Nanoscale structures and dynamics of a boundary liquid layer

Our long term scientific interest is the understanding of the interface properties of flowing liquids on a microscopic level. Various mechanisms have been introduced to explain the origin of slip at a solid-liquid interface like the formation of a thin depletion layer or a molecular ordering of the liquid near the interface. Reflectometry (using x-rays or neutrons) is a powerful technique to probe structures in this surface region. However, to date much less attention has been paid to the dynamical properties. In the first part of this paper we show that a different ordering of water exists next to a hydrophobic substrate in comparison to a hydrophilic interface. Furthermore, we find that shear has no effect on the depletion layer on hydrophobic substrates, while no depletion layer exists for hydrophilic surfaces. The second part of the paper addresses the dynamical properties of the boundary layer, and we present a new method which enables the observation of the diffusion dynamics of polymers next to a solid substrate. As a proof of concept, the dynamics of micelles next to the interface has been explored using grazing incidence neutron spin-echo spectroscopy. We were able to verify that investigation of the dynamics of the sample is feasible with this grazing incidence technique and we present data taken near the critical angle of total reflection. It appears that the diffusive motion of micelles at the hydrophobic (repulsive) interface is faster than at a hydrophilic interface or in the bulk. Furthermore, neutron spin-echo spectroscopy was extended to a first evaluation of the Doppler shift which occurs under flow.