Droplet Conductivity Strongly Influences Bump and Crater Formation on Electrodes during Charge Transfer

Aqueous droplets acquire charge when they contact electrodes in high voltage electric fields, but the exact mechanism of charge transfer is not understood. Recent work by Elton et al. revealed that electrodes are physically pitted during charge transfer with aqueous droplets. The pits are believed to result when a dielectric breakdown arc occurs as a droplet approaches the electrode and the associated high current density transiently locally melts the electrode, leaving distinct crater-like deformations on the electrode surface. Here we show that the droplet conductivity strongly modulates the pitting morphology but has little effect on the amount of charge transferred. Electron and atomic force microscopy shows that deionized water droplets yield no observable deformations, but as the salt concentration in the droplet increases above 10^–3 M, the deformations become increasingly large. The observed intensity of the flash of light released during the dielectric breakdown arc also increases with droplet conductivity. Surprisingly, despite the large difference in pitting morphology and corresponding arc intensity, droplets of any conductivity acquire similar amounts of charge. These results suggest that the energy transferred during dielectric breakdown is primarily responsible for electrode pitting rather than the total amount of energy released during charge transfer

Droplet Conductivity Strongly Influences Bump and Crater Formation on Electrodes during Charge Transfer

Aqueous droplets acquire charge when they contact electrodes in high voltage electric fields, but the exact mechanism of charge transfer is not understood. Recent work by Elton et al. revealed that electrodes are physically pitted during charge transfer with aqueous droplets. The pits are believed to result when a dielectric breakdown arc occurs as a droplet approaches the electrode and the associated high current density transiently locally melts the electrode, leaving distinct crater-like deformations on the electrode surface. Here we show that the droplet conductivity strongly modulates the pitting morphology but has little effect on the amount of charge transferred. Electron and atomic force microscopy shows that deionized water droplets yield no observable deformations, but as the salt concentration in the droplet increases above 10^–3 M, the deformations become increasingly large. The observed intensity of the flash of light released during the dielectric breakdown arc also increases with droplet conductivity. Surprisingly, despite the large difference in pitting morphology and corresponding arc intensity, droplets of any conductivity acquire similar amounts of charge. These results suggest that the energy transferred during dielectric breakdown is primarily responsible for electrode pitting rather than the total amount of energy released during charge transfer

Candidoses invasives en réanimation : données épidémiologiques, élaboration d’un score prédictif et mise au point de PCR pour le diagnostic

Patients in intensive care units (ICU) are at very high risk of invasive candidiasis associated with high mortality rate. Candida species are the third cause of septicemia. Clinical signs lack of specificity and blood cultures lack of sensitivity, and therefore the diagnosis remains a challenge. In order to improve the identification of patients with invasive candidiasis, predictive rules, biomarkers and PCR have been developed. The first part of this work describes the evolution over a ten years period in one ICU in Candida species distribution, susceptibility to antifungal drugs and consumption of antifungal agents. Changes in antifungal drug consumption were observed but they were not associated with significant changes in fungal ecology or with the emergence of resistant species. In a second part, we present a prospective, observational and bicentric study performed in 435 non-neutropenic patients in ICU. Several variables (risk factors of invasive candidiasis, Candida colonization, mannan antigen and anti-mannan antibodies) were analyzed and a predictive score of invasive candidiasis has been developed. Finally, the last part presents the development of Candida real-time PCR in blood, as well as the evaluation of a digital PCR.Les patients de réanimation sont des patients à très haut risque de survenue de candidoses invasives associées à une importante mortalité. Les espèces du genre Candida sont retrouvées en troisième position des agents infectieux les plus fréquemment isolés au cours des septicémies. Le diagnostic reste difficile en raison d’une clinique aspécifique et d’une sensibilité médiocre des hémocultures. Des scores prédictifs, des biomarqueurs ou encore des PCR ont été développés de manière à améliorer le diagnostic et l’identification des patients à risque. Dans ce travail, la première partie présente les données de l’évolution de l’écologie fongique, des candidoses invasives, des prescriptions d’antifongiques et des sensibilités aux antifongiques sur une période de dix ans dans un service de réanimation. Au cours de cette période, les changements observés dans la prescription d’antifongiques n’ont pas entrainé de modifications significatives de l’écologie fongique ni d’apparition de résistances. Dans une deuxième partie, nous présentons les résultats d’une étude prospective observationnelle bicentrique réalisée chez 435 patients non neutropéniques de réanimation. L’analyse de plusieurs variables (facteurs de risque de candidose invasive, colonisation à Candida sp., dosages d’antigène mannane et d’anticorps anti-mannane) a permis l’élaboration d’un score prédictif de survenue de candidose invasive. Finalement, la dernière partie du travail présente la mise au point de PCR Candida en temps réel dans le sang ainsi qu’une évaluation de la technologie de digital PCR

Measurement of Charge Transfer to Aqueous Droplets in High Voltage Electric Fields

The electric charge acquired by aqueous droplets when they contact an electrode is a crucial parameter in experimental and industrial applications where electric fields are used to manipulate droplet motion and coalescence. For unclear reasons, many investigators have found that aqueous droplets acquire significantly more positive than negative charge. Extant techniques for determining the droplet charge typically rely on a hydrodynamic force balance that depends on accurate characterization of the drag forces acting on the droplet. Here we present an alternative methodology for measuring the droplet charge via direct measurement of the electric current. As the droplet approaches the electrode the current is observed to gradually increase, followed by a large pulse when the droplet makes apparent contact. We interpret the transient current signals as the superposition of the natural response of an RLC circuit and an induced current described by the Shockley–Ramo theory. Nonlinear regression of the observed current to the theoretical model allows for the droplet charge to be extracted, independent of any assumptions about the force balance on the droplet. We demonstrate that regression of the current signal yields charge values that are on average within 4% of charges measured via a force balance. We use the chronocoulometric methodology to investigate how the charge varies with the applied potential, and we demonstrate that deionized water droplets contacting planar electrodes acquire on average 69% more positive charge than negative charge