Near-Surface Structure and Dynamics Explored by Grazing Incidence
Neutron Scattering
Marco Walz, University of
Erlangen-Nurrnberg, Germany
Even in simple fluids the conventional non-slip boundary condition
becomes microscopically void. Surface slip characterized by the slip
length may become macroscopic, and a large slip length implies a
pronounced anomaly in the shear flow adjacent to a solid surface,
where the anomaly itself is only present in a thin layer next to the
solid interface within the order of nanometers. For an understanding
of boundary slip, the anomalies in the structural and dynamical
properties in the interface layer need to be understood.
Neutron grazing incidence diffraction techniques can be tuned to
become surface sensitive and are thus ideally suited for this
question. The high contrast of neutrons on hydrocarbon chains combined
with a surface sensitive setup make neutron scattering techniques to a
particularly suitable method for the investigation of the near-
interface polymer structure and dynamics. To highlight the properties
of the boundary layer we carried out Grazing Incidence Small Angle
Neutron Scattering (GISANS) and, for the first time, a Neutron Spin-
Echo experiment under condition of Grazing Incidence (GINSE).
Our investigation of a concentrated tri-block copolymer solution shows
that the local structure depends on the distance to the interface and
the chemical termination of the solid boundary. However, the key for
the understanding of slip may also be related to a change in the local
dynamics of a liquid at an interface and under flow. GINSE appears to
be the appropriate technique to access the relevant internal
relaxation next to an interface. We have verified that the
investigation of the dynamics of the sample with the GINSE technique
is well feasible, and we present first data taken near the critical
angle of total reflection. It appears that the diffusive motion of
micelles at the hydrophilic (attractive) interface is reduced in
comparison to a hydrophobic (repulsive) interface.
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