Biomedical subjects

Susan J Altman

Publications and source records attributed to Susan J Altman.

4 recordsLinked to original sources

PubMed2006

Colloid dispersion in a uniform-aperture fracture.

This research investigates the dispersion of colloids through fracture systems by exploring experimentally and numerically the transport and dispersion of 1.0-, 0.11-, and 0.043-mum diameter fluorescent carboxylate-modified microspheres and chloride at various flow rates through variable-length, synthetic Plexiglas fractures (flow cells). A dimensionless number describing each experiment is varied by changing the colloid size, flow rate, and fracture length. Surface characteristics of the microspheres and Plexiglas favor repulsive interactions, thereby minimizing the chance of colloid filtration and remobilization. Full recovery of the colloids is typically observed, thereby supporting the assumption of negligible colloid filtration. In comparison to chloride transport, there is increased tailing for colloid plumes traveling through the flow cell. This increased tailing is attributed to Taylor dispersion phenomena (dispersion due to an advection gradient). In the synthetic fractures investigated here, colloid dispersion due to the velocity gradient is evident, but fully developed Taylor conditions are not realized. A particle-tracking algorithm is run inversely to estimate the effective dispersion rate for the colloid plume in each experiment as a function of the experimental parameters (flow rate, fracture length, and colloid size). Results suggest that the log of the effective dispersion rate of the colloid plume increases linearly with the log of the dimensionless number comprising experimental parameters.

Journal Article↗

PubMed2005

Characterization of adsorption sites on aggregate soil samples using synchrotron X-ray computerized microtomography.

Synchrotron-source X-ray computerized microtomography (CMT) was used to evaluate the adsorptive properties of aggregate soil samples. A linear relationship between measured mean mass attenuation coefficient (sigma) and mass fraction iron was generated by imaging mineral standards with known iron contents. On the basis of reported stoichiometries of the clay minerals and identifications of iron oxyhydroxides (1), we calculated the mass fraction iron and iron oxyhydroxide in the intergranular material. The mass fractions of iron were estimated to range from 0.17 to 0.22 for measurements made at 18 keV and from 0.18 to 0.21 for measurements made at 26 keV. One aggregate sample also contained regions within the intergranular material with mass fraction iron ranging from 0.29 to 0.31 and from 0.33 to 0.36 for the 18 and 26 keV measurements, respectively. The mass fraction iron oxyhydroxide ranged from 0.18 to 0.35 for the low-iron intergranular material and from 0.40 to 0.59 for the high-iron intergranular material. Using absorption edge difference imaging with CMT, we visualized cesium on the intergranular material, presumably because of adsorption and possible exchange reactions. By characterizing the mass fraction iron, the mass fraction iron oxyhydroxide, and the adsorptive capacity of these soil mineral aggregates, we provide information useful for conceptualization, development, and parametrization of transport models.

Adsorption↗

PubMed2005

Evaluation of synchrotron X-ray computerized microtomography for the visualization of transport processes in low-porosity materials.

Synchrotron-source X-ray computerized microtomography (CMT) is evaluated as a method to visualize transport processes. We conclude that CMT is adequate for visualization of transport experiments if the right conditions exist. Namely, 1) not much more than one-order-of-magnitude range in concentration data is needed for the study, 2) the pore space in the samples are greater than approximately 2--50 mum, depending on the sample size and system setup; 3) the sample is fine-grained enough so that a representative elemental volume (REV) can be contained by a 2--10 mm diameter sample, and 4) the transport process is slow enough that significant changes do not occur within the 25--50 min (and possibly less in the future) needed to collect data for one three-dimensional image. Absorption edge difference imaging (AEDI) in association with CMT is introduced as a method to enhance pore-space visualization. We successfully imaged the pore space in a low-porosity granodiorite, diorite and fine-grained granite cores and a higher-porosity soil aggregate sample. We found that the pore space important to transport in the core samples was smaller than what we were able to visualize with CMT. We also made rudimentary associations of minerals with pore-space location.

Absorption↗

PubMed2004

Use of X-ray absorption imaging to examine heterogeneous diffusion in fractured crystalline rocks.

Heterogeneous diffusion in different regions of a fractured granodiorite from Japan has been observed and measured through the use of X-ray absorption imaging. These regions include gouge-filled fractures, recrystallized fracture-filling material and hydrothermally altered matrix. With the X-ray absorption imaging technique, porosity, relative concentration, and relative mass of an iodine tracer were imaged in two dimensions with a sub-millimeter pixel size. Because portions of the samples analyzed have relatively low porosity values, imaging errors can potentially impact the results. For this reason, efforts were made to better understand and quantify this error. Based on the X-ray data, pore diffusion coefficients (Dp) for the different regions were estimated assuming a single diffusion rate and a lognormal multirate distribution of Dp. Results show Dp for the gouge-filled fractures are over an order of magnitude greater than those of the recrystallized fracture-filling material, which in turn is approximately two times greater than those for the altered matrix. The recrystallized fracture-filling material was found to exhibit the greatest degree of variability. The results of these experiments also provide evidence that diffusion from advective zones in fractures through the gouge-filled fractures and recrystallized fracture-filling material could increase the pore space available for matrix diffusion. This evidence is important for understanding the performance of potential nuclear waste repositories in crystalline rocks as diffusion is thought to be an important retardation mechanism for radionuclides.

Absorptiometry, Photon↗