Rotational friction at the molecular level

Abstract

Abstract. -We use second-harmonic generation measurements to follow the reorientation of liquid-crystal molecules in the first few molecular layers in contact with a surface. These measurements reveal four molecular relaxation times differing by orders of magnitude arising from different dissipation processes taking place at the interface with a solid. The slowest processes are due to the direct interaction of molecules with the solid, while the others concern molecules located just above this surface layer. This position-dependent dynamics explains some of the discrepancies previously observed in measurements of the surface reorientation dynamics of liquid crystals.

Reorientation dynamics in thin glassy films

“…The origin of the long relaxation times might be related to the fact that the non-polarisable sidegroups are polar mesogenic units. Ordinary non-glassy liquid crystals based on the same cyanobiphenyl units exhibit also a very slow reorientation dynamics in ultra-thin films (thickness below 10 nm) [48,52,53]. The reason for this slow dynamics is not clear.…”

Experimental testing of the Prandtl–Tomlinson model: Molecular origin of rotational friction

“…The rotation of a single molecule can be controlled by external stimuli, such as electrons, 41 temperature, 42 and mechanical stress. 43 By contrast, the dynamics of absorbing molecules can be probed collectively as surface viscosity 44 or solid friction torque. 45 However, previous research mainly focused on the molecule-substrate interaction, i.e., the term of corrugated surface potential in the PT model.…”