Depletion at Solid-Liquid Interfaces under Shear: Flowing Hexadecane on Functionalized Surfaces

Abstract

We present a neutron reflectivity study on the interface of flowing hexadecane, which is known to exhibit surface slip on functionalized solid surfaces. The single crystalline silicon substrates are either chemically cleaned or covered by octadecyl-trichlorosilane (OTS) which result in different interfacial energies in respect to hexadecane. The liquid is in situ sheared and the reflectivities are compared to the static case. Moreover, the temperature dependence is recorded. For both types of interface, density depleted layers are observed, but shear affects them differently. Whereas at the attractive surface the depletion is enhanced by shear, the low interfacial energy leads to a diminishment of the depleted layer when applying shear. This systematic change of the depletion layer indicates different structural rearrangements under shear load. Finally, the possible link of the depletion layer with surface slip is discussed.