E21 Van der Waals forces in mushrooms

Topic

The adhesive seta found on insects—whether on a beetle or the fly serving as the model here—consists of a specific microscopic structure that generates adhesion through van der Waals forces acting between the seta's tip and the contact surface.

At this nanoscale, induced-dipole fluctuations—specifically London forces—dominate over other adhesive forces; the Hamaker constant quantifies the strength of these London forces, which depends on the specific materials involved in the contact.

The Hamaker model incorporates these van der Waals interactions—introduced earlier in this document—along with the overall geometry of the seta and the contact surface.

The DMT model, applicable to small, rigid contacts, relates the seta's detachment force directly to the Dupré energy and the contact radius. Consequently, a larger contact radius between the seta and the surface results in a greater detachment force according to the DMT model.

The biological system's advantage lies in the distribution of contact across the millions of setae mentioned earlier: although each individual seta generates a relatively small force, the combined force of all the setae easily exceeds the insect's body weight. Consequently, this model—featuring adhesion driven by van der Waals forces between the tip of a mushroom-like structure and the contact surface—utilizes the structure's inclined geometry to enable the insect to easily detach in a direction perpendicular to the surface using minimal force, while simultaneously maintaining strong tangential adhesion. If the insect tilts the structure even further relative to the contact surface, the disparity between the low perpendicular detachment force and the high tangential adhesion becomes even more pronounced.

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