Influence of surface roughness on wet adhesion of biomimetic ...

Influence of surface roughness on wet adhesion of biomimetic adhesive pads with planar microstructures Kun Wang1, Bin He2, Run-Jie Shen2 1

College of Mechanical Engineering, Tongji University, Shanghai, People’s Republic of China Department of Control Science and Engineering, Tongji University, Shanghai, People’s Republic of China E-mail: [email protected]

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Published in Micro & Nano Letters; Received on 4th September 2012

Biological adhesive pads of some reptiles and insects, such as tree frogs and grasshoppers, are covered with planar microstructures and have strong and stable adhesive ability on both wet and dry substrates. These adhesion forces do not mainly come from van-der-Waals force but wet adhesion. In this study, the influence of substrates’ surface roughness on the wet adhesion of man-made adhesive pads inspired by tree frog toe pads is investigated experimentally. Biomimetic polydimethylsiloxane adhesive pads with planar hexagon microstructures with a microchannels width of 10 mm are fabricated by combining electroforming with soft lithography. Experiments of wet adhesive force between the pads and sandpaper slices with different average surface roughness are carried out at various preloads. Results show that the rougher surface leads to the decrement of wet adhesion force. It is also observed that if the microcosmic profile height of the substrates is near or less than the width of microchannels in the biomimetic adhesive pads, the microstructures and preloads can increase significantly the wet adhesive force, otherwise the microstructures and preloads do not contribute indistinctively. The experimental results can be explained by analysing the relation between the solid contact area and the area with a liquid bridge.

1. Introduction: Biological adhesive pads representative of reptiles and insects, which are covered with either fibrillar microstructures or planar microstructures, have strong and stable ability to cling to and detach from various substrates [1]. There is certain progress in the investigations of the mechanism of the biological adhesion. It has been found out that the surface of foot pads of many fibrillar adhesive systems, such as geckos, flies and beetles, are covered with very compliant microscale or nanoscale setae, and adhesive ability mainly comes from the van-der-Waals force [2, 3]. However, some animal’s foot pads with ‘smooth’ adhesive systems are covered with the planar microstructures (such as tree frogs, grasshoppers), which can copy the shape of the substrate surface under preload conditions [4, 5]. Tree frogs and grasshoppers squeeze or inject a liquid film into the pad –substrate contact area and generate a relatively long-range attractive interaction because of the formation of capillary bridges [6–8]. Furthermore, the model of a tree frog toe pad in contacting rough surfaces has also been investigated mathematically [6]. Understanding the mechanisms of biological adhesion has encouraged researchers to mimic the morphology and properties of biological adhesive pads in designing novel engineering systems. Progress of biomimetic adhesive pads mostly focus on developing and analysing micro and nanopillared ploymer surfaces inspired by the morphology of the biological fibrillar adhesive systems [4, 9]. Chemical vapour deposition was successfully used to obtain the high-density carbon nanotube arrays with width of 50 –500 mm so as to fabricate biomimetic fibrillar adhesive pads [10, 11]. In certain cases, slanting moulds were machined by mechanical microcutting, and millimetre scale slanting polyurethane fibrillar arrays with the adhesive intensity of 0.24 N/cm2 were fabricated by micro moulding [12, 13]. Meanwhile, because surface roughness is the main reason why solids usually do not adhere to each other with any measurable strength [6], there have also been many attempts to investigate the influence of surface roughness on adhesive force of biomimetic fibrillar adhesive pads [9, 14– 17]. Biomimetic pillar arrays increases the adhesive force on various rough surfaces than unpatterned controls [9, 14, 16, 17]. It has also been found that the relation between the pillar dimensions and microcosmic profile of rough surfaces crucially affects the scope of adhesive force [14, 15]. However, the study of biomimetic adhesive pads with planar microstructures is a relatively neglected research area. 1274 & The Institution of Engineering and Technology 2012

In this study, the influence of substrates’ surface roughness on the wet adhesion of man-made adhesive pads inspired by tree frog toe pads is investigated experimentally. Biomimetic adhesive pads with planar hexagon microstructures are fabricated by combining electroforming process with soft lithography. Experiments of adhesive force between the biomimetic adhesive pads and sandpaper with different average surface roughness are carried out at various preloads. This study provides new insights on the influence of roughness on the wet adhesion of microstructured biomimetic adhesive pads and the results can be utilised in designing novel adhesive systems.

