Scott Williams

Thermal neutron imaging is a powerful non-invasive diagnostic technique to study water management within a proton exchange membrane (PEM) fuel cell. To isolate the hydration behavior of membrane, significant increase in detecting thin water films is needed. A humidity cell is developed to study hydration of a PEM using cold neutron imaging. Spatial and temporal changes of PEM water uptake are quantified. Water film as small as 1 μm thick and corresponding to a volume containing 10 ng of liquid water at high spatial resolution is detected. This represents an order of magnitude improvement in water detection efficiency achievable at thermal neutron imaging facilities.

Cold neutrons have a wavelength that is in the same range as the lattice spacings of most polycrystalline metallic materials. Imaging of such materials with monochromatic cold neutrons of different wavelengths provides a unique contrast due to coherent Bragg scattering. Additionally, the spectral positions of the Bragg edges can be mapped for each point of an image by using the transmission data of corresponding wavelength scans. We present investigations of welded components with such energy-selective neutron radiography around specific Bragg-edges in the transmission spectrum. Features in the local microstructure of the weld have been visualized.

The problem of interface pattern selection in nonlinear dissipative systems is critical in many fields of science, occurring in physical, chemical and biological systems. One of the simplest pattern formations is the Saffman–Taylor finger pattern¹ that forms when a viscous fluid is displaced by a less viscous fluid. Such finger-shaped patterns have been observed in distinctly different fields of science^2,3,4 (hydrodynamics, combustion and crystal growth) and this has led to a search for a unified concept of pattern formation, as first proposed by the classic work of D'arcy Thomson⁵. Two-dimensional finger-shaped patterns, observed in flame fronts⁶ and the ensembled average shape of the diffusion-limited aggregation pattern, have been shown to be similar to Saffman–Taylor finger shapes^7,8. Here we present experimental studies that establish that the cell shapes formed during directional solidification of alloys can be described by the form of the Saffman–Taylor finger shape equation when a second phase is present in the intercellular region.