Diamagnetic orientation of a fluid of hard thin disks and anisotropy of the water diffusivity in the nematic phase of a suspension of clay platelets.

Monte Carlo simulations of the diamagnetic orientation of a fluid of hard thin disks in a magnetic field are presented. The particle density and magnetic field dependences of the eigenvalues of the order parameter tensor are calculated in the presence of a wall which promotes nematic order in a definite direction. The existence of a paranematic region is confirmed, and the approach to the Langevin regime at low densities in the paranematic regime is examined. A relationship between the eigenvalues of the tensor and the anisotropy of water diffusivity in the nematic phase of a colloidal suspension is proposed. This permits a comparison between the Monte Carlo simulations and diffusion-weighted magnetic resonance imaging, which shows consistency with the experimental data.

Anisotropic water diffusion in nematic self-assemblies of clay nanoplatelets suspended in water.

Diffusion-weighted magnetic resonance imaging provides a vivid description of the little understood role played by interfacial interactions with macroscopic bodies in the cooperative self-assembly of clay nanoplatelets suspended in water. The interfacial interaction between hydrophilic glass walls and clay platelets in a Na-fluorhectorite gel can produce, for dilute gels, a face-to-wall anchoring of the platelets that leads to a uniaxial nematic order with platelet faces parallel to the walls but with randomly distributed normals of the faces. The application of a magnetic field perpendicular to the walls transforms this uniaxial order to an extended biaxial nematic order with orthogonal alignment between normals and the field. Moreover, for apolar walls, this face-to-wall anchoring is considerably hindered, and the uniaxial nematic order can be substantially disrupted.

A reliable and specific enzyme-linked immunosorbent assay for the capture of IgM from human chagasic sera using fixed epimastigotes of Trypanosoma cruzi.

A rapid, sensitive, specific, and reliable enzyme-linked immunosorbent assay (ELISA) is proposed for determination of the levels of anti-Trypanosoma cruzi IgM in acute chagasic sera (ACD). The efficiency of this ELISA as a diagnostic method was compared with that of parasite DNA detection by polymerase chain reaction (PCR) and that of indirect immunofluorescence (iIF) anti-T. cruzi IgM detection. We tested whether this ELISA using fixed epimastigotes (epi) could detect anti-T. cruzi IgM in serum samples from two groups of children with acute Chagas' disease from a hyperendemic area in Bolivia. In a comparison of the ELISA method with other techniques, 95% and 71% of the results correlated with PCR and iIF findings, respectively. At the serum dilution applied (1:250), rheumatoid factor (RF) did not influence the results, and samples from patients carrying leishmaniasis or mixed Leishmania and T. cruzi infection could also be excluded from ACD. Highly specific and reliable results were obtained, a great number of the sera could be tested in only one assay, and a quantitative index of reactivity (IR) could be calculated without serial titration. Using test samples in triplicate, the method provides a useful tool for the detection of early acute-phase T. cruzi infection in humans.