Negative reflection of elastic guided waves in chaotic and random scattering media

The propagation of waves in complex media can be harnessed either by taming the incident wave-field impinging on the medium or by forcing waves along desired paths through its careful design. These two alternative strategies have given rise to fascinating concepts such as time reversal or negative refraction. Here, we show how these two processes are intimately linked through the negative reflection phenomenon. A negative reflecting mirror converts a wave of positive phase velocity into its negative counterpart and vice versa. In this article, we experimentally demonstrate this phenomenon with elastic waves in a 2D billiard and in a disordered plate by means of laser interferometry. Despite the complexity of such configurations, the negatively reflected wave field focuses back towards the initial source location, thereby mimicking a phase conjugation operation while being a fully passive process. The super-focusing capability of negative reflection is also highlighted in a monochromatic regime. The negative reflection phenomenon is not restricted to guided elastic waves since it can occur in zero-gap systems such as photonic crystals, chiral metamaterials or graphene. Negative reflection can thus become a tool of choice for the control of waves in all fields of wave physics.Comment: 9 pages, 6 figure

Negative refraction of Lamb modes: A theoretical study

This paper provides a theoretical investigation of negative refraction and focusing of elastic guided waves in a free-standing plate with a step-like thickness change. Under certain conditions, a positive phase velocity (forward) Lamb mode can be converted into a negative phase velocity (backward) mode at such interface, giving rise to negative refraction. A semi-analytical model is developed in order to study the influence of various parameters such as the material Poisson's coefficient, the step-like thickness, the frequency and the incidence angle. To this end, all the Lamb and shear horizontal propagating modes, but also a large number of their inhomogeneous and evanescent counterpart,s are taken into account. The boundary conditions applied to the stress-displacement fields at the thickness step yields an equation system. Its inversion provides the transmission and reflection coefficients between each mode at the interface. The step-like thickness and Poisson's ratio are shown to be key parameters to optimize the negative refraction process. In terms of material, Duralumin is found to be optimal as it leads to a nearly perfect conversion between forward and backward modes over broad frequency and angular ranges. An excellent focusing ability is thus predicted for a flat lens made of two symmetric thickness steps. Theoretical results are confirmed by a numerical FDTD simulation and experimental measurements made on an optimized Duralumin flat lens by means of laser interferometry. This theoretical study paves the way towards the optimization of elastic devices based on negative refraction, in particular for cloaking or super-focusing purposes.Comment: 8 pages, 6 figure

In vitro susceptibility of Actinobaculum schaalii to 12 antimicrobial agents and molecular analysis of fluoroquinolone resistance

Objectives To assess the in vitro susceptibility of Actinobaculum schaalii to 12 antimicrobial agents as well as to dissect the genetic basis of fluoroquinolone resistance. Methods Forty-eight human clinical isolates of A. schaalii collected in Switzerland and France were studied. Each isolate was identified by 16S rRNA sequencing. MICs of amoxicillin, ceftriaxone, gentamicin, vancomycin, clindamycin, linezolid, ciprofloxacin, levofloxacin, moxifloxacin, co-trimoxazole, nitrofurantoin and metronidazole were determined using the Etest method. Interpretation of results was made according to EUCAST clinical breakpoints. The quinolone-resistance-determining regions (QRDRs) of gyrA and parC genes were also identified and sequence analysis was performed for all 48 strains. Results All isolates were susceptible to amoxicillin, ceftriaxone, gentamicin, clindamycin (except three), vancomycin, linezolid and nitrofurantoin, whereas 100% and 85% were resistant to ciprofloxacin/metronidazole and co-trimoxazole, respectively. Greater than or equal to 90% of isolates were susceptible to the other tested fluoroquinolones, and only one strain was highly resistant to levofloxacin (MIC ≥32 mg/L) and moxifloxacin (MIC 8 mg/L). All isolates that were susceptible or low-level resistant to levofloxacin/moxifloxacin (n = 47) showed identical GyrA and ParC amino acid QRDR sequences. In contrast, the isolate exhibiting high-level resistance to levofloxacin and moxifloxacin possessed a unique mutation in GyrA, Ala83Val (Escherichia coli numbering), whereas no mutation was present in ParC. Conclusions When an infection caused by A. schaalii is suspected, there is a risk of clinical failure by treating with ciprofloxacin or co-trimoxazole, and β-lactams should be preferred. In addition, acquired resistance to fluoroquinolones more active against Gram-positive bacteria is possibl

