D21 Atomic force microscopy (AFM)

Topic

The interaction force between a tiny tip and a surface can be measured at the nanoscale using atomic force microscopy (AFM)—a technique applied directly to the insect's adhesive pads, its cuticle, and various substrates of interest.

In force-distance curve mode, the AFM tip approaches and then retracts from the surface; throughout this process, the deflection of the cantilever holding the tip is recorded relative to the tip's exact position as controlled by a piezoelectric element. Ultimately, this cantilever deflection constitutes the raw signal from which the interaction force is derived.

To convert this deflection into an actual force value, precisely calibrating the cantilever's spring constant is essential. Sader's method achieves this calibration in a particularly elegant way: it relates the spring constant to the cantilever's resonant frequency and quality factor while it vibrates within a fluid, eliminating the need for an external force reference standard for comparison.

Once the cantilever is calibrated, the approach phase of the force-distance curve can be fitted using the Hertz model to extract the sample's Young's modulus. This fit works because, in a spherical elastic contact—such as that between the AFM tip and the surface—the force increases with the penetration depth raised to the power of three-halves, a relationship characteristic of this type of contact. When the AFM tip is exceptionally small and rigid—with a radius of less than twenty nanometers—the Hertz model is no longer the most suitable; instead, the DMT model is applied. This model directly relates the force required to separate the tip from the surface (known as the pull-off force) to the Dupré adhesion energy between them, without requiring knowledge of the extent of surface deformation during contact.

When applied to individual adhesive setae of the insect, these force curves reveal something that no average measurement of the setal array could show: how the adhesion force varies from one seta to another within the same adhesive structure, and how that force changes depending on the orientation of the seta's contact with the surface—an effect known as adhesive anisotropy.

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