Neutron confinement cell for investigating complex fluids

Abstract

We describe an apparatus for measuring the molecular density and orientation of confined, ultrathin complex fluids under static and dynamic flow conditions. The device essentially couples the utility of the surface forces apparatus—ability to control surface separation and alignment under applied loads—with in situ structural characterization of the intervening material utilizing neutron reflectivity measurements. The apparatus is designed such that single crystal substrates of quartz or sapphire with areas up to tens of square centimeters can be kept parallel at controlled and well-defined separations from millimeters to less than 100 nm. The large substrate surface area enables direct structural measurements of the density profile of “soft” material placed between the aligned substrates. In addition, the cell is also designed to enable steady shear rates from 0.001 to 20 Hz to be applied in order to follow the dynamic structural response of the confined material, especially at the solid-solution interface. Faster shear rates of order 104 can be obtained using oscillatory motion. Current design specifications focus on the use of neutron reflectivity to characterize the structure of end-grafted polymer brush layers, but the device can be employed to probe the structure of any complex fluid of interest and is amenable to other characterization techniques.