designated as discretely-supported dry adhesive film, or namely, d-DAF for brevity. To manufacture it, double-side exposure lithography (Figure 1d) and soft molding (Figure 1e) process were used.49, 50 Before the curing process, another polydimethylsiloxane (PDMS) micropillar array replicated from a patterned silicon wafer was placed slightly on the uncured PDMS film (Figure 1f). Finally, the assembly was cured and the photoresist was sacrificed chemically, so that the d-DAF (Figure 1g) could be peeled-off from the substrate (Figure 1h) (detailed description of this procedure can be found in Experimental Section). The height, diameter, and center to center distance of the bottom-level pillars were about 55 µm, 20 µm, and 60 µm, while that of the top-level were about 14 µm, 6.2 µm, and 20 µm, respectively. The mushroom-shaped tip was about 12 µm in diameter and was 2–3 µm in thickness, depending on the back-side exposure time and development time. This dry adhesive film with a pattern area of 1.5 × 1.5 cm2 was compressed against a frosted slide and could suspend a 50 g counterweight (Figure 1i). Besides, it was found experimentally that samples covered with d-DAF showed stronger adhesion than those with only the top-level structures in the generally-adopted preload-pull and preload-drag-pull measurement cases (detailed experimental process and measurement data are supplied as Supporting Information, Figure S1).

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Figure 1. Discretely-supported dry adhesive film inspired by bending behavior of biological foundation element during attachment motion and its adhesion performance on rough surface. a) Photograph of gecko’s toe pads and SEM images of the bendable lamellar skin as well as finer contact elements at the end of the attachment system. b) Schematic illustration of bending behavior of lamellae in attachment motion. c) SEM image of d-DAF captured at a certain position with defect. d) Exposure of the photoresist spin-coated on a transparent substrate from the top-side with a mask present and from the bottom-side without any mask, in order to generate a microhole array with bottom undercut after the development. e) PDMS was spin-coated on the patterned photoresist to fill into the microhole array for the production of the middle-level thin film. f) Another patterned PDMS sample, fabricated via replicating the etched silicon mold, was placed slightly on the uncured PDMS to ensure a firm bonding between the two levels of structures. g) Demolding by dissolving the photoresist in ethyl alcohol. h) Peeling-off d-DAF from the substrate. i) Adhesion between a sample with 1.5 × 1.5 cm2 pattern area and a rough surface suspended 50 g counterweight. The insert was the height–distance curve of the ground glass slide.

Such adhesion performance of the proposed d-DAF on a rough surface was attributed to the introduction of the discretely-supported film foundation with a highly adaptive capacity towards the target surface asperity. To demonstrate this and reveal the possible mechanism allowing for the contact-based adhesion on rough surface, first, we investigated comparably 6 ACS Paragon Plus Environment

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the extent of the impact that the surface vertical drop (δ) could have on the adhesive force generated by d-DAF and single-level structures. Adhesion measurements were performed on five flat probes with different protrusion heights (i.e., hpr = 2.1 µm, 5.4 µm, 8.0 µm, 11.3 µm, and 15.4 µm), which were fabricated on a glass substrate by photolithography. Figure 2a displays the height–distance curves of the five probes measured using a step profiler. The protrusion region exhibits an array of square features with 400 µm side length and 200 µm gap. To ensure that only the protrusion height could influence the adhesion measurement, the alignment between the sample and probe had to be precisely controlled (detailed alignment method was described in Experimental Section).51 The saturated pull-off force values were then extracted and divided by the probe area (3 × 3 mm2) to obtain the pull-off strength σp. Figure 2b exhibits the pull-off strength vs. protrusion height data for two samples (the probe of hpr = 0 was the bare glass substrate). With the increase in the value of hpr to 15.4 µm, σp for the single-level and d-DAF sample decreased from the average level of about 22.5 KPa obtained on the bare glass substrate to about 6.1 KPa (72.8% reduction) and to 9.7 KPa (56.8% reduction), respectively. The results indicated a strong unevenness dependence of the adhesion; however, this dependence was much weaker for the sample covered by d-DAF. Moreover, noteworthy, the pull-off strength on the bare glass substrate (flat probe) of d-DAF was a little weaker than that of the single-level structure (see Figure 2b). This could be the result of the “indirect” contact achieved by the top-level pillars that were located between the bottom-level pillars. However, owing to the advantage of low preload requirement for generating strong adhesion of mushroom-shaped micropillar,8, 39, 43 the adhesion reduction was much smaller compared to that caused by the increase of probe unevenness. For an indicative understanding of the pillar buckling, Figure 2 also displays the intermediate snapshots of the contact interface between pillars on the top-level and the probe (hpr = 8.0 µm) in the preload application process. The top-view of the single-level sample when structures began to contact with the protrusion region is displayed in Figure 2c, where 7 ACS Paragon Plus Environment


