"Detachment of an inclined micro-pillar from a dissimilar substrate"

Many reversible adhesive systems have been proposed in the past that closely mimic the adhesion mechanisms in the gecko feet. These adhesive systems comprise an array of compliant micropillars that are fabricated on a substrate, which can be used to fasten two surfaces. Adhesive systems with directional adhesion have also been designed. One such system was proposed by Jeong et al. (PNAS, 2009, v106:5639-5644), where the compliant pillars were inclined with respect to the adherend surface. When the pillars were sheared along with the inclination, the adhesive strength was larger when compared to the case when the pillars were sheared opposite to the pillar orientation. The physical mechanisms that lead to this behavior have not been quantitively detailed yet, and this is the objective of our work. Using finite element simulations, we show that the difference in the strength is due to the nature of singularities in the elastic fields that exist near the interfacial pillar-substrate corners. We also show that this difference exists only when the pillar is compliant with respect to the substrate to which it is attached to. When the substrate is compliant with respect to the pillar, however, we do not observe a direction-dependent strength.