Direct observation of microcavitation in underwater adhesion of mushroom-shaped adhesive microstructure

Abstract

SummaryIn this work we report on experiments aimed at testing the cavitation hypothesis [Varenberg, M.; Gorb, S. J. R. Soc., Interface 2008, 5, 383–385] proposed to explain the strong underwater adhesion of mushroom-shaped adhesive microstructures (MSAMSs). For this purpose, we measured the pull-off forces of individual MSAMSs by detaching them from a glass substrate under different wetting conditions and simultaneously video recording the detachment behavior at very high temporal resolution (54,000–100,000 fps). Although microcavitation was observed during the detachment of individual MSAMSs, which was a consequence of water inclusions present at the glass–MSAMS contact interface subjected to negative pressure (tension), the pull-off forces were consistently lower, around 50%, of those measured under ambient conditions. This result supports the assumption that the recently observed strong underwater adhesion of MSAMS is due to an air layer between individual MSAMSs [Kizilkan, E.; Heepe, L.; Gorb, S. N. Underwater adhesion of mushroom-shaped adhesive microstructure: An air-entrapment effect. In Biological and biomimetic adhesives: Challenges and opportunities; Santos, R.; Aldred, N.; Gorb, S. N.; Flammang, P., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 2013; pp 65–71] rather than by cavitation. These results obtained due to the high-speed visualisation of the contact behavior at nanoscale-confined interfaces allow for a microscopic understanding of the underwater adhesion of MSAMSs and may aid in further development of artificial adhesive microstructures for applications in predominantly liquid environments.

Friction Contribution to Bioinspired Mushroom‐Shaped Dry Adhesives

“…For samples IV-VI, the values of (a − R i )/(R e − R i ) were about 0.4, 0.38, and 0.28, respectively, and then were used to inspect which one of the deflection curve family was the unique solution corresponding to the critical dynamic crack. As a result, the crack radius and height was about 7.4 µm and ≈123 nm, respectively, which was in good agreement with recent simulation results [52] (detailed method for calculating the critical dynamic crack shape supplied as Figure S4 in the Supporting Information). Interestingly, as for samples I-III, the U Π curves cannot reach obvious extreme points and were above 0 for the whole domain (see the inset in Figure 4c).…”

Comparative study of the fluid viscosity in tarsal hairy attachment systems of flies and beetles

“…Below h c , the secretion would be in tension, and the pressure inside the capillary bridge becomes negative. It is known that liquids can withstand high negative pressures (several times atmospheric pressure [45][46][47][48]), but this is physically a metastable state and even a small disturbance (gas residues in the secretion, surface roughness and/or particles) may lead to cavitation failure, giving rise to an unpredictable and catastrophic rupture between the plates. From the biological point of view, one may speculate that attachment systems of insects avoid functioning in such a regime.…”

Bioinspired reversible hydrogel adhesives for wet and underwater surfaces

“…Consequently, the water remaining on the substrate hinders close contact between the microstructure and substrate, leading to the remarkably reduced normal and shear adhesion strengths. 1,6,8,[18][19][20] In this case, the adhesion originated mainly from capillary adhesion. The capillary adhesion between two adjacent surfaces is expressed as: 69,70…”

“…[9][10][11][12][13][14][15][16][17] However, in wet environments, water molecules not only penetrate gaps between the surfaces easily, but also reduce the adhesion strengths of the adhesive materials by hydrating and decomposing them. Consequently, it is very challenging to ensure good adhesion under wet conditions 1,3,8,[18][19][20] Although cyanoacrylate (Super Gluet) is a strong tissue adhesive, it is cytotoxic, and incompatible with wet or underwater surfaces because the uncured adhesive rapidly solidifies into a glassy phase when it comes into contact with water. 21,22 In order to overcome such difficulties, synthetic wet adhesives emulating natural adhesive materials of marine organisms have been extensively investigated.…”

“…52 In this case, the entrapped water acts as a flaw for initiating debonding at the interface, 52 resulting in limited wet adhesion performance. 1,3,8,[18][19][20] In this regard, hydrogels have immense potential as reversible wet adhesives when they are integrated with biomimetic microstructures, based on the following factors. First, the hydrophilic nature of the hydrogels is beneficial to induce strong capillary adhesion under wet conditions.…”