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Biomedical subjects

T G Mason

Publications and source records attributed to T G Mason.

11 recordsLinked to original sources
PubMed2003

Asphaltene nanoparticle aggregation in mixtures of incompatible crude oils.

We study the structure and phase behavior of asphaltenes comprised of large polyaromatic molecules in blends of naturally occurring crude oils using small angle neutron scattering (SANS). When two compatible oils are blended together, the asphaltenes remain dispersed as colloidal nanoparticles; however, when two incompatible oils are blended together, these asphaltene nanoparticles can aggregate to form microscale structures. We show that SANS directly probes asphaltene aggregation in unmodified (i.e., nondeuterated) crude oil mixtures due to a significant neutron scattering length density difference between the hydrogen-poor asphaltenes and the surrounding oil. Moreover, the small length scales probed by SANS are ideally suited for studying asphaltene aggregation: SANS simultaneously provides the average size and concentration of nanoscale asphaltene particles and also the volume fraction of microscale asphaltene aggregates. These discoveries yield a practical means for directly assessing the compatibility of crude oils and for diagnosing refinery fouling problems resulting from blending incompatible oils.

Journal Article↗
PubMed2003

Rotational diffusion microrheology.

Examining the rotational diffusion of a microparticle suspended in a soft material opens up exciting new opportunities for locally probing the frequency-dependent linear viscoelastic shear modulus, G*(omega). We study the one-dimensional rotational diffusion of a wax microdisk in an aqueous polymer entanglement network using light streak tracking. By measuring the disk's time-dependent mean square angular displacement, , we predict the polymer solution's G*(omega) using a rotational generalized Stokes-Einstein relation. The good agreement of the predicted modulus with mechanical measurements confirms this new microrheological approach.

Journal Article↗
PubMed2002

Osmotically driven shape-dependent colloidal separations.

The thermally induced motion of nanometer-sized surfactant micelles in water is used to create strong attractive forces between micron-sized disks of wax in a mixed aqueous dispersion of microdisks and microspheres. The short-ranged attractive force due to the depletion of micelles from between the microdisks is much stronger than that between two microspheres of similar size, and is largest when the disks approach face to face, so columns of microdisks form. These columns cream, whereas the spheres remain dispersed, providing a means for shape-dependent colloidal separations driven by an applied micellar osmotic pressure.

Journal Article↗
PubMed2002

Light streak tracking of optically trapped thin microdisks.

Nonspherical particles can uniquely probe soft system dynamics. We show that laser tweezers stably trap thin coinlike microdisks in 3D with an edge-on orientation. Scattered light forms a streak that we track using a fast camera to measure the disk's angular displacement. Linearly polarized tweezers rotationally trap a birefringent disk, and we measure its harmonically bound Brownian rotation over 5 decades in time. Near a surface, the disk exhibits a translational-orientational switchback oscillation. Circularly polarized tweezers rotate the disk and streak, yielding a colloidal lighthouse.

Journal Article↗
PubMed1999

Critical angle of wet sandpiles.

We measure the increase in the maximum stable angle of a sandpile, theta(c), with the volume fraction, phi of a liquid added to cause cohesion between the grains. For two different liquids, tan theta(c) does not apparently scale with the air-liquid surface tension at low phi, whereas it does at higher phi. This suggests that the liquid forms menisci at asperities on the surfaces of the grains before filling cohesive menisci at intergrain contact points. In this cohesive limit, theta(c)(phi) agrees with a surface roughness theory. Electron and fluorescence microscopy of the dry and wet surfaces of the grains support this model.

Journal Article↗
PubMed1998

The current use and interpretation of rheumatologic tests.

The growing emphasis on primary care medicine has led to an increase in the number of physicians ordering and interpreting tests previously used by specialists. In order to prevent unnecessary and inaccurate testing, it is important to determine the necessity and accuracy of a test before it is performed. This chapter reviews several types of autoantibody (ANA) tests sensitive to rheumatologic diseases and explains how such tests may aid diagnosis.

Antibodies, Antineutrophil Cytoplasmic↗
PubMed1979

Competition between an Escherichia coli tyrosine auxotroph and a prototrophic revertant in glucose- and tyrosine-limited chemostats.

A tyrosine-requiring strain of Escherichia coli was grown in tyrosine-limited chemostats at a range of dilution rates between 0.08 h-1 and 0.42 h-1, conditions which always resulted in the selection of a prototrophic revertant population able to synthesise tyrosine. Analysis of the two-membered mixed cultures which arose showed that the prototrophic population outgrew the auxotroph since its growth rate was not restricted by the growth-limiting concentrations of exogenous tyrosine. During the take-over of the culture, the prototroph population grew exponentially but the specific growth rate increased with decreasing dilution rate of the competition experiments. In glucose-limited chemostats (in the presence of non-growth-limiting concentrations of tyrosine) of the tyrosine-requiring strain, prototrophs were never detected. Constructed two-membered mixed cultures with both populations competing for limiting amounts of glucose, showed that the prototroph was less competitive than the auxotroph.

Ecology↗