A24 Non-adhesive surfaces

Topic

Many surfaces, both natural and artificial, are optimized to do the exact opposite of what the insect's adhesive pads aim to achieve: rather than promoting contact, these anti-adhesive surfaces are designed to repel the insect and prevent it from gaining a foothold.

One common strategy employed by these anti-adhesive surfaces is a superhydrophobic, lotus-like texture, which drastically reduces the fraction of the surface area actually in contact with the pad; instead of resting on a continuous surface, the insect's adhesive pads touch only the tips of the surface's microstructures, and the resulting adhesion is reduced in direct proportion to the smallness of that actual contact area.

This same reduction in contact area determines the apparent contact angle water forms on the surface—as described by the Cassie-Baxter equation: when the angle exceeds 150 degrees, water ceases to wet the surface and simply beads up on it, while pad adhesion decreases in proportion to the increase in the contact angle.

Carnivorous plants, such as *Nepenthes*, take these anti-adhesive surfaces a step further by combining two strategies: on one hand, they coat their surface with a film of water that acts as a slippery layer, preventing the insect's adhesive pad from making direct contact with the plant's cuticle; on the other, they feature spines that snag and disorient any adhesive pads that do manage to get close.

When a pad becomes trapped on one of these spines, the insect can only free itself by exerting more elastic deformation energy than the spine requires to retain it—and the energy needed to escape can exceed the energy available in the insect's leg muscle. When that happens, the insect is immobilized by the very non-stick surface that was designed from the start to prevent it from gaining a foothold.

ID:2551