New Paper! Capillary adhesion of stick insects in Annals of the New York Academy of Sciences

Scientific progress within the last few decades has revealed the functional morphology of an insect’s sticky footpads—a compliant pad that secretes thin liquid films. However, the physico-chemical mechanisms underlying their adhesion remain elusive. Here, we explore these underlying mechanisms by simultaneously measuring adhesive force and contact geometry of the adhesive footpads of live, tethered Indian stick insects, Carausius morosus, spanning more than two orders of magnitude in body mass. We find that the adhesive force we measure is similar to the previous measurements that use a centrifuge. Our measurements afford us the opportunity to directly probe the adhesive stress in vivo and use existing theory on capillary adhesion to predict the surface tension of the secreted liquid and compare it to previous assumptions. From our predictions, we find that the surface tension required to generate the adhesive stresses we observed ranges between 0.68 and 12 mN m−1. The low surface tension of the liquid would enhance the wetting of the stick insect’s footpads and promote their ability to conform to various substrates. Our insights may inform the biomimetic design of capillary-based, reversible adhesives and motivate future studies on the physico-chemical properties of the secreted liquid.

(A) Schematic of the experimental setup combining tethered pulling measurements with frustrated total internal reflection (FTIR) imaging. (B) Pulling force measurement for a typical experiment with an insect of mass m = 390 mg. The discrete data points in yellow represent the experimental measurements, while the solid black line denotes the filtered data. The adhesive force F is taken as the peak force minus the insect’s weight. (C–E) Images of three adhesive pads, from the same insect as in panel B, using FTIR with (i) raw and (ii) binarized images.
Measurements of: (A) contact area A, (B) adhesive force F, (C) adhesive stress σ, (D) contact perimeter P, (E) liquid tension γ, and (F) sliding distance δ across body mass m. The solid lines denote power-law fits, provided in Table 1. Panel B also shows the whole-insect centrifuge measurements from Labonte et al.

Check out the paper here: https://nyaspubs.onlinelibrary.wiley.com/doi/epdf/10.1111/nyas.15195

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