X-ray transfocators: focusing devices based on compound refractive lenses.

This paper describes a tunable X-ray focusing apparatus, referred to as a transfocator, based on compound refractive lenses. By varying the number of lenses in the beam, the X-ray energy focused and the focal length can be varied continuously throughout a large range of energies and distances. The instrument can be used in both white and monochromatic beams to focus, pre-focus or collimate the beam. The transfocator can be used with other monochromators and/or other focusing elements, leading to significant increases in flux. Furthermore, the chromatic nature of the focusing means the transfocator suppresses harmonics and can also be used as an extremely high flux broad-band-pass monochromator. These devices have been installed in the first optics and second experimental hutches at the ID11 beamline at the ESRF.

Self-assembly of organic monolayers below the freezing threshold.

One measure that arctic fish and amphibians use to minimize damage to cellular membranes during cooling and freezing processes is the production of cryo-protective substances. We have mimicked this biological "trick" by using the surface of a cryo-protectant as a liquid subphase for the preparation of organic membranes. Following this innovative approach, quasi two-dimensional amphiphilic monolayers were cooled to -40 degrees C at a liquid/gas interface. To date, the low temperature region of the generic phase diagram for alkane chain molecules has been only "virtually" accessible by tuning the molecular chain length. By extending the temperature range well below the freezing point of water, we gained new insights into membrane stability, morphology, and reorganization at low temperatures. Upon cooling relaxed monolayers at a surface pressure of 4.5 mN/m, we find a transition from a mesophase with tilted chains at ambient temperature toward a crystalline phase with upright chains at low temperatures. Structure factor calculations reveal that the chain alignment in the crystalline phase differs from the classical herringbone configuration.

X-ray photon correlation spectroscopy under flow.

X-ray photon correlation spectroscopy was used to probe the diffusive dynamics of colloidal particles in a shear flow. Combining X-ray techniques with microfluidics is an experimental strategy that reduces the risk of X-ray-induced beam damage and also allows time-resolved studies of processes taking place in flow cells. The experimental results and theoretical predictions presented here show that in the low shear limit for a ;transverse flow' scattering geometry (scattering wavevector q perpendicular to the direction of flow) the measured relaxation times are independent of the flow rate and determined only by the diffusive motion of the particles. This is not generally valid and, in particular, for a 'longitudinal flow' (q parallel flow) scattering geometry the relaxation times are strongly affected by the flow-induced motion of the particles. The results here show that the Brownian diffusion of colloidal particles can be measured in a flowing sample and that, up to flux limitations, the experimental conditions under which this is possible are easier to achieve at higher values of q.