The elusive task of biomarkers of renal injury

Neutrophil gelatinase-associated lipocalin (NGAL) is one of the most promising candidate biomarkers of renal injury, with expression in renal tissue increasing dramatically after ischemia-reperfusion injury but not in the case of pure pre-renal failure. In a recent issue of Critical Care, Di Somma and colleagues reported that NGAL could improve the classification of acute kidney injury compared with clinical assessment and showed that NGAL was associated with poor prognosis. NGAL may therefore carry different information than biomarkers of renal function. This study finally provides additional evidence for the highly complex relationship between renal function and renal injury

Expression of Nestin, Vimentin, and NCAM by Renal Interstitial Cells after Ischemic Tubular Injury

This work explores the distribution of various markers expressed by interstitial cells in rat kidneys after ischemic injury (35 minutes) during regeneration of S3 tubules of outer stripe of outer medulla (OSOM). Groups of experimental animals (n = 4) were sacrificed every two hours during the first 24 hours post-ischemia as well as 2, 3, 7, 14 days post-ischemia. The occurrence of lineage markers was analyzed on kidney sections by immunohistochemistry and morphometry during the process of tubular regeneration. In postischemic kidneys, interstitial cell proliferation, assessed by 5-bromo-2′-deoxyuridine (BrdU) and Proliferating Cell Nuclear Antigen (PCNA) labeling, was prominent in outer medulla and reach a maximum between 24 and 72 hours after reperfusion. This population was characterized by the coexpression of vimentin and nestin. The density of -Neural Cell Adhesion Molecule (NCAM) positive interstitial cells increased transiently (18–72 hours) in the vicinity of altered tubules. We have also localized a small population of α-Smooth Muscle Actin (SMA)-positive cells confined to chronically altered areas and characterized by a small proliferative index. In conclusion, we observed in the postischemic kidney a marked proliferation of interstitial cells that underwent transient phenotypical modifications. These interstitial cells could be implicated in processes leading to renal fibrosis

Systematic study of the impact of MOF densification into tablets on textural and mechanical properties

Four different metal-organic framework powders (UiO-66, UiO-66-NH, UiO-67, and HKUST-1) were shaped into tablets. The effect of the applied pressure on porous properties, mechanical resistance and tablet bulk density is reported. We observe a linear relationship between densification and tensile strength for all four studied MOFs, with the slope being MOF-dependent. We also report conditions for improving significantly the volumetric uptake. Finally, we evaluated our tablets' stability over time in the presence of moisture

OH and RO 2 radicals at Dome C (East Antarctica): first observations and assessment of photochemical budget

International audienceMeasurements of OH and total peroxy RO 2 (HO 2 + organic peroxy) radicals were performed in December 2011/January 2012 at the Dome C Concordia station (East Antarctica, 75.1˚S / 123.3˚E) in the frame of the Oxi-dant Production over Antarctic Land and its Export (OPALE) project. The goal of these first on the East Antarctica plateau radical measurements was to estimate the oxidative capacity and assess the role of snow emissions on the radical budget in this part of Antarctica. The OH concentration levels were found to be in general similar to those observed at South Pole. However, based on the analysis of the OH sources and sinks derived from the available measurements of NO x , HONO, HCHO, H 2 O 2 and others, it has been concluded that, in contrast to South Pole, the photolysis of HONO is the major OH source at Dome C site. The role of HONO as the major source of OH is also supported by an excellent correlation of OH with the production rate of OH from the HONO photolysis. The observed diurnal profiles of OH and RO 2 are discussed in relation with boundary dynamics and the variability of photolysis and snow emissions rates