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Investigating the Adhesion of Gecko-Inspired Microstructures

Adhesion is present in many facets of life - glue on a surface, a tire on the road, dust on the display. It originates from two materials interacting in a certain way with each other at their interface. Thus, adhesion can be experienced macroscopically although it originates from forces acting at a considerably smaller scale.

Figure 1: Scanning electron microscopy micrographs of the investigated gecko-inspired flat punch (a) and T-shaped (b) micropillar structures. (c) Scheme of the experimental setup. The micropillar structures are attached to a micro-contact printing stamp (top) so that they can be approached to and retracted from the mechano-sensitive polymer brush. (d) Schematic depiction of the mechano-response mechanism. In equilibrium, the dye molecules on the polymer chains are fluorescing (green dots). Only a minor amount of dye molecules is quenched by the cationic charges on the polymer chains (grey dots). Compression leads to a change of the brush conformation so that more dye molecules are quenched, the fluorescence intensity is decreased. Tension leads to the opposite effect. The chains are more stretched, less dye molecules are quenched, the fluorescence intensity is increased. Adapted with permission from ref. [7]. Copyright 2016 American Chemical Society.

Figure 2: Schematic depiction of the detachment mechanisms of flat punch (top) and T-shaped pillars (bottom) from the mechano-sensitive polymer brush. The insets in the top right of each box show CLSM images taken during the stages of detachment. The scalebars correspond to 20 µm. Flat punch pillars detached with minor stresses indicating their weak adhesion. During the detachment of T-shaped pillars, tensile stresses were concentrated to the central contact area suppressing crack nucleation at the edges. Adapted with permission from ref. [7]. Copyright 2016 American Chemical Society.

In my PhD studies, I am utilizing mechano-responsive polymers as force sensors on this microscopic scale. As an example, we investigated the adhesion mechanism of gecko-inspired micropillar structures. For two differing micropillar geometries, we utilized mechano-responsive polymers to measure the stress distributions in the contact areas so that we could attribute them to the strong difference in adhesion.

The superior adhesion abilities of geckos originate from hairy fibrils covering their feet, the setae. These hairy fibrils split up into thinner fibrils with a certain contact geometry, the spatulae. They come in contact with the surface.

Splitting up the contact into numerous small contact areas can increase the ability for adapting to surface roughness. Even surfaces that appear smooth to the eye can feature a microscopic roughness. In addition to adaptability, each contact zone must be detached for complete adhesion failure. Without such contact splitting, the contact would have to be maintained over a large continuous area. As a consequence, a single crack could propagate over the continuous contact area, and might be sufficient to cause adhesion failure. Following the principle of contact splitting, arrays of micrometer-sized pillars were designed as bio-inspired adhesive structures. Their contact geometries were inspired by the setae of geckos and other animals. In previous work of del Campo et al., their adhesion performance was determined by bringing a macroscopic sphere in contact with the micropillar structures and measuring the pull-off force needed for detachment. [1] Significantly, simple flat punch pillars (Figure 1a) were outperformed by micropillars with annular overhangs (T-shapes, Figure 1b). [1,2]

To gain insight how these additional overhangs affect adhesion, detachment mechanisms of flat punches and T-shapes were derived from theoretical models. [3-5] For flat punches, theoreticians predicted maximal stress at the edges. This facilitates the formation of cracks which can propagate toward the center. Also, it reasonably explains the fast detachment of flat punches. For T-shapes, the stress was predicted to be lower at the edges. Accordingly, detachment cracks tend to form in the center of the T-shapes propagating toward the edges, as also observed in experiments. This hinders fast detachment.

These theoretical predictions of the contact stresses were already in good agreement with what had been observed during detachment in pull-off experiments. To bridge the gap between theory and bare observation, we measured the stress distributions in the contact zones of flat punch and T-shaped micropillars. To access the stresses acting directly between the adhesive structures and a surface, we used a mechano-sensitive surface coating based on a polymer brush.

