A Raman spectroscopic and combined analytical approach to the restoration of severely damaged frescoes: the Palomino project

The deterioration of art objects is normally relatively minor, controllable and attributable to environmental changes or bacterial invasion, and until now there has not been any recorded attempt to analyse an artwork that has been deliberately and significantly destroyed. The analytical problems are correspondingly larger but the potential reward from any information that can be forthcoming is thereby proportionately greater. The 17th Century Palomino frescoes on the vaulted ceiling of the Church of Sant Joan del Mercat in Valencia were largely destroyed by insurgents in the Spanish Civil War in 1936. The ensuing gunfire and a series of seven conflagrations inside the church had a devastating effect upon the artwork, and the surviving areas were also rendered unstable with respect to their detachment from the substrate. During the current restoration project being undertaken on these frescoes, an opportunity was provided for the application of several analytical techniques to secure information about the original pigment palette employed, the technology of application used by Palomino and the changes consequent upon the destruction process. Here, we report for the first time the use of analytical Raman spectroscopy, supported by scanning electron microscopy (SEM) and voltammetry of microparticles, for the combined identification of pigments, binders, substrate treatments and pigment alteration in an important, although badly damaged, wall painting for the informing of the ongoing conservation and restoration strategy. Copyright © 2008 John Wiley & Sons, Ltd.     

Multidimensional upwind convection schemes for flow in porous media on structured and unstructured quadrilateral grids

Standard reservoir simulation schemes employ first order upwind schemes for approximation of the convective fluxes when multiple phases or components are present. These convective flux schemes rely upon upwind information that is determined according to grid geometry. As a consequence directional diffusion is introduced into the solution that is grid dependent. The effect can be particularly important for cases where the flow is across grid coordinate lines and is known as cross-wind diffusion.Truly higher dimensional upwind schemes that minimize cross-wind diffusion are presented for convective flow approximation on quadrilateral unstructured grids. The schemes are locally conservative and yield improved results that are essentially free of spurious oscillations. The higher dimensional schemes are coupled with full tensor Darcy flux approximations.The benefits of the resulting schemes are demonstrated for classical test problems in reservoir simulation including cases with full tensor permeability fields. The test cases involve a range of structured and unstructured grids with variations in orientation and permeability that lead to flow fields that are poorly resolved by standard simulation methods. The higher dimensional formulations are shown to effectively reduce the numerical cross-wind diffusion effect, leading to improved resolution of concentration and saturation fronts.     

Flux compactifications of string theory on twisted tori

Global aspects of Scherk‐Schwarz dimensional reduction are discussed and it is shown that it can usually be viewed as arising from a compactification on the compact space obtained by identifying a (possibly non‐compact) group manifold 𝒢 under a discrete subgroup Γ, followed by a truncation. This allows a generalisation of Scherk‐Schwarz reductions to string theory or M‐theory as compactifications on 𝒢/Γ, but only in those cases in which there is a suitable discrete subgroup of 𝒢. We analyse such compactifications with flux and investigate the gauge symmetry and its spontaneous breaking. We discuss the covariance under O(d,d), where d is the dimension of the group 𝒢, and the relation to reductions with duality twists. The compactified theories promote a subgroup of the O(d,d) that would arise from a toroidal reduction to a gauge symmetry, and we discuss the interplay between the gauge symmetry and the O(d,d,ℤ T‐duality group, suggesting the role that T‐duality should play in such compactifications.     

Plasma-based X-ray laser at 21 nm for multidisciplinary applications

An overview of recent advances in applications of currently the most energetic X-ray laser at 21 nm is given. The unique parameters of this half-cavity based X-ray laser such as record output energy of 10 mJ, highly symmetric beam, robustness and reproducibility, have made it possible to carry out a number of multidisciplinary scientific projects featuring novel applications of intense coherent X-ray radiation. Selected results obtained in these experiments are reviewed, including X-ray laser probing of dense plasmas, measurements of transmission of focused soft X-ray radiation at intensities of up to 10¹² W cm^-2, measurements of infrared laser ablation rates of thin foils, and ablative microstructuring of solids.     

Mechanisms for soft‐tissue ablation and the development of alternative medical lasers based on investigations with mid‐infrared free‐electron lasers

Experimental evidence indicating the potential biomedical advantages of using a Mark‐III Free‐Electron Laser (FEL) for the ablation of soft tissue were first reported in 1994. Research progress since that time is reviewed, including: 1) successful human surgery using the Mark‐III FEL; 2) advances in understanding the physical mechanism for infrared tissue ablation and how these mechanistic features correlate with the preferential ablative properties; 3) the pursuit of table‐top, nanosecond‐pulsed laser technology that mimics the preferential ablation properties of the Mark‐III FEL with the aim of improving clinical acceptance of mid‐infrared laser ablation of soft tissue; and 4) current research challenges.     

