William A. Hamilton

Local membrane ordering of sponge phases at a solid-solution interface

We report a study of the ordering of the surfactant membranes of cetylpyridiniumchloride–hexanol in heavy brine sponge phase solutions in the proximity of a quartz surface by simultaneous neutron reflectometry (NR) and “near surface” small angle neutron scattering (NS-SANS) measurement in a reflection geometry sample cell. The NR results indicate layered surface ordering correlated with the solid–solution interface and decaying exponentially with depth over distances corresponding to a few membrane separations. The absolutely normalized NS-SANS results are consistent with conventional bulk SANS measurements, also indicating that the layered ordering established very near the surface does not constitute a phase of significant volume. We have compared this local surface ordering with the dilution behavior observed for sponge and lamellar phases in the bulk. At low membrane volume fraction the surface layering periodicity corresponds to the bulk sponge correlation peak, but approaches the smaller periodiciti...

Neutron confinement cell for investigating complex fluids

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 interfa...

Shear cell for the study of liquid-solid interfaces by neutron scattering

A cell for examining the density profile of sheared fluids at the solid‐liquid interface by neutron reflectometry is presented. This cell has also proven valuable in examining near‐surface bulk structures in the plane perpendicular to the shear flow using small angle neutron scattering. The shear rates can be controlled by changing the volume flow through the cell over three orders of magnitude. All components of the cell are designed to be chemically inert. A temperature‐controlled environment compatible with neutron studies is also briefly described. Preliminary neutron reflectivity and small angle neutron scattering results using this cell are presented, and potential applications are discussed.

“Over the horizon” SANS: Measurements on near-surface Poiseuille shear-induced ordering of dilute solutions of threadlike micelles

Abstract Although the behavior of a fluid under shear near a surface can be expected to be critically important to its drag and lubrication properties, most shear measurements to date have been of the bulk. This paper outlines the use of a specially developed Poiseuille shear cell at grazing incidence to measure the small-angle neutron scattering (SANS) signal from the first few tens of microns in the interfacial region. We illustrate the technique with measurements made on the near-surface ordering in flow past a quartz surface of dilute surfactant solutions comprising highly extended self-assembling “thread-like” micelles.

Neutron Reflectometry as Optical Imaging

Abstract In this paper we show how a specularly reflecting mirror sample at grazing incidence may be treated as a simple imaging element, akin to a slit camera. An analysis of the imaging properties leads to a new and efficient means of measuring neutron reflectivity. The resolution of a reflectivity curve obtained by this scheme depends only on the geometry of the sample-to-detector arrangement, and is largely independent of incident beam collimation. In the absence of significant off-specular scattering, this method of data reduction allows a neutron reflectometer equipped with a linear position sensitive detector to use the higher intensity of a loosely collimated beam while achieving excellent resolution. To illustrate the principle, we present raw and reduced reflection data taken on the first runs of the newly commissioned neutron reflectometer at the High-Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (OW).

A vapor barrier Couette shear cell for small angle neutron scattering measurements

We describe the design and operation of a temperature controlled Couette shear cell for small angle neutron scattering (SANS) studies of complex fluids under flow. This design incorporates a vapor barrier, which prevents sample evaporation to relatively high shear rates. This cell enables the investigation of systems which are highly sensitive to evaporation. Over the duration of a Couette SANS measurement composition phase transitions due to evaporation can be misinterpreted as true shear-induced transformations. We give a brief report of recent experiments performed on one such system: bicontinuous L3 sponge phases for which the surfactant membrane constituents are Cetylpyridinium chloride and hexanol. These clearly demonstrate the limitations of previous designs and the utility of the vapor barrier in measurements of a predicted shear induced sponge to lamellar phase transition. Using this cell we also describe and test a simple and effective way to put SANS data taken in the tangential Couette configu...

