Paper BI+NS-WeM13
Observing the Molecular Mechanisms of Insect Adhesion by Sum Frequency Generation Spectroscopy

Wednesday, November 1, 2017, 12:00 pm, Room 12

Many insects can walk on a range of natural surfaces through an adhesion process that combines an expansive array of hairy contacts on their feet, known as setae, and an adhesive fluid, forming contact between the setae and a substrate. Previous studies of this adhesion system have focused almost exclusively on the mechanical and kinematic aspects of adhesion, while ignoring the molecular interactions at the fluid – substrate interface. However, recent experiments illustrate that substrate chemistry does influence the adhesive forces produced by this fluid. Additionally, mass spectrometry results demonstrate that this adhesive fluid is a complex mixture containing both hydrophobic (i.e. fatty acids and lipids) and hydrophilic (i.e. sugars, alcohols, and carbohydrates) compounds. We hypothesize that the molecular structure at the adhesive fluid-substrate interface is dynamic, with different molecules within the fluid selectively organizing at the interface as a function of substrate hydrophobicity. In the work presented here we probe the molecular interactions between the adhesive fluid taken from lady bugs (Cocinella septempunctata) and three model substrates, polyethylene oxide, polystyrene and CaF₂ with vibrational sum frequency generation (SFG) spectroscopy and scanning electron microscopy (SEM). The observed water contact angles for the polyethylene oxide, polystyrene and CaF₂ substrates were 66^o, 92^o and 106^o, respectively. High-resolution SEM images of individual seta-fluid footprints on the surfaces indicate localized “water in oil” emulsion de-wetting with no sign of distinct patterning. SFG spectra collected, from the three substrates, at the C-H (2800-3100 cm^-1) contain peaks at 2850 cm^-1 and 2870 cm^-1, characteristic of symmetric CH₂ and CH₃ stretches, respectively. The presence of these peaks suggests an ordered hydrocarbon monolayer at the interface. However, subtle changes in ordering of these molecular groups at the interface were observed across substrates by comparing the ratio of the intensities of observed vibrational modes related to the CH₂ and CH₃ modes. Across the three different substrates this ratio increased with surface hydrophobicity suggesting that the fluid-surface interactions adapt to different substrate chemistries.