In situ observation of contact mechanisms in bioinspired adhesives at high magnification
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In situ observation of contact mechanisms in bioinspired adhesives at high magnification Dadhichi Paretkar, Andreas S. Schneider, Elmar Kroner, and Eduard Arzt, INM-Leibniz Institute for New Materials and Saarland University, Saarbrücken, Saarland, Germany Address all correspondence to Eduard Arzt at [email protected] (Received 20 July 2011; accepted 26 September 2011)
Abstract We analyzed the contact mechanisms of bioinspired microfibrillar adhesives using in situ scanning electron microscopy. During adhesion tests we observed that (i) the superior adhesion of mushroom-shaped fibrils is assisted by the stochastic nature of detachment, (ii) the aspect ratio of microfibrils influences the bending/buckling behavior and the contact reformation, and (iii) the backing layer deformation causes the microfibrils to elastically interact with each other. These studies give new insights into the mechanisms responsible for adhesion of bioinspired fibrillar adhesives.
Introduction During the last decade, researchers have developed sophisticated methods to fabricate adhesives inspired by the adhesion system of the gecko. Fibrillar structures were fabricated to mimic the natural system and to maximize adhesive performance.[1–6] Numerous experimental and theoretical studies were carried out to advance the understanding of the underlying contact mechanics.[4,7–13] However, to validate those models, in situ visualization of the contact phenomena at high magnification is necessary. Several groups have presented studies using optical microscopy combined with adhesion measurements.[14–17] These studies show, for example, the bending of fibrils, the actual contact area, or the attachment and detachment front. Although these experiments have given some insight into contact mechanisms, they are not suitable for investigating the processes at interfaces due to the limited magnification of the applied optical microscopy. Advanced bioinspired adhesives have features such as flaps (mushroom tips), which are in the sub-micrometer range.[1,18–20] To visualize the contact mechanisms at the length scale of these features, new measurement systems are required. Here, we propose scanning electron microscopy assisted contact mechanics visualization during an adhesion test with fibrillar surfaces.
Experimental methods Microfibrillar arrays of polydimethylsiloxane (PDMS) with pillars of different aspect ratios (height/diameter) and tip geometries were fabricated using photolithography and soft-molding processes.[21] SU-8 resists (2010, 2025, Micro Resist Technology, Berlin, Germany) were used to prepare templates in a standard photolithography process. The templates were then silanized using hexadecafluoro-1,1,12,2-tetrahydrooctyltrichlorosilane and
subsequently filled with PDMS (Sylgard 184, Dow Corning, 10:1 mixture). After cross-linking at 75 °C for at least 14 h, the samples were carefully demolded. For specific fabrication details, especially the fabrication of mushroom tip structures, see Refs.[4,21] Contact experiments were performe
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