Biomedical subjects

Raul Cruz Hidalgo

Publications and source records attributed to Raul Cruz Hidalgo.

6 recordsLinked to original sources

PubMed2006

Phase transitions of semiflexible hard-sphere chain liquids.

We present a density-functional theory for describing liquid-crystalline phase transitions in a fluid of semiflexible hard-sphere chain molecules based on the Onsager second-virial approximation for the free energy. Key ingredients of this theory are the generation of semiflexible chain conformations and calculation of the pair excluded volume and excluded area using a single-chain Monte Carlo enumeration method. First, we investigate the isotropic-nematic phase transition. Next, the theory is extended to account for a smectic-A phase by a bifurcation analysis around the nematic solution. The perturbation is calculated using a lowest-order Fourier series representation.

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PubMed2005

Smectic ordering of homogeneous semiflexible polymers.

A self-consistent-field theory for fluids of homogeneous wormlike polymers exhibiting a one-dimensional spatial variation is presented. We have extended the treatment of excluded-volume effects by adding an effective interaction term which describes the excluded volume between wormlike cylindrical segments and terminal (or end) segments of the polymer molecules. This enables us to find a smectic-A phase in the case of homogeneous semiflexible polymers. Using this framework, we have investigated the occurrence of smectic-A, nematic, and isotropic phases in the second-virial (Onsager) approximation. Phase diagrams are calculated for systems characterized by different rigidities (i.e., persistence lengths). For the case of infinitely rigid molecules, the nematic-smectic transition appears to be mostly second order. Systems of semiflexible molecules exhibit mainly a first-order smectic-nematic transition, and their isotropic-nematic-smectic triple points are accessed for different rigidity values. The nematic-smectic transition line is in good agreement with previous analytical calculations, which were also performed assuming the second-virial approximation. However, the values of the volume fraction at the nematic-smectic transition are large compared with computer simulation results, indicating limitations of the second-virial approximation.

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PubMed2003

Time evolution of damage under variable ranges of load transfer.

We study the time evolution of damage in a fiber bundle model in which the range of interaction of fibers varies through an adjustable stress transfer function recently introduced. We find that the lifetime of the material exhibits a crossover from mean-field to short-range behavior as in the static case. Numerical calculations showed that the value at which the transition takes place depends on the system's disorder. Finally, we have performed a microscopic analysis of the failure process. Our results confirm that the growth dynamics of the largest crack is radically different in the two limiting regimes of load transfer during the first stages of breaking.

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PubMed2003

Scaling laws of creep rupture of fiber bundles.

We study the creep rupture of fiber composites in the framework of fiber bundle models. Two fiber bundle models are introduced based on different microscopic mechanisms responsible for the macroscopic creep behavior. Analytical and numerical calculations show that above a critical load the deformation of the creeping system monotonically increases in time resulting in global failure at a finite time t(f), while below the critical load the system suffers only partial failure and the deformation tends to a constant value giving rise to an infinite lifetime. It is found that approaching the critical load from below and above the creeping system is characterized by universal power laws when the fibers have long-range interaction. The lifetime of the composite above the critical point has a universal dependence on the system size.

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PubMed2002

Fracture model with variable range of interaction.

We introduce a fiber bundle model where the interaction among fibers is modeled by an adjustable stress-transfer function that can interpolate between the two limiting cases of load redistribution, i.e., the global and the local load sharing schemes. By varying the range of interaction, several features of the model are numerically studied and a crossover from mean-field to short-range behavior is obtained. The properties of the two regimes and the emergence of the crossover in between are explored by numerically studying the dependence of the ultimate strength of the material on the system size, the distribution of avalanches of breakings, and of the cluster sizes of broken fibers. Finally, we analyze the moments of the cluster size distributions to accurately determine the value at which the crossover is observed.

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PubMed2002

Creep rupture of viscoelastic fiber bundles.

We study the creep rupture of bundles of viscoelastic fibers occurring under uniaxial constant tensile loading. A fiber bundle model is introduced that combines the viscoelastic constitutive behavior and the strain controlled breaking of fibers. Analytical and numerical calculations showed that above a critical external load the deformation of the system monotonically increases in time resulting in global failure at a finite time t(f), while below the critical load the deformation tends to a constant value giving rise to an infinite lifetime. Our studies revealed that the nature of the transition between the two regimes, i.e., the behavior of t(f) at the critical load sigma(c), strongly depends on the range of load sharing: for global load sharing t(f) has a power law divergence at sigma(c) with a universal exponent of 0.5, however, for local load sharing the transition becomes abrupt: at the critical load t(f) jumps to a finite value, analogous to second- and first-order phase transitions, respectively. The acoustic response of the bundle during creep is also studied.

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