Molecular dynamics simulations of electrophoresis of polyelectrolytes in nano confining cylindrical geometries.

We consider molecular dynamics simulations of the electrophoretic motion of charged polymers in straight cylinders and cylinders with a periodic variation (or modulation) of the diameter. The electric field is always orientated along the axis of the cylinders, and it does not change direction. The fluid is modeled explicitly, also the co-ions, counterions, and the charged polymer monomers. In straight geometries, we observe no separation effect for a wide range of applied electric fields. In modulated geometries, the charged polymer chains can be separated only at low fields. At large fields, we observe separation effects in modulated geometries only if the applied field is a dc pulse electric field. A simple scaling theory to explain the observed behavior is presented.

Simulations of polyelectrolyte dynamics in an externally applied electric field in confined geometry.

We consider the dynamics of charged polymers in free solution in a slit geometry under the influence of an electrical field, applied at an angle to the plane parallel walls of the confinement. The simulations are carried out using the Brownian dynamics method with explicit counterions and implicit hydrodynamics. The hydrodynamic interactions between all the particles and the plane parallel walls are taken into account using a diffusion matrix which depends on slit geometry and the actual polyelectrolyte-solute conformations. We observe a selective transport of the charged polymers, as a function of the degree of polymerization and slit height.

Single chain dynamics in polymer networks: a Monte Carlo study.

We investigated the dynamics of single chains diffusing in cross-linked polymer networks using the three-dimensional bond fluctuation method. We considered single chain dynamics in dry polymer networks, at monomer density of 0.51, and similarly, in networks swollen up to the maximum degree of swelling. In order to compare time scales at different degrees of network swelling we calculated the single monomer relaxation time tau(0), which showed a strong decrease with swelling. The scaling laws of terminal relaxation times tau(d) and the diffusion coefficients D as function of renormalized chain length covered both the Rouse and the entangled regime. We found that the entanglement length N(e) as function of monomer density of the network has similar values to those calculated for polymer melts, at the same concentration. The effect of fixed topological constraints in polymer networks on the concentration dependence of the entanglement length is discussed.

Electric and hydrodynamic stretching of DNA-polymer conjugates in free-solution electrophoresis.

The conjugation of an uncharged polymer to DNA fragments makes it possible to separate DNA by free-solution electrophoresis. This end-labeled free-solution electrophoresis method has been shown to successfully separate ssDNA with single monomer resolution up to about 110 bases. It is the aim of this paper to investigate in more detail the coupled hydrodynamic and electrophoretic deformation of the ssDNA-label conjugate at fields below 400 V/cm. Our model is an extension of the theoretical approach originally developed by Stigter and Bustamante [Biophys. J. 75, 1197 (1998)] to investigate the problems of a tethered chain stretching in a hydrodynamic flow and of the electrophoretic stretch of a tethered polyelectrolyte. These two separate models are now used together since the charged DNA is "tethered" to the uncharged polymer (and vice versa), and the resulting self-consistent model is used to predict the deformation and the electrophoretic velocity for the hybrid molecule. Our theoretical and experimental results are in good qualitative agreement.

[Evaluation of morbidity through dental decay in students by school stomatologic monitoring]. / Evaluarea morbiditatii prin carie dentara la elevi în cadrul actiunii de dispensarizare stomatologica scolara.

Starting from the fact that stomatologic dispensarization in schools is the best method for evaluating morbidity by bucco-dental disorders in students, the authors present the results of a longitudinal study which consisted in the permanent monitoring of 48 students for a period of 8 years (1980-1988). The children were from the I-st to the VIII-th class. It is stressed that the most relevant indexes of morbidity are those of incidence (for the evaluation of the present potential of developing a disorder), and that of severity (for the assessment of the morbidity status). The authors intend to take up again this study, with the same methodology, in a group of students to which a local fluorination procedure was applied from the I-st scholar cycle.