Rotational friction at the molecular level

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.