Digital hyperextension has no influence on the active self-drying of gecko adhesive subdigital pads.

The remarkable properties of the gecko adhesive system have been intensively studied. Although many gecko-inspired synthetic adhesives have been designed and fabricated, few manage to capture the multifunctionality of the natural system. Analogous to previously documented self-cleaning, recent work demonstrated that gecko toe pads dry when geckos take steps on dry substrates (i.e., self-drying). Whether digital hyperextension (DH), the distal to proximal peeling of gecko toe pads, is involved in the self-drying process, had not been determined. Here, the effect of DH on self-drying was isolated by preventing DH from occurring during normal walking locomotion of Gekko gecko after toe pads were wetted. Our initial analysis revealed low statistical power, so we increased our sample size to determine the robustness of our result. We found that neither DH nor the DH-substrate interaction had a significant effect on the maximum shear adhesive force after self-drying. These results suggest that DH is not necessary for self-drying to occur. Interestingly, however, we discovered that shear adhesion is higher on a surface tending hydrophobic compared to a hydrophilic surface, demonstrating that gecko adhesion is sensitive to substrate wettability during the subdigital pad drying process. Furthermore, we also observed frequent damage to the adhesive system during shear adhesion testing post-drying, indicating that water may compromise the structural integrity of the adhesive structures. Our results not only have behavioral and ecological implications for free-ranging geckos but also have the potential to influence the design and fabrication of gecko-inspired synthetic adhesives that can regain adhesion after fouling with water.

Effects of tether length on the behavior of amphiphilic bent-core molecules at water surfaces.

Alignment layers for bulk liquid crystalline phases can be created with monolayers formed by Langmuir-Schaefer techniques. Monolayer stability is a function of the propensity of the component molecule to effectively pack at a water interface; this propensity is enhanced when the molecule has an appropriate balance of hydrophilicity and hydrophobicity and the desired liquid crystalline order, as well as other structural factors. Our experiments show that molecules based on a bent-core with one hydrophilic and one hydrophobic end can form stable monolayers that act as effective alignment layers. However, the stable monolayers only form when the hydrophilic end has a sufficiently short chain. Molecular simulations carried out for both dilute concentrations (1 bent-core molecule) and high concentrations (25 bent-core molecules) on a water surface elucidate this behavior. The hydrophilic group acts to tether the molecule to the water surface, with a tether floppiness that depends on the tether length. At dilute concentrations, these molecules lay flat on the water surface (the molecular long axis approximately parallel to the surface), and the tether floppiness has little consequence. However, at high concentrations, the molecules pack with orientations approximately perpendicular to the surface; they stand upright on the tether, and the floppier tether leads to wobbly legs that cause large lateral fluctuations in the molecular positions and reduce monolayer stability.