Experimental testing of a prototype cantilevered liquid-nitrogen-cooled silicon mirror.

This report presents testing of a prototype cantilevered liquid-nitrogen-cooled silicon mirror. This mirror was designed to be the first mirror for the new soft X-ray beamlines to be built as part of the Advanced Light Source Upgrade. Test activities focused on fracture, heat transfer, modal response and distortion, and indicated that the mirror functions as intended.

Unexpected field-induced phase transitions between ferrielectric and antiferroelectric liquid crystal structures.

Liquid crystals are intriguing electrically responsive soft matter systems. We report previously unexplored field-induced changes in the structures of some frustrated liquid crystal phases and describe them theoretically. Specifically, we have discovered using resonant x-ray scattering that the four-layer intermediate smectic phase can undergo either a transition to the ferrielectric (three-layer) phase or to the ferroelectric phase, depending on temperature. Our studies of intermediate phases using electric fields offer a way to test theories that describe ferroelectricity in self-assembling fluids.

Deduction of the temperature-dependent structure of the four-layer intermediate smectic phase using resonant X-ray scattering.

A binary mixture of an antiferroelectric liquid-crystal material containing a selenium atom and a highly chiral dopant is investigated using resonant X-ray scattering. This mixture exhibits a remarkably wide four-layer intermediate smectic phase, the structure of which is investigated over a temperature range of 16K. Analysis of the resonant X-ray scattering data allows accurate measurement of both the helicoidal pitch and the distortion angle as a function of temperature. The former decreases rapidly as the SmC* phase is approached, whilst the latter remains constant over the temperature range studied at 8 degrees +/-3 degrees. We also observe that the senses of the helicoidal pitch and the unit cell of the repeating four-layer structure are opposite in this mixture and that there is no pitch inversion over the temperature range studied.

Templated biomineralization on self-assembled protein fibers.

Biological mineralization of tissues in living organisms relies on proteins that preferentially nucleate minerals and control their growth. This process is often referred to as "templating," but this term has become generic, denoting various proposed mineral-organic interactions including both chemical and structural affinities. Here, we present an approach using self-assembled networks of elastin and fibronectin fibers, similar to the extracellular matrix. When induced onto negatively charged sulfonated polystyrene surfaces, these proteins form fiber networks of approximately 10-mum spacing, leaving open regions of disorganized protein between them. We introduce an atomic force microscopy-based technique to measure the elastic modulus of both structured and disorganized protein before and during calcium carbonate mineralization. Mineral-induced thickening and stiffening of the protein fibers during early stages of mineralization is clearly demonstrated, well before discrete mineral crystals are large enough to image by atomic force microscopy. Calcium carbonate stiffens the protein fibers selectively without affecting the regions between them, emphasizing interactions between the mineral and the organized protein fibers. Late-stage observations by optical microscopy and secondary ion mass spectroscopy reveal that Ca is concentrated along the protein fibers and that crystals form preferentially on the fiber crossings. We demonstrate that organized versus unstructured proteins can be assembled mere nanometers apart and probed in identical environments, where mineralization is proved to require the structural organization imposed by fibrillogenesis of the extracellular matrix.

Surface roughness of supercooled polymer melts.

We report on in situ x-ray reflectivity measurements of the surface roughness of supercooled glass forming polymers in a temperature range from 190 to 330 K. The experimentally determined rms roughness has been found to obey the capillary wave model of a single liquid/vapor interface over the entire temperature range. An expression for the surface roughness below the bulk glass transition (T(G) approximately equal to 200 K) is deduced from the viscoelastic theory of surface fluctuations; however, no indication of a frozen-in surface roughness has been observed in the experiment. Additionally, it is shown that precise values of the surface tension of highly viscous liquids in the supercooled state can be determined by x-ray reflectivity.

Synchrotron x-ray scattering studies of water intercalation in a layered synthetic silicate.

