99 research outputs found
Hydrated Electron Dynamics at a Five Femtosecond Time Scale
Hydrated electrons are studied by frequency resolved pump probe with 5 fs time resolution in the spectral range from 600 nm to 1000 nm. A recurrence detected in the pumpprobe signal at —40 fs is tentatively assigned to coupling to librational motions in the electron's solvent cage
Active Membrane Fluctuations Studied by Micropipet Aspiration
We present a detailed analysis of the micropipet experiments recently
reported in J-B. Manneville et al., Phys. Rev. Lett. 82, 4356--4359 (1999),
including a derivation of the expected behaviour of the membrane tension as a
function of the areal strain in the case of an active membrane, i.e.,
containing a nonequilibrium noise source. We give a general expression, which
takes into account the effect of active centers both directly on the membrane,
and on the embedding fluid dynamics, keeping track of the coupling between the
density of active centers and the membrane curvature. The data of the
micropipet experiments are well reproduced by the new expressions. In
particular, we show that a natural choice of the parameters quantifying the
strength of the active noise explains both the large amplitude of the observed
effects and its remarkable insensitivity to the active-center density in the
investigated range. [Submitted to Phys Rev E, 22 March 2001]Comment: 14 pages, 5 encapsulated Postscript figure
Relatório de estágio profissional
O presente Relatório foi realizado no âmbito do Estágio Profissional I e II. Trata-se de um trabalho elaborado com base na observação de aulas e na experimentação didática; os materiais obtidos (como, por exemplo, horários, dados sobre as turmas, fichas, etc.) foram submetidos a uma análise documental
RNA reference materials with defined viral RNA loads of SARS-CoV-2—A useful tool towards a better PCR assay harmonization
SARS-CoV-2, the cause of COVID-19, requires reliable diagnostic methods to track the circulation of this virus. Following the development of RT-qPCR methods to meet this diagnostic need in January 2020, it became clear from interlaboratory studies that the reported Ct values obtained for the different laboratories showed high variability. Despite this the Ct values were explored as a quantitative cut off to aid clinical decisions based on viral load. Consequently, there was a need to introduce standards to support estimation of SARS-CoV-2 viral load in diagnostic specimens. In a collaborative study, INSTAND established two reference materials (RMs) containing heat-inactivated SARS-CoV-2 with SARS-CoV-2 RNA loads of ~107 copies/mL (RM 1) and ~106 copies/mL (RM 2), respectively. Quantification was performed by RT-qPCR using synthetic SARS-CoV-2 RNA standards and digital PCR. Between November 2020 and February 2021, German laboratories were invited to use the two RMs to anchor their Ct values measured in routine diagnostic specimens, with the Ct values of the two RMs. A total of 305 laboratories in Germany were supplied with RM 1 and RM 2. The laboratories were requested to report their measured Ct values together with details on the PCR method they used to INSTAND. This resultant 1,109 data sets were differentiated by test system and targeted gene region. Our findings demonstrate that an indispensable prerequisite for linking Ct values to SARS-CoV-2 viral loads is that they are treated as being unique to an individual laboratory. For this reason, clinical guidance based on viral loads should not cite Ct values. The RMs described were a suitable tool to determine the specific laboratory Ct for a given viral load. Furthermore, as Ct values can also vary between runs when using the same instrument, such RMs could be used as run controls to ensure reproducibility of the quantitative measurements.Peer Reviewe
Electrohydrodynamic model of vesicle deformation in alternating electric fields
We develop an analytical theory to explain the experimentally-observed
morphological transitions of giant vesicles induced by AC electric fields (1).
The model treats the inner and suspending media as lossy dielectrics, while the
membrane as an ion-impermeable flexible incompressible-fluid sheet. The vesicle
shape is obtained by balancing electric, hydrodynamic, and bending stresses
exerted on the membrane. Considering a nearly spherical vesicle, the solution
to the electrohydrodynamic problem is obtained as a regular perturbation
expansion in the excess area.
The theory predicts that stationary vesicle deformation depends on field
frequency and conductivity conditions. If the inner fluid is more conducting
than the suspending medium, the vesicle always adopts a prolate shape. In the
opposite case, the vesicle undergoes a transition from a prolate to oblate
ellipsoid at a critical frequency, which the theory identifies with the inverse
membrane charging time. At frequencies higher than the inverse Maxwell-Wagner
polarization time, the electrohydrodynamic stresses become too small to alter
the vesicle's quasi-spherical rest shape. The analysis shows that the evolution
towards the stationary vesicle deformation strongly depends on membrane
properties such as viscosity. The model can be applied to rationalize the
transient and steady deformation of biological cells in electric fields
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