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Browsing by Author "Lower, Steven K."

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    Experimentally Derived Sticking Efficiencies of Microparticles using Atomic Force Microscopy: Toward a Better Understanding of Particle Transport
    Cail, Tracy (Virginia Tech, 2003-12-18)
    It is estimated that there are 5x1030 microorganisms on Earth and that approximately 50% live in unconsolidated sediment on the terrestrial subsurface. Subsurface disturbances caused by the constant search for natural resources and our dependence on groundwater make the abundance and diversity of these organisms a global concern. It is vital to many environmental fields, including bioremediation, water purification, and contaminant transport, that we understand how microorganisms and other colloidal particles attach to and detach from natural sediments and ultimately how they travel through porous media. Sticking efficiency (alpha) is a major component of most particle transport theories. It is defined as the ratio of particles that adhere to a collector surface compared to the total number of particles that collide with that surface. In this study, the Interaction Force Boundary Layer (IFBL) model was used to determine the sticking efficiencies of inorganic colloidal particles and Enterococcus faecalis cells against a silica glass collector surface. Sticking efficiencies were derived from intersurface potential energies that were determined from integrated force-distance data measured by Atomic Force Microscopy (AFM). Force data were measured in buffered aqueous solutions of varying pH and ionic strength to determine the influence of solution chemistry on particle removal from solution. Zeta-potentials were measured to determine the impact of particle and collector surface charge on force measurements. The results of this study indicate that alpha is strongly influenced by solution chemistry. The response of alpha to small changes in solution pH and ionic strength may be several orders of magnitude. Zeta-potential measurements imply that sticking efficiencies are strongly influenced by the electrical charges on both the particle and collector surfaces. Zeta-potentials of bacteria did not vary significantly with changing solution pH, but did respond to changing solution ionic strength. Historically, alpha has been very difficult to predict. This study is the first to report sticking efficiencies measured using AFM and the first to successfully apply the IFBL model to colloidal particles. Æ nThe incorporation of empirical nanoscale interactions into the measurement of alpha promises to more successfully describe particle adhesion and, thus, particle transport.
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    Mineral-Microbe Interactions Probed in Force, Energy, and Distance Nanospace
    Lower, Steven K. (Virginia Tech, 2001-02-22)
    Biological force microscopy (BFM) was developed to quantitatively measure pico- to nano-Newton forces (10-9 to 10-12 N) as a function of the nanoscale distance (nanometers) between living bacteria and mineral surfaces, in aqueous solution. Native cells were linked to a force-sensing probe, which was used in a force microscope to measure attractive and repulsive forces as a mineral surface approached, made contact with, and subsequently withdrew from a bacterium on the probe. The resulting data were used to interpret the interactive dynamics operative between bacteria and mineral surfaces under environmentally relevant conditions. BFM was used to study bacterial adhesion to mineral surfaces. In the case of Escherichia coli interactions with goethite, graphite, and muscovite, attractive and repulsive forces were detected at ranges up to 400 nanometers, the magnitude and sign depending on the ionic strength of the intervening solution and the mineral surface charge and hydrophobicity. Adhesion forces, up to several nanoNewtons in magnitude and exhibiting various fibrillation dynamics, were also measured and reflect the complex interactions of structural and chemical functionalities on the bacteria and mineral surfaces. In the study of Burkholderia cepecia interactions with mica, it was found that the physiological condition of the cell affected the observed adhesion forces. Cells grown under oligotrophic conditions exhibited an increased affinity for the mineral surface as opposed to cells grown under eutropic conditions. BFM was also used to characterize the transfer of electrons from biomolecules on Shewanella oneidensis to Fe(III) in the structure of goethite. Force measurements with picoNewton resolution were made in aqueous solution under aerobic and anaerobic conditions. Energy values (in attoJoules) derived from these measurements show that the affinity between S. oneidensis and goethite rapidly increases by two to five times under anaerobic conditions where electron transfer from bacterium to mineral is expected. Specific signatures in the force curves, analyzed with the worm-like chain model of protein unfolding, suggest that the bacterium recognizes the mineral surface such that a 150 kDa putative, iron reductase is quickly mobilized within the outer membrane of S. oneidensis and specifically interacts with the goethite surface to facilitate the electron transfer process.