Synchrotron x-ray diffraction studies were performed on a synthetic layered silicate Fluorohectorite clay. Diffraction patterns along the stacking direction were obtained in surface reflection and bulk transmission geometries on bulk pressed samples under controlled temperature and relative humidity. One-dimensional structure factors modeling the positions of the intercalant atoms have been obtained for three stable hydration states. From the narrow (00l) peak widths we conclude that well-crystallized domains consist of stacks of about 100 platelets, forming crystallites of the order of 0.1 microm thick. These crystallites have an orientational angular distribution of about 24 degrees around the stacking direction and represent the solid framework for microporosity in these samples.

Tetra point wetting at the free surface of liquid Ga-Bi.

A continuous surface wetting transition, pinned to a solid-liquid-liquid-vapor tetra coexistence point, is studied by x-ray reflectivity in liquid Ga-Bi binary alloys. The short-range surface potential is determined from the measured temperature evolution of the wetting film. The thermal fluctuations are shown to be insufficient to induce a noticeable breakdown of mean-field behavior, expected in short-range-interacting systems due to their d(u) = 3 upper critical dimensionality.

Orientational order in gravity dispersed clay colloids: a synchrotron x-ray scattering study of Na fluorohectorite suspensions.

Colloidal suspensions of clay particles in aqueous salt solutions make ideal model systems for the study of interactions between plate-shaped particles, due to the ease in tuning their electrostatic repulsion with the concentration of the salt. Numerous gel and sol structures are possible, including nematic liquid crystalline order, although only qualitative identification of the latter in clay colloids has been available so far. We present synchrotron x-ray diffraction from gravity dispersed solutions of Na fluorohectorite, a synthetic swelling clay, over a large NaCl concentration range. Our use of liquid scattering techniques allows us to identify regions in which particles reorient from horizontal to vertical alignments in strata coexisting at different heights within the sample. We identify two distinct gel regions characterized by differences in orientational anisotropy and domain size. Our results provide direct evidence for nematic order, as well as unique structural information regarding particle morphology and alignment within each of the colloid phases.

Pairing interactions and Gibbs adsorption at the liquid bi-in surface: a resonant X-ray reflectivity study.

Resonant x-ray reflectivity measurements from the surface of liquid Bi(22)In(78) find only a modest surface Bi enhancement, with 35 at. % Bi in the first atomic layer. This is in contrast to the Gibbs adsorption in all liquid alloys studied to date, which show surface segregation of a complete monolayer of the low surface tension component. This suggests that surface adsorption in Bi-In is dominated by attractive interactions that increase the number of Bi-In neighbors at the surface. These are the first measurements in which resonant x-ray scattering has been used to quantify compositional changes induced at a liquid alloy surface.

Positional order and thermal expansion of surface crystalline N-alkane monolayers.

We report a high-resolution synchrotron grazing incidence x-ray diffraction measurement of a surface crystalline monolayer at the liquid-vapor interface of the n-alkane eicosane (C20H42) just above its melting temperature. The peak width of the surface monolayer rotator phase is shown to be resolution limited and implies positional correlations of at least approximately 1 microm. The high resolution allowed determination of the temperature dependence of the peak position over the narrow (3 degrees C) temperature range of the surface crystal phase. The two-dimensional thermal expansion was determined to be (dA/dT)/A=1.8(+/-0.1)x10(-3) degrees C-1, which is comparable to the expansion in similar chain length bulk n-alkane rotator phases. Our data are consistent with the power-law shaped scattering tails expected from quasi-long-range order in two dimensions.

Microscopic structure of the wetting film at the surface of liquid Ga-Bi alloys

X-ray reflectivity measurements of the binary liquid Ga-Bi alloy reveal a dramatically different surface structure above and below the monotectic temperature T(mono) = 222 degrees C. A Gibbs-adsorbed Bi monolayer resides at the surface in both regimes. However, a 30 A thick, Bi-rich wetting film intrudes between the Bi monolayer and the Ga-rich bulk for T>T(mono). The wetting film's internal structure, not hitherto measured, is determined with A resolution, showing a concentration gradient not predicted by theory and a highly diffuse interface with the bulk phase.