Figure 2.

The surface layer of silicone elastomers exhibits higher apparent stiffness because of solid surface tension. (A) Apparent Young’s moduli were determined by fitting F-d curves from AFM indentation measurements with the standard Hertz model (red data points) or a modified Hertz model that includes a linear term for the solid surface tension (blue data points). An indentation speed of 1 μm/s and a setpoint force of 10 nN were used for the measurements. The x marks indicate independent experiments, and the circles mean values and error bars SEM. (B) Apparent Young’s moduli were determined with the standard Hertz model as a function of indentation speed for elastomers of varying ζ. Data represent the average and error bars the SD from 75 measurements at different locations of the elastomers (one of two independent experiments shown). Data were fitted with a power law equation, with the fitted parameters given in the table. (C) Apparent Young’s moduli were determined with the standard Hertz model as a function of indentation depth for elastomers of varying ζ. The inset shows the data as a function of setpoint force. Data represent the average and error bars the SD from 75 measurements (one of two independent experiments shown). (D) A typical F-d curve derived from indentation of an elastomer with ζ = 0.7 along with best fits for the standard Hertz model and a modified Hertz model that includes a linear surface tension term. (E) Solid surface tension of silicone elastomers were derived from the modified Hertz model. The x symbols indicate independent experiments and the column the average value. To see this figure in color, go online.