29Si Hyperfine Structure of the E'_\alpha Center in Amorphous Silicon Dioxide

We report a study by electron paramagnetic resonance (EPR) on the E'_\alpha point defect in amorphous silicon dioxide (a-SiO2). Our experiments were performed on gamma-ray irradiated oxygen-deficient materials and pointed out that the 29Si hyperfine structure of the E'_alpha consists in a pair of lines split by 49 mT. On the basis of the experimental results a microscopic model is proposed for the E'_alpha center, consisting in a hole trapped in an oxygen vacancy with the unpaired electron sp3 orbital pointing away from the vacancy in a back-projected configuration and interacting with an extra oxygen atom of the a-SiO2 matrix.Comment: 4 page

Structural relaxation of E' gamma centers in amorphous silica

We report experimental evidence of the existence of two variants of the E' gamma centers induced in silica by gamma rays at room temperature. The two variants are distinguishable by the fine features of their line shapes in paramagnetic resonance spectra. These features suggest that the two E' gamma differ for their topology. We find a thermally induced interconversion between the centers with an activation energy of about 34 meV. Hints are also found for the existence of a structural configuration of minimum energy and of a metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let

Online fabrication and characterization of capsule populations with a flow-focusing microfluidic system

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.We have designed a microfluidic system that combines a double flow-focusing setup for calibrated capsule fabrication with a microchannel for the characterization of their mechanical properties. The double flow-focusing system consists of a first Y junction to create the microdroplets and of a second Y junction to introduce the cross-linking agent allowing the membrane formation. The human serum albumin (HSA) aqueous solution for the dispersed solution, hydrophobic phase for the continuous solution and cross-linking agent solution are introduced by means of syringe pumps. A wavy channel after the second junction allows to control the reticulation time. A cylindrical microchannel then enables to deform and characterize the capsules formed. The mechanical properties of the capsule membrane are obtained by inverse analysis (Chu et al. 2011). The results show that the drop size increases with the flow rate ratio between the central and lateral channels and does not change much regardless of the flow rate of the reticulation phase. The mean shear modulus of the capsules fabricated after 23 s of reticulation is of the order of the surface tension of HSA solution with Dragoxat indicating that the reticulation time is too short to form an elastic membrane around the droplet. When the reticulation time is increased to 60 s, the membrane shear modulus is multiplied by a factor of 3 confirming that a solid membrane has formed around the drop

Delocalized Nature of the E'-delta Center in Amorphous Silicon Dioxide

We report an experimetal study by Electron Paramagnetic Resonance (EPR) of E'-delta point defect induced by gamma ray irradiation in amorphous SiO2. We obtained an estimetion of the intensity of the 10 mT doublet characterizing the EPR spectrum of such a defect arising from hyperfine interaction of the unpaired electron with a 29Si (I=1/2) nucleus. Moreover, determining the intensity ratio between this hyperfine doublet and the main resonance line of E'-delta center, we pointed out that unpaired electron wave function of this center is actually delocalized over four nearly equivalent silicon atoms.Comment: approved for publication in Physical Review Letter

Characterization of E'delta and triplet point defects in oxygen deficient amorphous silicon dioxide

We report an experimental study by electron paramagnetic resonance (EPR) of gamma ray irradiation induced point defects in oxygen deficient amorphous SiO2 materials. We have found that three intrinsic (E'gamma, E'delta and triplet) and one extrinsic ([AlO4]0) paramagnetic centers are induced. All the paramagnetic defects but E'gamma center are found to reach a concentration limit value for doses above 10^3 kGy, suggesting a generation process from precursors. Isochronal thermal treatments of a sample irradiated at 10^3 kGy have shown that for T>500 K the concentrations of E'gamma and E'delta centers increase concomitantly to the decrease of [AlO4]0. This occurrence speaks for an hole transfer process from [AlO4]0 centers to diamagnetic precursors of E' centers proving the positive charge state of the thermally induced E'gamma and E'delta centers and giving insight on the origin of E'gamma from an oxygen vacancy. A comparative study of the E'delta center and of the 10 mT doublet EPR signals on three distinct materials subjected to isochronal and isothermal treatments, has shown a quite general linear correlation between these two EPR signals. This result confirms the attribution of the 10 mT doublet to the hyperfine structure of the E'delta center, originating from the interaction of the unpaired electron with a nucleus of 29Si (I=1/2). Analogies between the microwave saturation properties of E'gamma and E'delta centers and between those of their hyperfine structures are found and suggest that the unpaired electron wave function involves similar Si sp3 hybrid orbitals; specifically, for the E'delta the unpaired electron is supposed to be delocalized over four such orbitals of four equivalent Si atoms.Comment: Approved for publication in Physical Review

