FIGURE 7.

Elasticity versus indentation (δ) for the evaluation of the data represented in Fig. 4. After an initially nonlinear response at low force (very near initial contact), the shear modulus calculation approaches a steady value. The horizontal line indicates the minimal-error value of elasticity (G = 1.45 kPa) reported by the program. In the horizontal region, the data indicate the independence of elasticity and indentation. The meaning of the nonhorizontal region has not been fully explored and may reflect the interaction of repulsive surface forces on the cantilever, noise in the vicinity of initial contact, or both.