RESUMO
An experimental method based on the rotational dynamics of a magnetic probe is reported to measure the local viscoelasticity of soft materials on microscopic scales. The technique is based on the alignment of dipolar chains of submicrometer magnetic particles in the direction of an applied magnetic field. On one hand, light scattering is used to detect the chains' oscillations over a 0.001-100 Hz frequency range when submitted to an oscillating magnetic field and leads to global microrheological measurements. On the other hand, the chains' rotation toward a permanent magnetic field is observed with a microscope, allowing a local determination of viscoelastic properties on the scale of the chains of particles. We demonstrate the accuracy of both assays with a micellar Maxwellian solution and validate theoretical predictions.
RESUMO
We present an experimental method allowing a direct measurement of the local rheological behavior of complex fluids. A magnetic probe is inserted into the bulk of the fluid and submitted to a controlled magnetic force or torque, which induces a mechanical perturbation of the fluid. The geometry of the perturbation can be varied using two kinds of probes: a magnetic bead submitted to a homogeneous magnetic force in one direction, and a magnetic needle that can turn inside the material under the effect of an applied magnetic torque. Two complex viscoelastic fluids are investigated. First, a surfactant solution, which has a linear mechanical behavior in the range of the applied stresses, is used to test and validate the experimental methodology. We then use the local probes to investigate a Laponite colloidal suspension, which exhibits nonlinear behavior such as thixotropy, shear thinning, and aging. In this latter fluid, we find an exponential growth of the rheological relaxation time versus the system age, a power-law dependence of the fluid viscosity on the applied stress, and a dynamical yield stress which saturates with the fluid aging time.
RESUMO
The nucleotide sequence of a 2.2 kb region downstream of the hydrogenase structural genes in Bradyrhizobium japonicum was determined. Four genes encoding predicted polypeptides of 27.8 (HupC), 21.4 (HupD), 10.6 (HupF) and 15.8 (HupG) kDa were identified, of which the first three probably belong to the same operon as the hup structural genes, hupS and hupL. HupC is homologous to the hydrophobic polypeptides with four potential transmembrane regions that are encoded by open reading frames following the hydrogenase structural genes in Rhodobacter capsulatus, Escherichia coli, Azotobacter vinelandii, Wolinella succinogenes, Rhizobium leguminosarum and Alcaligenes eutrophus. Also HupD, HupF and HupG are homologous to genes involved in processing, maturation, functioning and regulation of hydrogenase activity in various hydrogen-oxidizing bacteria.
Assuntos
Genes Bacterianos , Hidrogenase/genética , Rhizobiaceae/genética , Sequência de Aminoácidos , Sequência Conservada , Dados de Sequência Molecular , Fases de Leitura Aberta , Rhizobiaceae/enzimologia , Homologia de Sequência de AminoácidosRESUMO
The authors report a case of meningitis, presenting all characteristics of the disease described in 1944 by P. Mollaret, occurring in a child aged 2 years and 8 months. They stress the rarity of cases on childhood. The cells found in the cerebrospinal fluid, photographs of which are included, are fragile and may undergo early lysis. They are present in association with lymphocytes and polynuclears, giving the slide examined a highly characteristic mottled appearance. The authors raise the question of the morphological similarity of these large cells, with their polygonal contour and small central nucleus, with an epithelium.
