There have been many attempts to characterize and mimic natural fluid-based adhesive systems. However, very few of these studies have examined surface or interfacial interactions between adhesive and substrate. Furthermore, until now no study has used a combination of surface analytical and kinematic techniques to determine the precise chemical mechanisms that govern the success or failure of a natural adhesive in different environments. The work presented here demonstrates how the composition, order and relative orientation of adhesive molecules at functional interfaces can be determined using complementary surface analytical techniques.
Initially, the chemical makeup and pressure-sensitive nature of frog mucus was examined with Near Edge X-Ray Absorption Fine Structure (NEXAFS) Spectroscopy and Sum Frequency Generation (SFG) Vibrational Spectroscopy to determine if mucus glycoprotein molecules located at the mucus-prey interface formed fibrils during prey capture. NEXAFS sampled photons associated with bond orbitals of molecular bonds within the top few nanometers of the interface while SFG measured order-sensitive intensity of vibrational modes at this same surface with sub-monolayer resolution. NEXAFS images at the N1s and C1s K-edges at two incident x-ray angles were collected by rastering the x-ray beam at across the surface of the mucus, showed that surface chemical bonds associated with glycoproteins were uniformly distributed. Angle-resolved NEXAFS experiments and SFG spectra indicated a glycoprotein tertiary structure
consistent with fibril formation and methyl and methylene groups on amino acid side chains oriented normal to fibrils on the surface.
Next, insect tarsal fluid adhesive was investigated, utilizing SFG spectroscopy to probe the presence of ordered molecular bonds at the functional interface between adhesive fluid and substrate. Fluid footprints of a model species, the seven-spotted ladybird beetle, were collected on three substrates with varying surface energies and hydrophobicities. Film thickness and roughness were determined with profilometry and atomic force microscopy respectively to ensure only smooth surfaces were tested. Resulting SFG spectra revealed that the chemical environment at the interface was not affected by substrate hydrophobicity. However, the ordering of observed hydrocarbon groups within the fluid increased with a corresponding increase in substrate hydrophobicity. The interfacial layer of beetle adhesive fluid was determined to promote lubrication of foot contact, rather than adhesion.
Building upon these results, a biomimetic adhesive fluid inspired by ladybird beetle foot adhesive was designed. A water-in-oil emulsion was formulated using 3 components - squalane, deuterated stearic acid and water. SFG spectra of the mimetic fluid on both hydrophilic and hydrophobic surfaces revealed that molecular vibrations in each case closely resembled those of the natural fluid, with squalane the only surface-active component. Preexisting molecular dynamics simulation of squalane at solid surfaces showed that it prefers to orient parallel to the surface with its methyl branches pointing in the surface normal direction. Spectra of pure squalane and a squalane/deuterated stearic acid mixture on the same substrates revealed the that the surface-inactive components (d-stearic acid and D2O) have an observable effect on the ordering of these squalane layers at the interface. Pull-off force, traction force and viscometry measurements of each fluid showed an increase in viscosity and pull-off force and a decrease in traction force with increasing fluid complexity. Based upon this, it is concluded that the surface-inactive components of ladybird beetle adhesive created a more cohesive fluid which allowed it to accurately mimic the functional properties of natural ladybird beetle adhesive.