2. Materials and methods 2.1. Fabrication of adhesive pads: Soft materials with microstructures were usually treated by integrating polydimethylsiloxane (PDMS) in SU-8 structures [14, 18, 19]. For biomimetic adhesive pads needing to be replicated repeatedly, the pads inspired by tree frog toe pads were fabricated by demoulding PDMS from electroformed Cu templates in this Letter. The approach consists of three fabrication steps: (i) fabrication of SU-8 photoresist mould; (ii) electroforming process; (iii) soft lithography replication using PDMS [20]. SU-8 50 photoresist was chosen for the process, and the 100 mm-thick SU-8 film was used as a coating on a stainless steel substrate. The temperature was raised to 958C with a 108C/ min ramping rate, and this temperature was kept for 30 min. After that, the substrate was cooled down to room temperature. Then electroforming of Cu was carried out in an acid sulphate cuprum solution. The electrolyte contained CuSO4†5H2O 250 g/l, H2SO4 60 g/l, NaCl 0.08 g/l. Pulses supply with ton of 50 ms, toff of 50 ms and frequency of 3333 Hz were used in the process. After electroforming, the product with the SU-8 photoresist was immersed in an 808C NANOTW Remover PG solution for 2 h to separate the Cu mould, as shown in Fig. 1a. A 10:1 ratio of Sylgard 184 prepolymer and cross-linker were mixed and degassed in a desiccator for 10 min to remove bubbles, and then the mixture was poured on the Cu mould. All this, as a whole, was treated at 658C for 2 h in a furnace to achieve a PDMS prepolymer. The PDMS biomimetic adhesive pads with planar hexagon microstructures with width of 100 mm and height of 25 mm have been obtained, as shown in Fig. 1b. The width of the microchannels of the microstructures is 10 mm. Micro & Nano Letters, 2012, Vol. 7, Iss. 12, pp. 1274–1277 doi: 10.1049/mnl.2012.0683

Figure 1 SEM micrographs of microstructures a Cu template by electroforming b PDMS biomimetic adhesive pad

2.2. Preparation of rough surfaces: Four sandpaper slices were cut into 25 mm2 pieces and glued on the flat aluminium substrates, and were treated by filing or grinding to obtain surfaces with markedly discrepant roughness. The average roughness parameters of the slices were measured and characterised by surface roughness apparatus and three-dimensional profile microscopy; see Fig. 2. These surfaces are shown with randomly distributed scratches and grooves. 2.3. Methods: The wet adhesive force of biomimetic pads with/ without planar microstructures are test-contradistinctive on rough surfaces at various preloadings to investigate the influence of surface roughness on wet adhesion, as shown in Fig. 3. First, a liquid film was formed between the pads and rough surfaces; see Fig. 3a. The wet adhesive pad was approaching the surface until the given preloads between the pads and the rough surfaces were obtained. At the same time of preloading, the biomimetic adhesive pads deformed on the rough surfaces and the liquid squeezed in the micro gap; see Fig. 3b. Then the direction of the motion was reversed, and the pads were detached from the rough surfaces, whereas a micro force sensor with the resolution of l mN was collecting signals during the detaching process. The maximum force could be track-recorded and be characterised as wet adhesion force; see Fig. 3c. Fig. 3d illustrates the model of wet adhesion, in which a liquid bridge is formed between the wet adhesive pad and the rough surfaces [7, 8]. In Fig. 3d, R is the radius of the liquid bridge, h is the average height of the liquid film, u and w are contact angles. The liquid volume is constant, and so when the pad is pulled or squeezed, the liquid bridge will shrink or expand. The wet adhesive force mainly comes from capillary force fc and the surface tension fst [7, 8]. Taking no account of both the surface roughness of substrates

Figure 2 Microscopy images of rough surfaces

Micro & Nano Letters, 2012, Vol. 7, Iss. 12, pp. 1274–1277 doi: 10.1049/mnl.2012.0683

Figure 3 Schematic diagram showing the experimental methods

and the microstructures of biomimetic adhesive pads, wet adhesive force F is [7, 8] F = fc + fst = −pR2 g( cos u + cos w)/h + 2pRg cos u

(1)

where g is the surface tension of the liquid. Equation (1) shows that the normal wet adhesive force increases with the increase of the contact area. Taking no account of the surface roughness, when the contact area of the biomimetic adhesive pads was divided into a uniform sub-area by planar microstructures, wet adhesive force FM is FM = fcM + fstM  

  R 2 R = n −p √ g( cos u + cos w)/h + 2p √ g cos u n n √ = fc + nfst

(2)

where n is the number of the microstructures. Equations (1) and (2) show that the microstructures on the surface of adhesive pads can effectively improve the wet adhesive ability. 3. Experiments: Experiments of the wet adhesive force of biomimetic adhesive pads with/without planar microstructures were carried out by changing the surface roughness of the substrates at a preload of 2.0 N; see Fig. 4. Surface roughness (Ra) is from 0.18 to 14.79, as shown in Fig. 2. A 30% honey solution was daubed on the surface to form a liquid film with the thickness of l0 mm. The wet adhesive pad was approaching the different rough surfaces vertically, at the speed of 0.5 mm/s, and then the pads detached the surfaces with the velocity of 0.3 mm/s. Moreover,

