Nanoscale friction mechanisms at solid-liquid interfaces.

1 IMETUM und Physik Department, TUM, Boltzmannstrasse 11, 85748 Garching. 2 Ernst-Berl Institut für Makromolekulare Chemie, TU Darmstadt, Petersenstrasse 22, 64287 Darmstadt, Germany. Fig. 1: Observed nanoscale friction mechanisms. A single polymer molecule at a solidliquid interface can respond in different ways to a lateral external force, here exerted by an AFM cantilever tip (depicted in yellow). (a) Previously theoretically described slip occurs when the polymer has a very high mobility and undetectably low friction. In this case, the vertical force (Fz) versus lateral extension (x) plot shows a plateau of constant force, before the polymer detaches from the surface and Fz becomes zero. (b) The most frequent motif is desorption stick. It is characteristically a decrease in the force Fz with increasing lateral pulling extension x. Here the static friction coefficient is high enough to favor desorption over slipping. While the polymer does not move laterally, it leaves the surface continuously with very low internal friction, that is, the polymer is confined in two dimensions. (c) In several cases the polymer is stuck to the surface owing to strong directional bonds (indicated as orange sticks); that is, confined in three dimensions (cooperative stick). A lateral external force elastically stretches the polymer. In the images, one sphere corresponds to at least 100 monomers [1]. Nanoscale Friction Mechanisms at Solid–Liquid Interfaces