Meeting Abstract
Common orb weaving spiders don’t catch many moths. Moths evade capture by shedding the scales on their body and wings when those microstructures contact adhesive on the web. Orb weavers of the genus Cyrtarachne have evolved a solution that allows them to be moth-catching specialists: a low-viscosity glue that flows quickly under the moth’s scales, gluing the scales and the web attached to them to the underlying cuticle before the scales can be shed. The glue from one species, Cyrtarachne akirai, is unremarkable when its properties are measured on glass, the classic substrate for bioadhesion test, but when this glue is tested on biomimetic substrates made from scaled moth wings, its behavior changes dramatically, spreading three times farther and required eight times the force to remove. We hypothesize that this ‘hyper-wetting’ is caused by the interaction of the glue and the topology of the scales. Using the physics of droplets, capillary forces, and pipe flow, we propose several spreading models. The models are tested and compared using the spreading behavior measured in the experimental situation. We propose that the glue of Cyrtarachne is of a particular viscosity that interacts with the scales favorably, creating a porous material, wicking water using capillary action. We tested this by comparing the spreading behavior of (1) variable viscosity liquids brought into contact with scaled surfaces and (2) glue droplets spread on surfaces of various porosities. Furthermore, our model predicts this ‘hyper-wetting’ leads to a separation of the water soluble components and glycoproteins within the droplet, leading to glue hardening. We used Raman spectroscopy to test this by measuring the distribution of salts and proteins along the radius of droplets spread on glass and moth wings.
