Linear ripples and traveling circular ripples produced on polymers by thermal AFM probes

We discuss the time and temperature evolution of the nanometer-scale surface undulations (ripples) produced by a heated atomic force microscope (AFM) tip scanning across surfaces of several amorphous polymers. During linear zigzag scanning we obtain pseudolinear ripples approximately perpendicular to the fast scan direction in a range of scan rates and probe temperatures. As expected, the size of the ripples increases massively in the vicinity of the glass temperature for each polymer. We also examine a different case in which the AFM tip follows a circular path. Contrary to the "steady" linear ripples we obtain circular ripples which rotate along the scanning path during consecutive scans. The group velocity of the circular ripples is 2 orders of magnitude lower than the scan speed. We interpret the experimental data using a phenomenological model accounting for erosion and smoothing effects caused by the probing tip.