New light on an old problem: The boundary of flowing liquids

Max Wolff (Uppsala University, Sweden)

The solid-liquid interface introduces anisotropy to a fluid and represents a singularity that may result in distinct properties as compared to the volume. For a static liquid this fact manifests in, for example, tuneable absorption for molecules at functionalized interfaces. For flowing liquids the discontinuity at the interface is quantified by a phenomenological number called slip length. To understand the microscopic origin of slip the knowledge on static samples has to be complemented by measurements under shear. Neutron scattering under grazing incident beam geometry will certainly play a major role in this context. In this presentation the possibilities offered by surface sensitive scattering techniques for the field are evaluate. In the first part slip in simple liquids will be related to the structure at the solid-liquid interface. For hexadecane the amount of slip measured by complementary techniques can not be explained by a depleted layer (two-fluid model). However, very first results from gracing incidence diffraction reveal an alignment of the molecules close to the interface that may explain slip. Additional inelastic scattering experiments will allow to incorporate the dynamics of the single molecules in the microscopic picture. First experiments done with triple axis under grazing incidence are very promising in this context. In the second part the focus will be on complex liquids. For a micellar system information over a large range of length scales is extracted from data collected under grazing incident beam geometry. The anisotropy introduced by the interface enforces a rearrangement of the micellar structure. It turns out that micelles prefer to grow epitaxial at an interface terminated attractively for the micelles corona. From diffuse scattering information on the lateral correlation length involved can be extracted. Micelles crystallizing at different germs start orienting but loose long range correlations with increasing overlap of the crystallites. This structural information is related to changes in the viscosity. For in situ measurements under shear load, we have combined the technique of neutron reflectivity with rheology. It turns out that shear is aligning the crystallites but decreases the long range correlations. After stopping the shear a slower relaxation of the crystalline structure is found close to the attractive interface and is additionally sensitive to the crystalline structure at the interface. This effect relates well to the molecular dynamics that were found faster in the vicinity of a repulsive interface.

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