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regular circles appear in the contact region (right side of the optical image) and are darker than those in the non-contact region (left side) under the microscope.52, 53 These structures were then subjected to a larger compression as the probe approached downwards further, and finally slanted heavily with irregular circles being observed, when the contact within the valley region (left side) was reached, as shown in Figure 2d. Pillars in such case lost a significant contact area, thus contributing poorly to the adhesion.27, 35 Differently, when the same procedure was performed on the d-DAF sample, the patterned film foundation with reduced effective stiffness could compensate a part of the normal displacement so as to prevent the top-level pillars from being destabilized easily (or slanting heavily). As a result, both the structures underneath the valley and protrusion regions were compressed moderately and conformably formed contact with the uneven probe, as shown in Figures 2e and 2f. In view of actual functionality, the patterned film foundation was similar to gecko’s lamellar skin, for both of them could sustain a part of the vertical drop in the attachment process48. Besides, these observations revealed that the pillar buckling could be the basic mechanism causing the adhesion difference in the single-level and d-DAF structures when a step-like uneven probe was compressed on them. Noteworthy, pillars on the bottom-level could also lose stability, although not observed from the top view, in particular, if a huge preload was applied on the sample with unreasonable dimensions. Herein, we proposed a simple calculation method for optimizing the structure dimension by quantitatively modeling the pillar buckling behaviors that probably occurred on any of the two structural levels. (detailed calcualtion process and results are supplied as Supporting Information, Figures S2 and S3). Using this method, the critical compression displacement, hcal, beyond which pillar could buckle, was calculated to be about 6.5 µm and 12.6 µm for the single-level and d-DAF samples, respectively (vertical dotted line, Figure 2b). It quantitatively explained the reason for the dramatic loss in adhesion in single-level sample for hpr = 8.0 µm (larger than hcal of the single-level but smaller than that of the d-DAF). For greater unevenness, the single-level 8 ACS Paragon Plus Environment

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sample would be easier to buckle with weaker adhesion; however, the d-DAF one would maintain a strong adhesion until the vertical drop was too large to compensate (e.g. hpr = 15.4 µm).

Figure 2. Influence of probe unevenness on the pull-off strength as well as the structure deformation behavior. a) Height–distance curves corresponding to about 1.3 mm length of five probes were measured using step-profiler. b) Pull-off strength, σp, as a function of protrusion height, hpr, for the single-level (square) and d-DAF (circle) samples, respectively. The vertical dotted line presents the calculated critical compression displacement of the single-level (black) and d-DAF (red) samples, beyond which pillar buckling could occur. c)–f) Top views of the contact status when a step-like uneven surface was compressed on the two samples. When the contact within the valley region was reached as a result of the further downwards displacement, pillars of the single-level sample underneath the protrusion region were compressed excessively, thus slanting heavily. In contrast, as the patterned foundation compensated a part of the vertical drops, pillars of the d-DAF sample were all compressed moderately and were still functional. The scale bar was 30 µm.

