Non-uniform breaking of molecular bonds, peripheral morphology and releasable adhesion by elastic anisotropy in bio-adhesive contacts

Abstract

Biological adhesive contacts are usually of hierarchical structures, such as the clustering of hundreds of sub-micrometre spatulae on keratinous hairs of gecko feet, or the clustering of molecular bonds into focal contacts in cell adhesion. When separating these interfaces, releasable adhesion can be accomplished by asymmetric alignment of the lowest scale discrete bonds (such as the inclined spatula that leads to different peeling force when loading in different directions) or by elastic anisotropy. However, only two-dimensional contact has been analysed for the latter method (Chen & Gao 2007 J. Mech. Phys. Solids 55, 1001-1015 (doi:10.1016/j.jmps.2006.10.008)). Important questions such as the three-dimensional contact morphology, the maximum to minimum pull-off force ratio and the tunability of releasable adhesion cannot be answered. In this work, we developed a three-dimensional cohesive interface model with fictitious viscosity that is capable of simulating the de-adhesion instability and the peripheral morphology before and after the onset of instability. The two-dimensional prediction is found to significantly overestimate the maximum to minimum pull-off force ratio. Based on an interface fracture mechanics analysis, we conclude that (i) the maximum and minimum pull-off forces correspond to the largest and smallest contact stiffness, i.e. 'stiff-adhere and compliant-release', (ii) the fracture toughness is sensitive to the crack morphology and the initial contact shape can be designed to attain a significantly higher maximum-to-minimum pull-off force ratio than a circular contact, and (iii) since the adhesion is accomplished by clustering of discrete bonds or called bridged crack in terms of fracture mechanics terminology, the above conclusions can only be achieved when the bridging zone is significantly smaller than the contact size. This adhesion-fracture analogy study leads to mechanistic predictions that can be readily used to design biomimetics and releasable adhesives.

Effects of geometric factors and shear band patterns on notch sensitivity in bulk metallic glasses

“…(2) is invalid and these flaws, since they are smaller than the plastic zone, will not reduce the tensile strength noticeably. For the other example, the inelastic process zone can be a bridging zone arising from atomic cohesion in brittle solids or fiber pull-out in composites [13][14][15]. As shown in Fig.…”

“…In this case, microscopic flaws have virtually no effects on the composite strength. 5 For a third example, we note that the long-chain molecular bonds in biological materials govern the interface adhesion [15]. For applications like cell adhesion, the cohesive zone size can indeed be very large, so the use of LEFM should be cautious.…”

Fracture mechanics modeling of popping event during daughter cell separation

“…The frac-ture mechanics model developed in the article to explain the crack propagation-driven popping event in S. Aureus daughter cell separation can be easily generalized to understand other popping events driven by crack propagation, such as those observed in various plants and fungi mentioned at the beginning of the article. At last, we are thankful to one of the anonymous reviewers for pointing out that the cohesive zone model (Gao and Gao 2016;Liu and Gao 2015) may be a better choice for quantitative understanding fracture phenomena in cells. We believe the elastic fracture mechanics model developed in this article provides a mechanism which links the "popping" event of the cell and unstable crack growth in the peripheral ring.…”

Modelling actin polymerization: the effect on confined cell migration

“…However, as far as we know, it is still not clear if it plays an important role in cell migration from a mechanical point of view. Second, the friction coefficient between the cell and the channel wall is assumed constant; nevertheless, some works point out a variable friction coefficient in the channel wall (Liu and Gao 2015). Third, myosin contraction is assumed constant over time, although some research groups (Olsen et al.…”