Biomedical subjects

Daniel A Vega

Publications and source records attributed to Daniel A Vega.

5 recordsLinked to original sources

PubMed2006

Lifshitz-Safran coarsening dynamics in a 2D hexagonal system.

The coarsening process in a two-dimensional hexagonal system in the region close to both spinodal and order-order transitions was investigated through the Cahn-Hilliard model. We found a distinctive region of the phase diagram where the pinning of dislocations plays only a minor role and the dynamics is led by the triple points. In this region, we found configurations of domains with the same features as those proposed by Lifshitz. As a consequence, different correlation lengths grow logarithmically in time, in good agreement with the predictions of coarsening at low temperatures proposed by Safran.

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PubMed2005

Arm retraction potential of branched polymers in the absence of dynamic dilution.

We study the stress relaxation of model polymer networks containing low contents of star shaped and linear dangling polymers. As compared with their melts, the behavior of star and dangling polymers leads to a dynamic response with unprecedented large relaxation times. By comparing data of star melts with those corresponding to stars and dangling chains residing in polymer networks, we were able to identify the effects of dynamic dilution clearly. Since in polymer networks the dynamic dilution effect is suppressed, we were able by the first time to experimentally test the validity of the potential for arm retraction proposed by Pearson and Helfand.

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PubMed2005

Ordering mechanisms in two-dimensional sphere-forming block copolymers.

We study the coarsening dynamics of two-dimensional hexagonal patterns formed by single microdomain layers of block copolymers, using numerical simulations. Our study is focused on the temporal evolution of the orientational correlation length, the interactions between topological defects, and the mechanisms of coarsening. We find no free disclinations in the system; rather, they are located on large-angle grain boundaries, commonly where such boundaries bifurcate. The correlation lengths determined from the scattering function, from the density of dislocations, and from the density of disclinations exhibit similar behavior and grow with time according to a power law. The orientational correlation length also grows following a power law, but with a higher exponent than the other correlation lengths. The orientational correlation length grows via annihilation of dislocations, through preferential annihilation of small-angle grain boundaries due to poor screening of the strain field around dislocations located on small-angle grain boundaries. Consequently, the patterns are characterized by large-angle grain boundaries. The most commonly observed mechanism of coarsening is the collapse of smaller grains residing on the boundary of two larger grains delimited by large-angle grain boundaries. Simulations agree remarkably well with experimental results recently obtained.

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PubMed2002

Dynamics of pattern coarsening in a two-dimensional smectic system.

We have followed the coarsening dynamics of a single layer of cylindrical block copolymer microdomains in a thin film. This system has the symmetry of a two-dimensional smectic. The orientational correlation length of the microdomains was measured by scanning electron microscopy and found to grow with the average spacing between +/-1/2 disclinations, following a power law xi2(t) approximately t(1/4). By tracking disclinations during annealing with time-lapse atomic force microscopy, we observe dominant mechanisms of disclination annihilation involving tripoles and quadrupoles (three and four disclinations, respectively). We describe how annihilation events involving multiple disclinations result in similarly reduced kinetic exponents as observed here. These results map onto a wide variety of physical systems that exhibit similarly striped patterns.

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PubMedDate not supplied

Correction for piezoelectric creep in scanning probe microscopy images using polynomial mapping.

We describe a method for using polynomial mapping to correct scanning probe microscope images for distortion due to piezoelectric creep. Because such distortion varies from image to image, this method can be used when the actual locations of some features within an image are known absolutely, or in a series of images in which the actual locations of some features are known not to vary. While the general case of polynomial mapping of degree N requires the determination of 2(N+ 1)2 matrix elements by regression, we find that by understanding the mechanism by which piezoelectric creep distorts scanning probe microscope images, we can fix most of these coefficients at 0 or 1 a priori, leaving only 2(N+ 1) coefficients to be determined by regression. We describe our implementation of this strategy using the Interactive Data Language (IDL) programming language, and demonstrate our technique on a series of atomic force microscopy (AFM) images of diblock copolymer microdomains. Using our simplified scheme, we are able to reduce the effects of distortion in an AFM image from 5% of the scan width to a single pixel, using only five reference points.

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