Khai Trung Le talks to René Hensel on developing a structure that improves adhesion on rough surfaces.
A new adhesive structure has been developed at the Leibniz Institute for New Materials (INM), Germany, using a combination of hard and soft materials to counter the negative effect roughness has on adhesion. Roughness reduces contact area and results in higher elastic strains in the contact zone, counteracting adhesion.
The team looked to common fibrillar dry adhesive structures including the footpads of geckos that allow for easy adaption for roughness with little strain energy. The INM structure comprises 2mm composite pillars of either polyethylene glycol dimethacrylate or polydimethylsiloxane, with a softer tip coating made from polyurethane. The structure was applied to glass substrates, and peeled off to evaluate interface geometry and Young’s modulus in relation to surface roughness, preload and hold time.
The paper, Composite pillars with a tunable interface for adhesion to rough substrates, published in ACS Applied Materials & Interfaces, details how the ‘pull-off forces of composite pillars can significantly exceed the values of conventional pillar structures’, noting that composite pillars retain similar adhesion to both smooth and rough surfaces, while conventional pillars lost around 50% adhesion.
René Hensel, Deputy Head of Functional Microstructures at INM, told Materials World, ‘The pillars that we used were made of a hard material, but their ends had a layer of soft plastic. In order to peel away this pillar, we needed to apply a force that was five times greater compared to peeling away a pillar composed entirely of the soft material. Therefore, it clearly adheres better.’
The strength of adhesion was linked to the softness and thinness of the coating applied to the ends of the pillars, with the team noting that the softer the material, the more adaptable it is to rough surfaces, as thinner coatings delay the formation of cracks on contact due to reduced stress peaks. Hensel added, ‘Surprisingly, the thinner the coating is, the more pronounced this phenomenon.’
Shape also informs adhesion, with the thickness of the soft coating also impacting on the formation of cracks. Hensel said, ‘The surface of woodchip wallpaper is far rougher than skin, for instance, so in order for something to adhere to woodchip wallpaper, a much thicker soft coating must be selected compared to adhesion to skin.’
One unusual finding was the connection between the difference of strength between composite pillars with hemispherical interfaces and thin tips over composites with flat interfaces and conventional pillar sizes. Hensel said, ‘The higher adhesion probably results from larger contact areas that were most likely induced by the high centre stresses under compressive pre-loads, which translate into high centre stresses in tension during detachment. These are more beneficial than high stresses at the perimeter, in the case of conventional pillars, due to edge stress intensities.’
Current applications include industrial handling, and the team aims to further hone the structure to be compatible with skin, hoping to further the development of wearable technology and wound treatment.
You can read Composite pillars with a tunable interface for adhesion to rough substrates at bit.ly/2S1q9yT