12 research outputs found
Rheological Chaos in a Scalar Shear-Thickening Model
We study a simple scalar constitutive equation for a shear-thickening
material at zero Reynolds number, in which the shear stress \sigma is driven at
a constant shear rate \dot\gamma and relaxes by two parallel decay processes: a
nonlinear decay at a nonmonotonic rate R(\sigma_1) and a linear decay at rate
\lambda\sigma_2. Here \sigma_{1,2}(t) =
\tau_{1,2}^{-1}\int_0^t\sigma(t')\exp[-(t-t')/\tau_{1,2}] {\rm d}t' are two
retarded stresses. For suitable parameters, the steady state flow curve is
monotonic but unstable; this arises when \tau_2>\tau_1 and
0>R'(\sigma)>-\lambda so that monotonicity is restored only through the
strongly retarded term (which might model a slow evolution of material
structure under stress). Within the unstable region we find a period-doubling
sequence leading to chaos. Instability, but not chaos, persists even for the
case \tau_1\to 0. A similar generic mechanism might also arise in shear
thinning systems and in some banded flows.Comment: Reference added; typos corrected. To appear in PRE Rap. Com
Adsorption of mono- and multivalent cat- and anions on DNA molecules
Adsorption of monovalent and multivalent cat- and anions on a deoxyribose
nucleic acid (DNA) molecule from a salt solution is investigated by computer
simulation. The ions are modelled as charged hard spheres, the DNA molecule as
a point charge pattern following the double-helical phosphate strands. The
geometrical shape of the DNA molecules is modelled on different levels ranging
from a simple cylindrical shape to structured models which include the major
and minor grooves between the phosphate strands. The densities of the ions
adsorbed on the phosphate strands, in the major and in the minor grooves are
calculated. First, we find that the adsorption pattern on the DNA surface
depends strongly on its geometrical shape: counterions adsorb preferentially
along the phosphate strands for a cylindrical model shape, but in the minor
groove for a geometrically structured model. Second, we find that an addition
of monovalent salt ions results in an increase of the charge density in the
minor groove while the total charge density of ions adsorbed in the major
groove stays unchanged. The adsorbed ion densities are highly structured along
the minor groove while they are almost smeared along the major groove.
Furthermore, for a fixed amount of added salt, the major groove cationic charge
is independent on the counterion valency. For increasing salt concentration the
major groove is neutralized while the total charge adsorbed in the minor groove
is constant. DNA overcharging is detected for multivalent salt. Simulations for
a larger ion radii, which mimic the effect of the ion hydration, indicate an
increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure
The influence of the external ankle support on the dynamic balance in volleyball athletes
Detecção do provírus da Imunodeficiência Felina em gatos domésticos pela técnica de Reação em Cadeia da Polimerase Detection of feline immunodeficiency provirus in domestic cats by polymerase chain reaction
A infecção de gatos domésticos pelo Vírus da Imunodeficiência Felina (FIV) é um dos modelos mais promissores para o estudo da infecção pelo vírus da imunodeficiência humana (HIV) que causa a Síndrome de Imunodeficiência Adquirida (AIDS). O FIV causa, em gatos, uma enfermidade similar àquela observada em pacientes com AIDS, sobretudo no que diz respeito ao aumento da susceptibilidade a infecções oportunistas. No presente estudo, utilizou-se a Reação em Cadeia da Polimerase (PCR), com o objetivo de detectar o provírus do FIV em gatos com sinais clínicos de imunodeficiência. O fragmento de DNA escolhido como alvo para amplificação situa-se no gene gag do lentivírus felino, o qual é conservado entre as diferentes amostras do vírus. O DNA utilizado foi extraído a partir de amostras de sangue e de tecidos de animais com suspeita clínica de imunodeficiência. Das 40 amostras analisadas, 15 foram positivas, das quais 4 foram submetidas à hibridização, confirmando a especificidade dos fragmentos amplificados. Esses resultados demonstram a presença do FIV na população de gatos domésticos do Rio Grande do Sul, Brasil.<br>Feline immunodeficiency virus (FIV) infection of domestic cats is one of the most promising animal models for the infection by the human immunodeficiency virus (HIV) which causes acquired immunodeficiency syndrome (AIDS). Infected cats may develop a disease similar to that observed in AIDS patients, with increased susceptibility to opportunistic infections. In this study we used the polymerase chain reaction (PCR) to detect proviral DNA of feline immunodeficiency virus on the blood and tissue samples from cats with a clinical diagnosis of immunodeficiency. The PCR primers were used to amplify the gag gene, which is conserved among different isolates. From 40 samples analyzed, 15 were positive and 4 of them were submitted to hybridization to confirm the specificity of the amplified fragments. These results confirm the presence of FIV in domestic cats in Rio Grande do Sul, Brazil
Cellulose microfibril orientation and cell shaping in developing guard cells of Allium: The role of microtubules and ion accumulation
Atomistic calculations on interfaces: Bridging the length and time scales
Phase field simulations suitable to describe interfacial phenomena at the mesoscale require as input parameters accurate interfacial energies as well as the interface mobilities. However, this information is not directly accessible by experiment. Furthermore, phenomena such as impurity segregation cannot be decoupled and their independent role in interfacial cohesion and mobility cannot be deduced. On the other hand ab-initio calculations and/or classical interatomic potentials are suitable tools which can provide an on-atomic-scale description of the interfaces. However, there are a number of challenges that one encounters: multidimensional phase space of the interfacial misorientation degrees of freedom, suitable driving forces, and large length and time scales just to mention a few. In the present report we provide an extended review on the atomistic calculations and the simulation strategies proposed to tackle the corresponding problems