Superconducting magnetic Wollaston prism for neutron spin encoding

A magnetic Wollaston prism can spatially split a polarized neutron beam into two beams with different neutron spin states, in a manner analogous to an optical Wollaston prism. Such a Wollaston prism can be used to encode the trajectory of neutrons into the Larmor phase associated with their spin degree of freedom. This encoding can be used for neutron phase-contrast radiography and in spin echo scattering angle measurement (SESAME). In this paper, we show that magnetic Wollaston prisms with highly uniform magnetic fields and low Larmor phase aberration can be constructed to preserve neutron polarization using high temperature superconducting (HTS) materials. The Meissner effect of HTS films is used to confine magnetic fields produced electromagnetically by current-carrying HTS tape wound on suitably shaped soft iron pole pieces. The device is cooled to ~30 K by a closed cycle refrigerator, eliminating the need to replenish liquid cryogens and greatly simplifying operation and maintenance. A HTS film ensures that the magnetic field transition within the prism is sharp, well-defined, and planar due to the Meissner effect. The spin transport efficiency across the device was measured to be ~98.5% independent of neutron wavelength and energizing current. The position-dependent Larmor phase of neutron spins was measured at the NIST Center for Neutron Research facility and found to agree well with detailed simulations. The phase varies linearly with horizontal position, as required, and the neutron beam shows little depolarization. Consequently, the device has advantages over existing devices with similar functionality and provides the capability for a large neutron beam (20 mm × 30 mm) and an increase in length scales accessible to SESAME to beyond 10 μm. With further improvements of the external coupling guide field in the prototype device, a larger neutron beam could be employed.

Spin echo modulated small-angle neutron scattering using superconducting magnetic Wollaston prisms

The spin echo modulated small-angle neutron scattering technique has been implemented using two superconducting magnetic Wollaston prisms at a reactor neutron source. The density autocorrelation function measured for a test sample of colloidal silica in a suspension agrees with that obtained previously by other neutron scattering methods on an identically prepared sample. The reported apparatus has a number of advantages over competing technologies: it should allow larger length scales (up to several micrometres) to be probed; it has very small parasitic neutron scattering and attenuation; the magnetic fields within the device are highly uniform; and the neutron spin transport across the device boundaries is very efficient. To understand quantitatively the results of the reported experiment and to guide future instrument development, Monte Carlo simulations are presented, in which the evolution of the neutron polarization through the apparatus is based on magnetic field integrals obtained from finite-element simulations of the various magnetic components. The Monte Carlo simulations indicate that the polarization losses observed in the experiments are a result of instrumental artifacts that can be easily corrected in future experiments.

Application of small-angle neutron scattering to the study of forces between magnetically chained monodisperse ferrofluid emulsion droplets

The optical magnetic chaining technique (MCT) developed by Leal-Calderon, Stora, Mondain-Monval, Poulin & Bibette [Phys. Rev. Lett. (1994), 72, 2959–2962] allows precise measurements of force profiles between droplets in monodisperse ferrofluid emulsions. However, the method lacks an in situ determination of droplet size and, therefore, requires a combination of separately acquired measurements of droplet chain periodicity versus an applied magnetic field from optical Bragg scattering and droplet diameter inferred from dynamic light scattering (DLS) to recover surface force–distance profiles between the colloidal particles. Compound refractive lens (CRL) focused small-angle neutron scattering (SANS) MCT should result in more consistent measurements of droplet size (form factor measurements in the absence of field) and droplet chaining period (from structure factor peaks when the magnetic field is applied), and, with access to shorter length scales, extend force measurements to closer approaches than possible by optical measurements. This article reports on CRL-SANS measurements of monodisperse ferrofluid emulsion droplets aligned in straight chains by an applied field perpendicular to the incident beam direction. Analysis of the scattering from the closely spaced droplets required algorithms that carefully treated resolution and its effect on mean scattering vector magnitudes in order to determine droplet size and chain periods to sufficient accuracy. At lower applied fields, scattering patterns indicate structural correlations transverse to the magnetic field direction owing to the formation of intermediate structures in early chain growth.

Comparison of critical adsorption scaling functions obtained from neutron reflectometry and ellipsometry.

Carpenter et al. [Phys. Rev. E 59, 5655 (1999); 61, 532 (2000)] managed to explain ellipsometric critical adsorption data collected from the liquid-vapor interface of four different critical binary liquid mixtures near their demixing critical temperature using a single model. This was the first time a single universal function had been found which could quantitatively describe the surface critical behavior of many different mixtures. There have also been various attempts to investigate this surface critical behavior using neutron and x-ray reflectometries. Results have been mixed and have often been at variance with Carpenter et al. In this paper, the authors show that neutron reflectometry data collected from a crystalline quartz-critical mixture interface, specifically deuterated water plus 3-methylpyridine, can be quantitatively explained using the model of Carpenter et al. derived from ellipsometric data.