Monitoring the Contact Stress Distribution of Gecko-Inspired Adhesives Using Mechano-Sensitive Surface Coatings

Abstract

The contact geometry of microstructured adhesive surfaces is of high relevance for adhesion enhancement. Theoretical considerations indicate that the stress distribution in the contact zone is crucial for the detachment mechanism, but direct experimental evidence is missing so far. In this work, we propose a method that allows, for the first time, the detection of local stresses at the contact area of biomimetic adhesive microstructures during contact formation, compression and detachment. We use a mechano-sensitive polymeric layer, which turns mechanical stresses into changes of fluorescence intensity. The biomimetic surface is brought into contact with this layer in a well-defined fashion using a microcontact printer, while the contact area is monitored with fluorescence microscopy in situ. Thus, changes in stress distribution across the contact area during compression and pull-off can be visualized with a lateral resolution of 1 μm. We apply this method to study the enhanced adhesive performance of T-shaped micropillars, compared to flat punch microstructures. We find significant differences in the stress distribution of the both differing contact geometries during pull-off. In particular, we find direct evidence for the suppression of crack nucleation at the edge of T-shaped pillars, which confirms theoretical models for the superior adhesive properties of these structures.

Luminescent Surface‐Tethered Polymer Brush Materials

“…Neubauer et al. expanded on this work demonstrating the use of mechanosensitive polymer brushes incorporating a carboxyfluorescein fluorophore that enabled the detection of the contact distributions of biomimetic structures by CLSM [95] …”

Adhesion between oppositely charged polyelectrolytes

“…the edge of the circular contact. Contact stresses are a longstanding problem in contact mechanics [34] and although their measurement has been of longstanding interest [2] new techniques for the in situ measurement of the stress are continually being developed [35]. The stress is given by…”

Remarkable Mechanochromism in Blends of a π‐Conjugated Polymer P3TEOT: The Role of Conformational Transitions and Aggregation

“…A clear color‐shift, which requires a shift in the absorption band, would provide a fast, visible warning sign for mechanical stress or deformations in the material. Moreover, mechanochromic polymers might be useful in many emerging materials and applications, e.g., in artificial skin, wearable, camouflage systems, and attoreactor sensors, and for anti‐counterfeiting …”

“…There are two major approaches to build up polymer‐based mechanochromic materials. The first is the incorporation of so‐called aggregachromic dyes into polymer matrixes to form thermodynamically stable micro‐/nano‐sized aggregates . Aggregachromic dyes are planar, rigid π‐conjugated small molecules, which form J‐ or H‐aggregates through the π‐stacking interactions and exhibit distinct optical properties as compared to the monomeric species (bathochromically or hypsochromically shifted absorptions/emissions).…”

“…The second approach involves a covalent linking of chromogenic units to polymer chains . The colorimetric transition can originate from changing molecular interactions, e.g., aggregation, or caused by chemical transformations . Common chromogenic units are organic molecules or organometallic complexes .…”