Multi-Technique Investigation of a Biomimetic Insect Tarsal Adhesive Fluid
There is substantial motivation to develop novel adhesives which take advantage of the superior adhesive strength and adaptability of many natural animal adhesives; however, the tools typically used to study these mechanisms are incapable of determining the precise interactions of molecules at an adhesive interface. In this study, a surface speciﬁc, order sensitive vibrational spectroscopy called sum frequency generation (SFG) is, for the ﬁrst time, combined with multiple bulk characterization techniques to examine a novel, simple biomimetic adhesive ﬂuid inspired by tarsal ﬂuid of insects. Insects perform complex adhesive demands, including sticking, climbing vertically and running upside-down with little difﬁculty. Thus, we hypothesize that both bulk and surface speciﬁc properties of the ﬂuid contribute to the success of this wet adhesive mechanism. SFG spectra of biomimetic emulsion exhibited similar hydrocarbon organization on hydrophobic and hydrophilic substrates to natural beetle ﬂuid previously studied with the same method. Bulk characterization techniques indicated that the emulsion had a shear-thinning proﬁle with the ability to enhance traction forces during climbing and low surface tension ideal for surface wetting on the majority of natural surfaces. Multi-technique comparisons between emulsion and pure squalane revealed that a hydrocarbon only based ﬂuid could not replicate the traction promoting properties of the emulsion. We conclude that the insect tarsal ﬂuid adhesive mechanism relies upon contributions from both surface-speciﬁc properties optimizing traction force and bulk properties promoting rapid surface wetting and maintaining pull-off force for fast detachment.