Neutron confinement cell for investigating complex fluids

We describe an apparatus for measuring the molecular density and orientatio n of confined, ultrathin complex fluids under static and dynamic flow condi tions. The device essentially couples the utility of the surface forces app aratus-ability to control surface separation and alignment under applied lo ads-with in situ structural characterization of the intervening material ut ilizing neutron reflectivity measurements. The apparatus is designed such t hat single crystal substrates of quartz or sapphire with areas up to tens o f square centimeters can be kept parallel at controlled and well-defined se parations from millimeters to less than 100 nm. The large substrate surface area enables direct structural measurements of the density profile of "sof t" 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 appli ed in order to follow the dynamic structural response of the confined mater ial, especially at the solid-solution interface. Faster shear rates of orde r 10(4) can be obtained using oscillatory motion. Current design specificat ions focus on the use of neutron reflectivity to characterize the structure of end-grafted polymer brush layers, but the device can be employed to pro be the structure of any complex fluid of interest and is amenable to other characterization techniques. (C) 2001 American Institute of Physics.