Mesostructures of cobalt nanocrystals. 2. Mechanism

Mesoscopic patterns of cobalt nanocrystals produced by applying a magnetic field perpendicular to the substrate during the deposition process are presented. These mesoscopic patterns markedly differ with the size distribution of the cobalt nanocrystals. Well-defined columns are produced when the size distribution of cobalt nanocrystals is low; conversely, the coalescence of columns with formations of labyrinths occurs for a large size distribution. A formation mechanism of these structures is proposed.     

Experimental study of the near-field impact of an oyster table on the flow

Oyster farming structures are artificial obstacles which disturb tidal flow and wave propagation. These effects can induce modifications of erosion and sedimentation patterns, turbidity changes, local silting up and can be threatening for the shellfish farming itself. The understanding of the impact of these structures in terms of hydrodynamics and sediment dynamics in the far-field, i.e. at the scale of a bay, is a very challenging task.In order to investigate the far-field impact, it is very important to understand in the first place all the changes which occur at a smaller scale, i.e. at the scale of a single table for a farm consisting of oyster tables made of metallic wire structures on which porous bags of oysters are laid. This work is carried out through the idealized representation of the in-situ flow in a free surface flume tank. The flow characteristics around the overall structure are determined from velocity measurements obtained by laser velocimetry. The results highlight an asymmetric development of the boundary layers which suggest the existence of preferential areas for silting up and suspended matter fragmentation under the table.     

Design of a photostabilizer having built-in antioxidant functionality and its utility in obtaining broad-spectrum sunscreen formulations

Di-2,2'-diethylhexyl-3,5-dimethoxy-4-hydroxy-benzylidenemalonate (INCI name diethylhexyl syringylidene malonate, DESM), the target photostabilizer, was synthesized in one step by condensation of 3,5-dimethoxy-4-hydroxy benzaldehyde (Syringaldehyde) with di-2,2'-diethylhexyl malonate. Photostability data in sunscreen formulations showed that DESM is photostable and improves the photostability of avobenzone significantly when compared to control (without a photostabilizer). Photostable broad-spectrum sunscreen formulations with high SPF (>30) have been achieved by combining avobenzone, DESM and UV-B sunscreens, such as homosalate, octisalate or other UV-B sunscreens. It seems that (a) triplet-state energy transfer from avobenzone to DESM and (b) scavenging of reactive species are responsible for the observed stabilization of avobenzone. In vitro study of the two formulations containing DESM clearly showed critical wavelength of well over 370 nm and can thus be categorized as broad-spectrum sunscreens. DESM does not have any contribution to in vivo SPF; instead it boosts SPF by about 5 units in high-SPF products. DESM was found to be an excellent singlet-oxygen quencher, thereby reducing photodegradation of avobenzone caused by singlet oxygen. In short, the multiplicity of effects and formulation benefits seen with DESM makes it an ideal choice as a unique antioxidant photostabilizer for a variety of cosmetic products targeting young and mature skin alike.     

Quenching mechanism of Zn(salicylaldimine) by nitroaromatics

Nitroaromatics and nitroalkanes quench the fluorescence of Zn(Salophen) (H2Salophen = N,N'-phenylene-bis-(3,5-di- tert-butylsalicylideneimine); ZnL(R)) complexes. A structurally related family of ZnL(R) complexes (R = OMe, di-tBu, tBu, Cl, NO2) were prepared, and the mechanisms of fluorescence quenching by nitroaromatics were studied by a combined kinetics and spectroscopic approach. The fluorescent quantum yields for ZnL(R) were generally high (Phi approximately 0.3) with sub-nanosecond fluorescence lifetimes. The fluorescence of ZnL(R) was quenched by nitroaromatic compounds by a mixture of static and dynamic pathways, reflecting the ZnL(R) ligand bulk and reduction potential. Steady-state Stern-Volmer plots were curved for ZnL(R) with less-bulky substituents (R = OMe, NO2), suggesting that both static and dynamic pathways were important for quenching. Transient Stern-Volmer data indicated that the dynamic pathway dominated quenching for ZnL(R) with bulky substituents (R = tBu, DtBu). The quenching rate constants with varied nitroaromatics (ArNO2) followed the driving force dependence predicted for bimolecular electron transfer: ZnL* + ArNO2 --> ZnL(+) + ArNO2(-). A treatment of the diffusion-corrected quenching rates with Marcus theory yielded a modest reorganization energy (lambda = 25 kcal/mol), and a small self-exchange reorganization energy for ZnL*/ZnL(+) (ca. 20 kcal/mol) was estimated from the Marcus cross-relation, suggesting that metal phenoxyls may be robust biological redox cofactors. Electronic structure calculations indicated very small changes in bond distances for the ZnL --> ZnL(+) oxidation, suggesting that solvation was the dominant contributor to the observed reorganization energy. These mechanistic insights provide information that will be helpful to further develop ZnL(R) as sensors, as well as for potential photoinduced charge transfer chemistry.     

Finite Energy Scattering for the Lorentz–Maxwell Equation

In the case where the charge of the particle is small compared to its mass, we describe the asymptotics of the Lorentz–Maxwell equation (Abraham model) for any finite-energy data. As time goes to infinity, we prove that the speed of the particle converges to a certain limit, whereas the electromagnetic field can be decomposed into a soliton plus a free solution of the Maxwell equation. It is the first instance of a scattering result for general finite energy data in a field-particle equation. Submitted: October 31, 2007. Accepted: April 21, 2008.     

Strong solutions and weak-strong uniqueness for the nonhomogeneous Navier-Stokes system

This article is devoted to the study of the nonhomogeneous incompressible Navier-Stokes system in dimension d ≥ 3. We use new a priori estimates, which enable us to deal with low-regularity data and vanishing density. In particular, we prove new well-posedness results which improve the results of Danchin [6] by considering a less regular initial density, without a lower bound. Also, we obtain the first uniqueness criterion for weak solutions which is at the scaling of the equation.