The use of high aspect ratio photoresist (SU-8) for super-hydrophobic pattern prototyping

In this work we present a reliable technique for the production of large areas of high aspect-ratio patterns and describe their use as model super-hydrophobic systems. The high thickness and straight sidewalls possible with SU-8 were used to generate dense patterns of small pillars. These photoresist patterns could be used directly, without the need for micromoulding. A method is given allowing resist thickness to be varied over a wide range and a bottom antireflective layer was used to simplify patterning on reflective substrates. This patterning technique allows rapid testing of wetting theories, as pattern size and depth can be varied simply and samples can be produced in sufficient numbers for laboratory use. We show how the static contact angle of water varies with pattern height for one sample-pattern and how static and dynamic contact angles vary with dimension using high aspect-ratio patterns

Three-dimensional in vitro culture models in oncology research

Cancer is a multifactorial disease that is responsible for 10 million deaths per year. The intra- and inter-heterogeneity of malignant tumors make it difficult to develop single targeted approaches. Similarly, their diversity requires various models to investigate the mechanisms involved in cancer initiation, progression, drug resistance and recurrence. Of the in vitro cell-based models, monolayer adherent (also known as 2D culture) cell cultures have been used for the longest time. However, it appears that they are often less appropriate than the three-dimensional (3D) cell culture approach for mimicking the biological behavior of tumor cells, in particular the mechanisms leading to therapeutic escape and drug resistance. Multicellular tumor spheroids are widely used to study cancers in 3D, and can be generated by a multiplicity of techniques, such as liquid-based and scaffold-based 3D cultures, microfluidics and bioprinting. Organoids are more complex 3D models than multicellular tumor spheroids because they are generated from stem cells isolated from patients and are considered as powerful tools to reproduce the disease development in vitro. The present review provides an overview of the various 3D culture models that have been set up to study cancer development and drug response. The advantages of 3D models compared to 2D cell cultures, the limitations, and the fields of application of these models and their techniques of production are also discussed

Proof of the thermodynamical stability of the E' center in SiO2

The E' center is a paradigmatic radiation-induced defect in SiO2 whose peculiar EPR and hyperfine activity has been known since over 40 years. This center has been traditionally identified with a distorted, positively-charged oxygen vacancy V_O+. However, no direct proof of the stability of this defect has ever been provided, so that its identification is still strongly incomplete. Here we prove directly that distorted V_O+ is metastable and that it satisfies the key requirements for its identification as E', such as thermal and optical response, and activation-deactivation mechanisms.Comment: RevTeX 4 pages, 2 figure

Conservation must capitalise on climate’s moment

The health of the natural environment has never been a greater concern, but attention to biodiversity loss is being eclipsed by the climate crisis. We argue that conservationists must seize the agenda to put biodiversity at the heart of climate policy

An optical fibre rereadable radiation dosimeter for use at high doses and at elevated temperature

A new type of radiation dosimeter for large radiation doses is described, which is based on silica fibre material. Conventional radioluminescence or thermoluminescence of silica produces emission in the blue region of the spectrum. However, in this new material irradiation, in conjunction with a heat treatment, generates a green emission band. The intensity of the green band can be monitored by either radioluminescence or thermoluminescence using a test dose. The signals are directly related to the total irradiation history of the material. The dosimeter is therefore rereadable. The production mechanism of the green emission centre requires a thermal processing stage, with an activation energy of 0.52 eV. Further, the dosimeter is effective at recording radiation during high-temperature exposure, to at least 400°C, with the subsequent dosimetry being performed below 200°C