John B. Hayter

Neutron scattering from charge stabilized suspensions undergoing shear

Small angle neutron scattering(SANS) is used to investigate the local order in aqueous, charge stabilized suspensions of 103 A diameter polystyrene latexspheres at ∼14 wt. % solids. These samples which evidence a close packed close packed structure in equilibrium are observed to undergo a transition to an amorphous order when sustaining a sufficiently large and steady shear. The shear melting phenomonology is different from that observed by light scattering for very dilute (0.16 wt. % solids) suspensions of polystyrene particles which form bcc lattices in equilibrium. Furthermore, neutron scattering reveals more detail than light scattering for concentrated suspensions and indicates a larger degree of three‐dimensional ordering under steady shear flow than is implied by sliding layer models for similar systems.

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.

On the structure of liquid alkali metals

The authors calculate a structure factor for the liquid alkali metals assuming that the soft core of the potential plays the dominant structure-determining role. The structure factor is calculated in an analytic form suitable for fitting to experimental data. They obtain good agreement with existing measurements on Na, K and Rb.

Neutron optics at the Advanced Neutron Source (Invited Paper)

The Advanced Neutron Source (ANS), currently in conceptual design at Oak Ridge National Laboratory, will be a multipurpose neutron research laboratory serving the needs of academic and industrial users throughout the nation in chemistry, biology, physics, materials science, and many other fields. It will be centered on the worlds highest flux neutron beam reactor, but that is only the starting point for ensuring high productivity, which will be enhanced further by careful design of the neutron beam delivery systems. Many instruments will be situated in low backgrounds at distances up to 80 m from the reactor, using neutron guides with tailored neutron optical coatings for beam transport. Where appropriate, multilayer optical coatings will also be used in beam focussing (and sometimes defocussing) devices. Certain applications, such as prompt-(gamma) activation analysis, may also use neutron fiber optics to split beams between multiple samples. Neutron optical techniques such as interferometry and reflectometry will form a basic part of the research instrument complement at the ANS. Finally, very long wavelength neutron beams, in which the optical potential becomes very large, will be provided for specialized research.

Transmission optical device to produce intense polarized neutron beams

It is shown how a device can be constructed using a stack of magnetic supermirrors to produce high‐intensity polarized neutron beams with a large area. The device makes use of the fact that single‐crystal Si is transparent to neutrons. A prototypical device has been constructed which provides 64% neutron efficiency with 20 min collimation.

Use of complementary neutron scattering techniques in studying the effect of a solid/liquid interface on bulk solution structures

By appropriate combination of neutron scattering techniques, it is possible to obtain structural information at various depths from a solid/liquid interface and thus probe in some detail how the surface structures evolve into bulk structures. We have used neutron reflectometry (NR) with a newly developed shear cell, near-surface small-angle neutron scattering (NSSANS) again in combination with the new shear cell, and regular small-angle neutron scattering (SANS) with a standard Couette shear cell to probe the structures formed in our aqueous surfactant systems and how they react to a flow field, particularly in the near-surface region of a solid/liquid interface. We present data for a 20 × 10–3M aqueous solution of 70% cetyltrimethyl-ammonium 3,5-dichlorobenzoate (CTA3,5ClBz) and 30% CTAB. This system forms a very viscoelastic solution containing long thread-like micelles. NR only probes to a depth of ca. 0.5 µm from the surface in these systems and clearly indicates that adsorbed onto the surface is a surfactant layer which is insensitive to shear. The depth probed by the NSSANS is of the order of 20–30 µm and is determined by the transmission of the sample, the angle of incidence and the wavelength. In this region, the rods align under shear into a remarkably well ordered hexagonal crystal. The SANS from the Couette cell averages over the entire sample, so that the signal is dominated by scattering from the bulk. While the near-surface hexagonal structure is clearly visible, these data are not consistent with the crystal structure persisting throughout the bulk, leading to the postulate that the bulk structure is a 2D liquid where the rods align with the flow, but do not order in the other two dimensions.

Role of fractal structure on thin-film processing of YBa2Cu3O7 –x using alkoxide sols

An alkoxide sol–gel route has been developed for producing YBa2Cu3O7 –x superconducting powder and oriented thin films. CuII methoxyethoxide, Y methoxyethoxide and Ba methoxyethoxide precursors were synthesized and processed into stable sols with methoxyethanol (MOE), methyl ethyl ketone (MEK), toluene and diisopropyl ketone (DIK) solvents. Pyridine (C5H5N), lutidine (C13H21N) and toluene-2,4-disulphonic acid were used as organic additives to modify the polymeric network structure and corresponding sol behaviour. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were used to study the structural modifications in sols and gels with changes in solution chemistry parameters, viz. acidity/basicity, moisture exposure and solvent system. Mass-fractal dimensions in the range 2.0–2.5 were obtained for 0.01–0.1 mol kg–1 YBa2Cu3 sols in methoxyethanol–methyl ethyl ketone–toluene solvent. Significant effects of two organic base additives, pyridine and lutidine, on the sol structure were observed as a function of sol concentration. The ability of the different solutions to cover SrTiO3, ZrO2, sapphire, SiO2 and silicon substrates was measured by contact-angle measurements. Thin films produced via spin casting, dip coating and spray coating were characterized using optical, scanning electron microscopy (SEM) and X-ray diffraction techniques, and the results are discussed in terms of the sol structures.

Ultracold neutron turbine for the advanced neutron source

Abstract The paper describes a design for an ultracold neutron turbine source which has been developed for the planned Advanced Neutron Source at Oak Ridge. An “S”-shaped neutron guide transports very cold neutrons from a horizontal Cold Source to the turbine location. For the design neutron velocity of 40 m/s, this guide approaches the efficiency of a straight guide, while it filters strongly against neutrons with velocities above 60 m/s. The proposed turbine is designed for operation at a peripheral velocity of 20 m/s. This design speed, slower than that of existing turbines, has been chosen to minimize the effect of the circular turbine motion. The lower speed, coupled with the use of 58 Ni as the turbine blade material, allows a significant increase in performance with fewer blades.

A theory of correlations in a magnetically aligned ferrofluid

Abstract We have obtained an analytic form for the structure factor of a colloidal ferrofluid in which the particle moments are completely aligned by an applied magnetic field. The results obtained show the particle chaining first proposed by de Gennes and Pincus.