Figure 1.

Schematics of the microsphere indentation method. A microsphere with radius R is in contact with a soft gel layer of thickness h. The bottom of the gel is adhered to a glass slide. Heavy dotted lines indicate the surface of the elastic substrate and the sphere (rigid) just before contact. The geometry after contact is indicated by solid lines. The point P is at distances that are very large compared with the contact radius a. In Hertz theory, h = ∞ and δ₂ is determined by a small-angle approximation, as illustrated in the inset.