In addition to vertical drop, d-DAF was also effective in adapting another morphological factor that could reduce the adhesion, i.e., the angular missing (θ), resulting from the local misorientation of pillar face to surface profile. To demonstrate this and understand the 9 ACS Paragon Plus Environment


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significant role of the bendable foundation in rough surface adhesion, as shown in Figure 3, real-time force recording experiment combined with contact and deformation statuses observation was performed. Figure 3a displays the schematic illustration of the contact between a rough surface and an adhesive sample. It was impossible for all the asperities to contact synchronously with adhesive features; therefore, a sloped surface with a pre-defined angular missing (Figure 3b) was used as a probe to be approached to and retracted from a dry adhesive sample (single-level or d-DAF, Figure 3c). This facilitated the investigation of how a single peak would influence the local bending behavior, the contact status in its neighboring region, and the pull-off force. Figures 3d and 3e present the force-time curves in the case of ~40 mN preload for θ = ~4.5° and θ = ~9°, respectively. The pull-off force of the single-level and d-DAF samples was about 20 mN and 22.5 mN for θ = 4.5°, and was about 2 mN and 13 mN for θ = 9°, respectively. Clearly, with the increase in θ, indicating greater misorientation of probe face to pillar face, the generation of a conformal contact interface would become more and more difficult, causing an obvious reduction in the pull-off force. On the other hand, the difference between pull-off forces of the two samples for the smaller misorientation case (@ θ = 4.5°) was not as significant as that for the larger case (@ θ = 9°), which showed a factor of about six enhancement in pull-off force by the d-DAF sample. The results indicated that the additional thin film foundation enabled the adhesive features on the top-level to have better adaptive capacity to the local misorientation, which became more significant with the increase in the misorientation degree. Considering that a real rough surface was constituted of numerous peaks with wide ranges of face misorientations, d-DAF was supposed to perform better in applications. To demonstrate that this enhanced property was caused by the bending behavior of the film foundation, the preload application and detachment processes were recorded by a charge coupled device camera and the intermediate snapshots are presented in Figures 3f–3i (videos recording the interface behaviors of the two types of adhesive films can be found in Supporting Information). The contact length, lc, defined as the distance from 10 ACS Paragon Plus Environment

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probe peak to contact boundary (white arrows in optical images), of the single-level @ θ = 4.5° and the d-DAF @ θ = 4.5° was about 550 µm (Figure 3f II) and 680 µm (Figure 3g III), which respectively decreased to about 130 µm @ θ = 9° (Figure 3h II) and 420 µm (Figure 3i III) @ θ = 9°, wherein the preload reached its maximum value of about 40 mN. For ease of comparison, the time histories of lc were also plotted in Figures 3j and 3k corresponding to the force-time curves shown in Figures 3d and 3e, respectively. A significant reduction in lc (76.3%) of the single-level sample was observed for the two cases of different θs, which was deduced as the reason for the loss in the pull-off force during measurement. Although the dDAF could not entirely eliminate this reduction, the pressure induced bending behavior (Figures 3g II and 3i II, dotted circles) weakened the influence of the interfacical stress singularity at the probe peak on the contact status in neighboring region, thus causing a slight reduction in the contact length (38.2%) and a high level pull-off force. Interestingly, it was found that stable and unstable crack propagations appeared alternately to damage the contact interface of d-DAF sample in the detachment process, which correspondingly exhibited a smooth decrease followed by a rapid drop in the time history of contact length (dotted circles in Figures 3j and 3k). This phenomenon was also the consequence of the bending behavior in the detachment process (see the dotted circles in Figures 3g IV, V and 3i IV, V) and was very similar to the results reported by Glassmaker et al.,46 who utilized the continuous film as a contact element and demonstrated that the crack trapping effect could enhance the adhesion. Moreover, different preloads were applied in the measurement and the pull-off force vs. preload data are presented in Figure 3l, showing that in a wide range of preload values (

Discretely Supported Dry Adhesive Film Inspired by Biological

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