Figure 6.

Autocorrelation function R_xx(x) of a pristine sidewall surface measured with AFM, and theoretical exponential fit with a correlation length of 83 nm. The standard deviation on the height is 10.3 nm and the distribution is almost Gaussian. The long-range order is caused by the larger scale ripples. The result is that only one ripple may stick out significantly more than the others and hence friction is more localised than on a surface with a purely exponentially decreasing autocorrelation function. The sidewall measured with AFM is similar but not identical to the one used for the friction measurements.