Figure 4 Maximum adhesive force variation with surface roughness

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experiments of the wet adhesive force with microstructures were performed by changing the surface roughness and the preload under the same conditions; see Fig. 5. All the error bars result from five measurements at one randomly chosen location on the corresponding rough surfaces. Figs. 4 and 5 show that the wet adhesive force of biomimetic adhesive pads with planar microstructures increases with the decreases of the surface roughness and with the increase of the preloads. Fig. 4 shows that the wet adhesive force of those pads without microstructures is independent of surface roughness. In addition, on a smoother surface (such as Ra ¼ 0.18, Ra ¼ 2.62), the microstructures and the preloads can increase significantly the wet adhesion force, the trend of which was the consequence of more increment of adhesive force at the smooth surface. However, on a rougher surface (such as Ra ¼ 6.2, Ra ¼ 14.79) the microstructures and the preloads do not contribute indistinctively. 4. Discussion: From (1) and (2), improvement of wet adhesive force essentially comes from an increase of the actual area with the liquid bridge (AL) [9]. The microstructures and the preloads can result in increases of the solid contact area between pads and substrates (As); see Fig. 3b. Without regard to surface roughness As is equal to AL , so fabricating microstructures and increasing preloads are effective approaches to enhance wet adhesive ability. However, it is a different story when considering surface roughness. In this case, AL depends on two factors: (i) As; (ii) the microcosmic profile of the rough surface. As shown in Fig. 6b, when the pads with planar microstructures come into contact with a rough surface, liquid is pulled out from the microchannels because of capillary suction [6]. If the separation between the rough surface and the pads is smaller than the width of the channels, the pressure in the liquid film will be lower than in the grooves, resulting in the flow of liquid into the micro gap and formation of the liquid bridge [6]. So the relation between the microcosmic profile of the rough surface and the width of channels on biomimetic adhesive pads is the crucial factor of difference between As and AL . The profiles of the four rough surfaces in this study (see Fig. 2), which averaged three profiles of randomly chosen sections, were traced with the width of channels of the microstructures, as shown in Fig. 6a. On rough surface (Ra ¼ 2.62), most of the profiles are smaller than 10 mm, so AL is almost near As . On rough surface (Ra ¼ 0.18) all the profiles are less than 10 mm, then AL is equal to As . However, on the other two rough surfaces, most of the profiles are greater than 10 mm; therefore AL is much smaller than As . So in this case, fabricating microstructures and increasing preloads can increase As , but does not indistinctively contribute to the increase

Figure 6 Profiles of rough surfaces and schematic of area with liquid bridge

of AL . The analytical results are in good agreement with the experimental results in Section 3. According to the proposed experiments and analysis, designing channels of microstructures to adapt the given rough surface in wet adhesive systems is very important. It is worth mentioning that there are self-similar and multi-stage planar microstructures in a tree frog toe pad [4], so multi-scale microchannels form objectively to adapt different surface roughness. 5. Conclusions: The influence of substrates’ surface roughness on the wet adhesion of man-made adhesive pads inspired by tree frog toe pads has been investigated experimentally. The rougher surface leads to the decrement of wet adhesion force, and the wet adhesive force of pads without microstructures is independent of surface roughness. If the microcosmic profile height of the substrates is near or less than the width of microchannels in the biomimetic adhesive pads, the microstructures and preloads can increase significantly the wet adhesion force, otherwise the microstructures and preloads do not contribute indistinctively. The experimental results can be explained by analysing the relation between the solid contact area (As) and the area with the liquid bridge (AL) when considering surface roughness. 6. Acknowledgments: The work was financially supported by the Natural Science Foundation of China (grant numbers 51105279, 51275360, 61040056), the Shanghai Key Project of Foundation Funding (grant number 09JC1414600), the Huo Yingdong Foundation for Basic Research (grant number 121064), the National Basic Research Programme of China (973 Programme: 2011CB013800), the Specialised Research Fund for the Doctoral Programme of Higher Education (grant number 20100101120017), the Railway Ministry Science and Technology Research and Development Programme (grant number J2011Z004) and the ‘Chen Guang’ project supported by the Shanghai Municipal Education Commission and Shanghai Education Development Foundation (grant number 10CG20). 7

Figure 5 Maximum adhesive force variation with preloads on various rough surfaces

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