Gecko adhesion has attracted considerable scientific and public attention over the past two decades. Much effort is going into developing mimetics and gecko-inspired adhesives to replicate the gecko's adhesive mechanisms. Although much research has focused on the hierarchical micrometer and submicrometer structures of the gecko foot leading to the extraordinary gecko adhesion, the molecular-level mechanisms have remained largely unexplored. This work presents a novel multiscale simulation approach that allows us to study gecko adhesion at multiple length scales, from the atomistic to the mesoscale. We investigate the effect of humidity, the role of electrostatic interactions, and the impact of surface roughness on gecko adhesion. Finally, we reconcile a long-standing scientific debate regarding the primary mechanism of gecko adhesion under humid conditions, providing clarity and confirming the significant role of a previously undiscovered molecular mechanism in enhancing adhesion.

Our simulations go beyond previous computational and theoretical studies of gecko adhesion, which treated the spatula and surface as continuum bodies and modeled their interaction using the Hamaker theory. Instead, we bridge the gap in our understanding of gecko adhesion by using molecular dynamics simulations to investigate the molecular mechanism, uncovering a previously unknown effect called water-mediation. This mechanism involves small numbers of water molecules filling the gaps between keratin and the surface and increasing the number of keratin-surface contacts by partially absorbing into the keratin at the interface, thereby increasing the local density and the effective surface energy of the spatula.

Parallel to the endeavor of designing synthetic gecko-inspired adhesives, the remarkable impact of relative humidity (or the presence of water) on the stickiness of gecko spatulae and setae has been discovered. Through single-spatula atomic force microscopy experiments, it was found that the pull-off forces required to separate two surfaces increased with the relative humidity and decreased with the water contact angle of the surface. Despite years of investigation into the adhesion phenomena, the molecular mechanisms behind humidity-enhanced adhesion had remained elusive due to limitations in resolution and length scale. This work addresses this gap by combining molecular dynamics simulations, a true multiscale protocol, with experimental literature, providing a deeper understanding of the molecular mechanisms in gecko adhesion and its response to relative humidity.

The present work extends the understanding of attachment to rough surfaces. It shows that the stickiness decreases as the size of the surface features falls below the spatula contact area. We demonstrate that spatula-softening, the process by which the stiffness of the spatula decreases due to the presence of water, only assists adhesion at near saturation, meaning that the ambient relative humidity is close to 100%. While previous studies have suggested capillary forces as an alternative mechanism to spatula-softening, superhydrophobic surfaces lack attractive capillary forces. Nevertheless, we still observe increased adhesion under humid conditions compared to dry conditions on superhydrophobic surfaces. We conclude that water-mediation determines humidity-enhanced adhesion on hydrophobic flat and rough surfaces and that spatula-softening assists adhesion when the surface is rough and the ambient relative humidity is well above 80%.

Unlike on hydrophobic surfaces, keratin shows a remarkable difference on hydrophilic surfaces. The folding of polar and charged amino acids towards the hydrophilic surface maximizes electrostatic and van-der-Waals interactions, leading to a stronger attachment, even in dry conditions. The keratin attaches closer to flat hydrophilic surfaces than hydrophobic surfaces, even in dry conditions. These observations suggest that the keratin exhibits a functional-polymer-like ability to selectively change its tertiary structure upon contacting a hydrophilic surface. Moreover, we find that the presence of water enhances adhesion to hydrophilic surfaces due to water-mediated interactions between the keratin and the surface.

Overall, this work provides new insights into the molecular mechanisms of gecko adhesion and may be useful in finding new directions for developing gecko-inspired adhesives that can function effectively under a wide range of humidity conditions, by optimizing for water-mediated adhesion. Our work highlights the importance of considering the molecular-level interactions and the role of water in adhesion and provides a new direction for future research in this field.