Quantifying the Initial Unfolding of Bacteriorhodopsin Reveals Retinal Stabilization

The forces that stabilize membrane proteins remain elusive to precise quantification. Particularly important, but poorly resolved, are the forces present during the initial unfolding of a membrane protein, where the most native set of interactions is present. A high‐precision, atomic force microscopy assay was developed to study the initial unfolding of bacteriorhodopsin. A rapid near‐equilibrium folding between the first three unfolding states was discovered, the two transitions corresponded to the unfolding of five and three amino acids, respectively, when using a cantilever optimized for 2 μs resolution. The third of these states was retinal‐stabilized and previously undetected, despite being the most mechanically stable state in the whole unfolding pathway, supporting 150 pN for more than 1 min. This ability to measure the dynamics of the initial unfolding of bacteriorhodopsin provides a platform for quantifying the energetics of membrane proteins under native‐like conditions.     

Investigation of an Unusual Crystal Habit of Hydrochlorothiazide Reveals Large Polar Enantiopure Domains and a Possible Crystal Nucleation Mechanism

The observation of an unusual crystal habit in the common diuretic drug hydrochlorothiazide (HCT), and identification of its subtle conformational chirality, has stimulated a detailed investigation of its crystalline forms. Enantiomeric conformers of HCT resolve into an unusual structure of conjoined enantiomorphic twin crystals comprising enantiopure domains of opposite chirality. The purity of the domains and the chiral molecular conformation are confirmed by spatially revolved synchrotron micro‐XRD experiments and neutron diffraction, respectively. Macroscopic inversion twin symmetry observed between the crystal wings suggests a pseudoracemic structure that is not a solid solution or a layered crystal structure, but an unusual structural variant of conglomerates and racemic twins. Computed interaction energies for molecular pairs in the racemic and enantiopure polymorphs of HCT, and the observation of large opposing unit‐cell dipole moments for the enantiopure domains in these twin crystals, suggest a plausible crystal nucleation mechanism for this unusual crystal habit.     

Coupling mechanism of an EMAT

Electro-magnetic acoustic transducers (EMATs) generate ultrasonic waves in metals through an electromagnetic coupling mechanism. A concept for EMAT generation, using a coil alone without a permanent magnet, but with a pulse generator and a sample, is introduced. A simplified equivalent coil circuit is given, and has been validated by experimental measurements. Such an equivalent circuit is useful for excitation current calculations, which have often been neglected in previous publications in this area but have proved to be of great importance in considering the efficiency and frequency characteristics of ultrasonic generation. Due to the coil sample coupling, the equivalent coil inductance is dependent on the electrical conductivity and magnetic permeability of the metal sample, the lift-off, the coil orientation relative to the metal sample and the coil configuration. The use of a coil alone to generate ultrasound has limited applications as a suitable ultrasonic detector must also be used, but where appropriate this type of generation source provides a robust, non-contact and inexpensive means of ultrasonic generation without worrying the permanent magnet attracting ferromagnetic particles that can prevent from practical on-line application of EMATs.     

A PL and PLE Study of High Cu Content Cu2ZnSnSe4 Films on Mo/Glass and Solar Cells

Physics of the Solid State - Cu2ZnSnSe4 (CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and...     

Effects of Pressure and pH on the Physical Stability of an I-Motif DNA Structure

The thermodynamic stability of a cytosine(C)-rich i-motif tract of DNA, which features pH-sensitive [C..H..C]⁺ moieties, has been studied as function of both pressure (0.1–200 MPa) and pH (3.7–6.2). Careful attention was paid to correcting citrate buffer pH for known variations that stem from changes in pressure. Once pH-corrected, (i) at pH >4.6 the i-motif becomes less stable as pressure is increased (K_D decreases), giving a small negative volume change for dissociation (Δ_DV°) of the i-motif – a conclusion opposite to that which would be drawn if the buffer pH was not corrected for the effects of pressure; (ii) the i-motif's melting temperature increases by more than 30 K between pH 6.5 and 4.5, the consequence of an enthalpy for dissociation (Δ_DH°) of 77(3) and 90(3) kJ (mol H⁺)⁻¹ at 0.1 and 200 MPa, respectively; (iii) below pH 4.6 at 0.1 MPa (pH 4.3 at 200 MPa) the melting temperature decreases as a result of double protonation of cytosine pairs, and Δ_DH° and Δ_DV° change signs; and (iv) the combination of Δ_DH° and Δ_DV° lead to the melting temperature at pH 4.3 being 3 K higher at 200 MPa than at 0.1 MPa.