In a polymer brush, polymer chains are bond to a surface at one end while the free chain end is stretched into solution, resembling bristles of a brush. For mechano-sensing, we generated cationic polyelectrolyte chains in such a brush conformation on glass. These chains carried positive charges on each repetition unit. Further, fluorescein dye molecules were attached to the chains. Their fluorescence could be quenched by the cationic charges of the chains when they come close to each other, reporting local stresses. [6]

Consequently, the polymer brush could serve as a tool to visualize mechanical stresses from the local fluorescence intensity. Local compression lead to locally lower fluorescence intensity, as the quenching of dye molecules was increased locally. Vice versa, local tension stretched the chains so that the local fluorescence intensity was higher than in equilibrium because of reduced quenching (Figure 1d).

Utilizing this surface as a mechano-sensor, the location of the stresses is limited by the optical resolution of the fluorescence microscope. To achieve micrometer resolution, we used confocal laser scanning microscopy (CLSM).

In the experiments, the gecko-inspired microstructures were approached to the mechano-sensing polymer brush with a micro-contact printing device, and retracted from the surface again (Figure 1c). Simultaneously, the local fluorescence was monitored with CLSM through the glass substrate. [7]

From the fluorescence intensity of the CLSM images, compressive and tensile stresses in the contact area of the flat punch and T-shaped micropillars could be deduced and localized. During approach, compression was observed over the whole contact area for both geometries. Expectedly, more distinct differences were found upon retraction (Figure 2). The compression under the flat punch micropillars decreased until the flat punches had been retracted to the position of contact formation. Then, the flat punches were detached with minor tensile stresses, indicating their weak adhesion and quick detachment. For T-shaped micropillars, tensile stress built up at the center of the contact area when the T-shapes were retracted above the position of contact formation. Simultaneously, the contact appeared to be maintained at the edges of the T-shapes.

Being able to resolve these contact stresses before detachment, the results indicated evidence for the postulated detachment mechanism that had been derived from theory. [7] This further supported that the underlying mechanism for the enhanced adhesion of T-shapes was an effect of contact mechanics. Therefore, this method has demonstrated to be a valuable tool in this field. Ongoing studies focus on the development of mechano-sensitive polymer structures.

References
[1] A. del Campo, C. Greiner, E. Arzt, Contact shape controls adhesion of bioinspired fibrillar surfaces, Langmuir. 23 (2007) 10235–10243.
[2] L. Xue, J. Iturri, M. Kappl, H.-J. Butt, A. del Campo, Bioinspired Orientation-Dependent Friction, Langmuir. 30 (2014) 11175–11182.
[3] G. Carbone, E. Pierro, S.N. Gorb, Origin of the superior adhesive performance of mushroom-shaped microstructured surfaces, Soft Matter. 7 (2011) 5545–5552.
[4] B. Aksak, K. Sahin, M. Sitti, The optimal shape of elastomer mushroom-like fibers for high and robust adhesion, Beilstein J. Nanotechnol. 5 (2014) 630–638.
[5] R.G. Balijepalli, M.R. Begley, N.A. Fleck, R.M. McMeeking, E. Arzt, Numerical simulation of the edge stress singularity and the adhesion strength for compliant mushroom fibrils adhered to rigid substrates, Int. J. Solids Struct. 85-86 (2016) 160–171.
[6] J. Bünsow, J. Erath, P.M. Biesheuvel, A. Fery, W.T.S. Huck, Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes, Angew. Chem. Int. Ed. 50 (2011) 9629–9632.
[7] J.W. Neubauer, L. Xue, J. Erath, D.-M. Drotlef, A. del Campo, A. Fery, Monitoring the Contact Stress Distribution of Gecko-Inspired Adhesives Using Mechano-Sensitive Surface Coatings, ACS Appl. Mater. Interfaces. 8 (2016) 17870–17877.


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