A new technique for fluid velocimetry based on near field scattering

We show that the time evolution of near-field scattering speckles, originated by a fluid suspension of particles, provides information about the velocity field in the fluid. This information can be extracted from a statistical analysis of speckle fields taken at different times, either by measuring their cross-correlation function or by recovering the power spectrum corresponding to the difference between the two speckle fields. Experimental data are in accordance to the expected behaviors. The results are independent of the scatterer's size, allowing one to exploit the technique also with sub-wavelength tracking particles.     

Sedimentary process inferences from Humic substances analysis and deposition rates (Western Ross Sea,Antarctica)

The first 40?cm of sediment of three basins in the Ross Sea were sampled using a box corer. Site Y1 was located close to the coast in Terra Nova Bay; the sediment of site Y3 was collected in a more distal basin in the central sector of the Ross Sea; finally site Y5 was sampled in the deepest zone of the Joides Basin.

Sediment cores were sliced and analysed with a depth resolution of 2–4.5?cm. The distribution of humic substances and their structural features along the cores were determined and related to the pattern of Total Organic Carbon (TOC) and sedimentological data. The grain size distribution and the ²¹⁰Pb inventories allow the sediment of the study sites to be characterised.

The humic substance content in the sediment decreases, with a change in slope between 23 and 26?cm at Y1, between 12 and 15?cm at Y3 and constant values with further depth. At Y5 the depth profile of humic substance content shows constant values in the upper 17?cm and values decrease with further depth. The pattern of humic substance yield is similar to that found for TOC. The analysis of the elemental composition of the humic acids extracted from different sediment depths shows an increasing C/N atomic ratio at sites Y1 and Y3 and constant values along the Y5 core. The depth profile of the C/N atomic ratio is confirmed by the changes observed in the structural characterisation and indicates a shift from the freshly deposited organic matter on the sediment surface to more humified material (humin). The results obtained highlight a different sedimentation rate at the three sites as deduced from sedimentological analysis.     

Study of the interaction between an optical probe and micelles of sodium deoxycholate

The interaction between aggregates of sodium deoxycholate and an optical probe, sensitive to the nature of the medium surrounding it, has been studied by circular dichroism and NMR measurements. The results indicate that the molecules of the probe are embedded in a polar medium and interact with the apolar face of sodium deoxycholate. These findings seem to be consistent with a structural model of the micelles different from that currently accepted.     

Transient oscillations in Soret-driven convection in a colloidal suspension

We present measurements of the transient stage of Soret-driven convective instability. The sample is a diluted colloidal suspension of silica spheres in water with an unusually large negative Soret coefficient S_T. A large temperature gradient (heating from above) is rapidly applied over the sample, while a shadowgraph imaging technique provides images of the convective flow. From the processing of the variance of the intensity of the images we are able to recover the time evolution of the overall intensity of the convective flow. A typical evolution of such signal exhibits, after a latency time, a peak followed by some damped oscillations leading to a steady-state value. Both the onset time _p (the temporal position of the first peak), and the oscillation period _osc show a power law dependence as a function of the solutal Rayleigh number R_s. The exponents found are compared with predictions from existing models.     

Dipolar interactions and origin of spin ice in ising pyrochlore magnets

Recent experiments suggest that the Ising pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 display qualitative properties of the nearest-neighbor "spin ice" model. We discuss the dipolar energy scale present in both these materials and discuss how spin-ice behavior can occur despite the presence of long-range dipolar interactions. We present results of numerical simulations and a mean field analysis of Ising pyrochlore systems. Based on our quantitative theory, we suggest that the spin-ice behavior in these systems is due to long-range dipolar interactions, and that the nearest-neighbor exchange in Dy2Ti2O7 is antiferromagnetic.     

Space intensity correlations in the near field of the scattered light: A direct measurement of the density correlation function g(r)

We show that the two point intensity correlation in the near field of the scattered light is directly related to the two point density correlation g(r). Preliminary measurements on two sets of calibrated random pinholes of 140 and 300 &mgr;m diameters, and on aqueous solutions of latex spheres of 5, 10, and 40 &mgr;m are reported. A discussion on the desirability of the technique as a simple and powerful alternative to low angle scattering will be presented.     

Hydrolytic and microbial degradation of multi-block polyurethanes based on poly(?-caprolactone)/poly(ethylene glycol) segments

Biodegradable amphiphilic multi-block poly(ether-ester-urethane)s were prepared by one-step bulk polycondensation of PEG and PCL macro-diols and HMDI. For biomedical or environmental applications of the proposed materials, one of the critical steps is the study of the degradation characteristics under physiological or environmental conditions. Different ratios of PEG/PCL and molecular weights of the resultant copolymers allowed for tuning their hydrophilicity, as evaluated by water uptake measurements. The swelling results were further supported by microgravimetric measurements performed by QCM-D. In vitro hydrolytic biodegradation was carried out under different conditions (i.e., in phosphate buffer solution - with and without Lipase). Total mineralization in the presence of microorganisms from a sample of river water was observed to follow different kinetics, depending upon the composition and the molecular weight of the copolymer.     

Metal-free, aerobic dioxygenation of alkenes using simple hydroxamic acid derivatives

The dioxygenation of alkenes using molecular oxygen and a simple hydroxamic acid derivative has been achieved. The reaction system consists of readily prepared methyl N-hydroxy-N-phenylcarbamate and molecular oxygen with a radical initiator, offering an alternative to common dioxygenation processes catalyzed by precious transition metals. This transformation capitalizes on the unique reactivity profile of hydroxamic acid derivatives in radical-mediated alkene addition processes.