Biomedical subjects

Béla Joós

Publications and source records attributed to Béla Joós.

6 recordsLinked to original sources

PubMed2006

Shear-induced overaging in a polymer glass.

A phenomenon recently coined as overaging implies a slowdown in the collective (slow) relaxation modes of a glass when a transient shear strain is imposed. We are able to reproduce this behavior in simulations of a supercooled polymer melt by imposing instantaneous shear deformations. The increase in relaxation times Delta(tau(1/2)) rises rapidly with deformation, becoming exponential in the plastic regime, and is accompanied by significant changes in the distribution of these relaxation times throughout the system. This overaging is distinct from standard aging. We find increases in pressure, bond-orientational order, and in the average energy of the inherent structures ( ) of the system, all dependent on the size of the deformation. The observed change in behavior from elastic to plastic deformation suggests a link to the physics of the "jammed state."

Journal Article↗

PubMed2005

Elasticity and stability of a helical filament.

We derive the general shape equations in terms of Euler angles for a uniform elastic rod with spontaneous torsion and curvatures and subjected to external force and torque. Our results based on an analytic formalism show that the extension of a helical rod may undergo a one-step discontinuous transition with increasing stretching force. This agrees quantitatively with experimental observations for a helix in a chemically defined lipid concentrate. The larger the twisting rigidity, the larger the jump in the extension. The effect of torque on the jump is, however, dependent on the value of the spontaneous torsion. In contrast, increasing the spontaneous torsion encourages the continuous variation of the extension. An "over-collapse" behavior is observed for the rod with asymmetric bending rigidity, and an intrinsic asymmetric elasticity under twisting force is found.

Biopolymers↗

PubMed2004

Rigidity transition in polymer melts with van der Waals interaction.

We study the onset of rigidity near the glass transition (GT) in a short-chain polymer melt modelled by a bead-spring model, where all beads interact with Lennard-Jones potentials. The properties of the system are examined above and below the GT. In order to minimize high-cooling-rate effects and computational times, equilibrium configurations are reached via isothermal compression. We monitor quantities such as the heat capacity CP, the short-time diffusion constants D, the viscosity eta , and the shear modulus; the time-dependent shear modulus Gt is compared with the shear modulus mu obtained from an externally applied instantaneous shear. We give a detailed analysis of the effects of such shearing on the system, both locally and globally. It is found that the polymeric glass displays long-time rigid behavior only below a temperature T1 , where T1 < TG. Furthermore, the linear and nonlinear relaxation regimes under applied shear are discussed.

Journal Article↗

PubMed2003

Elasticity of randomly diluted central force networks under tension.

We study the rigidity of two-dimensional site-diluted central force triangular networks under tension. We calculate the shear modulus micro directly and fit it with a power law of the form mu approximately (p-p*)(f), where p is the concentration of sites, p* its critical value, and f the critical exponent. We find that the critical behavior of mu is quite sensitive to tension. As the tension is increased there is at first a sharp drop in the values of both p* and f, followed by a slower decrease towards the values of the diluted Gaussian spring network (or random resistor network). We find that the size of the critical region is also sensitive to tension. The tension-free system has a narrower critical regime with the power law failing for p>0.8. In contrast, a small tension is sufficient to extend the power law to near p=1. The physical basis for these behaviors is discussed.

Journal Article↗

PubMed2003

Lattice model for the kinetics of rupture of fluid bilayer membranes.

We have constructed a model for the kinetics of rupture of membranes under tension, applying physical principles relevant to lipid bilayers held together by hydrophobic interactions. The membrane is characterized by the bulk compressibility (for expansion) K, the thickness 2h(t) of the hydrophobic part of the bilayer, the hydrophobicity sigma, and a parameter gamma characterizing the tail rigidity of the lipids. The model is a lattice model which incorporates strain relaxation, and considers the nucleation of pores at constant area, constant temperature, and constant particle number. The particle number is conserved by allowing multiple occupancy of the sites. An equilibrium "phase diagram" is constructed as a function of temperature and strain with the total pore surface and distribution as the order parameters. A first-order rupture line is found with increasing tension, and a continuous increase in protopore concentration with rising temperature till instability. The model explains current results on saturated and unsaturated phosphatidylcholine lipid bilayers and thicker artificial bilayers made of diblock copolymers. Pore size distributions are presented for various values of area expansion and temperature, and the fractal dimension of the pore edge is evaluated.

Biophysical Phenomena↗

PubMed2003

Model for gelation with explicit solvent effects: structure and dynamics.

We study a two-component model for gelation consisting of f-functional monomers (the gel) and inert particles (the solvent). After equilibration as a simple liquid, the gel particles are gradually cross linked to each other until the desired number of cross links have been attained. At a critical cross-link density, the largest gel cluster percolates and an amorphous solid forms. This percolation process is different from ordinary lattice or continuum percolation of a single species in the sense that the critical exponents are new. As the cross-link density p approaches its critical value p(c), the shear viscosity diverges: eta(p) approximately (p(c)-p)(-s) with s a nonuniversal concentration-dependent exponent.

Journal Article↗