Showing papers by "David A. Kessler published in 2004"

Mesenchymoma: an unusual tumour of the lip.

Abstract: The purpose of this paper is to present a case report of a benign mesenchymoma involving the lip. A brief review of relevant literature is presented.

Crack-microcrack interactions in dynamical fracture.

Abstract: We address the interaction of fast moving cracks in stressed materials with microcracks on their way, considering it as one possible mechanism for fluctuations in the velocity of the main crack (irrespective whether the microcracks are existing material defects or they form during the crack evolution). We analyze carefully the dynamics (in two space dimensions) of one macrocrack and one microcrack, and demonstrate that their interaction results in a large and rapid velocity fluctuation, in qualitative correspondence with typical velocity fluctuations observed in experiments. In developing the theory of the dynamical interaction we invoke an approximation that affords a reduction in mathematical complexity to a simple set of ordinary differential equations for the positions of the crack tips; we propose that this kind of approximation has a range of usefulness that exceeds the present context.

Distribution functions for filaments under tension.

Abstract: We develop a biased Monte Carlo simulation technique to measure the distribution functions of the extension and the end-to-end distance of fluctuating filaments stretched by external force The method is applicable for arbitrary ratio of the persistence length to the contour length and for arbitrary forces, and also for the case of steric constraints, such as an external wall The fundamental idea underlying the algorithm is to account explicitly for the length-scale dependence of the effective elastic moduli We find that orientational fluctuations and wall effects produce non-Gaussian distributions for nearly rigid filaments in the small to intermediate force regime The simulation results are tested against analytic expressions for the force-extension curves, both in the semiflexible and nearly stiff limits

Analytical study of the effect of recombination on evolution via DNA shuffling.

Abstract: DNA shuffling is an evolutionary protocol wherein cycles of selection, recombination, mutation, and amplification are employed to evolve proteins and DNA sequences. Experiments have shown its superiority to traditional protocols which do not employ recombination. Motivated by DNA shuffling, we investigate a multilocus evolutionary model that incorporates selection, recombination, and point mutations. Due to simplicity of the model, for the case of an infinite population we can obtain a full analytical treatment of both its dynamical and equilibrium properties, and study the benefit of recombination explicitly and quantitatively. We also briefly discuss finite-population size corrections.

Investigation of Cyclodextrin-Enhanced Electrokinetic Soil Remediation. Fate and Transport of Nitroaromatic Contaminants and Cyclodextrin Amendments in Expansive Clays

Abstract: : Nitroaromatic-based compounds can pose a serious threat to ecosystems and human health. These compounds, which have been used in munitions and pesticides, are present in soil and groundwater at military installations and other sites. The presence of these contaminants in clayey soils poses a difficult and potentially costly remediation problem. The low hydraulic conductivity of these soils prohibits the use of conventional pump-and-treat remediation techniques, and high energy costs make the process of excavating and thermally treating the soil highly inefficient and damaging to the environment. For these reasons, much research has been done to advance the field of electrokinetic remediation as a viable cleanup technique. Although some studies have reported that organic molecules can be removed from clayey soils by electro-osmosis alone using a purging solution, the addition of amendments to the solution has increased removal efficiencies. The objective of this research was to advance the state of knowledge on a novel technique for cost-effective remediation of nitroaromatic compounds using cyclodextrin molecules as solubilizing agents and electrokinetically transporting them through the contaminated zone. The results provided direct evidence that it is possible to remove nitrobenzene and 2,4-DNT from expansive montmorillonite clay using electro-osmosis both with and without cyclodextrin amendments.

Fluctuation-regularized Front Propagation Dynamics

Abstract: We introduce and study a new class of fronts in finite particle number reaction-diffusion systems, corresponding to propagating up a reaction rate gradient. We show that these systems have no traditional mean-field limit, as the nature of the long-time front solution in the stochastic process differs essentially from that obtained by solving the mean-field deterministic reaction-diffusion equations. Instead, one can incorporate some aspects of the fluctuations via introducing a density cutoff. Using this method, we derive analytic expressions for the front velocity dependence on bulk particle density and show self-consistently why this cutoff approach can get the correct leading-order physics.

Crack (Non)Propagation in Amorphous Media

Abstract: We study Mode I crack propagation in amorphous material via a molecular dynamics simulation of a binary alloy with pairwise central-force interactions We find that when the system is subjected to constant displacement after introduction of a seed crack, the crack does not propagate This was found to be true both for the modified Lennard-Jones potential of Falk and a shorter-range potential of our own devising Crystalline samples prepared with these potentials exhibit normal brittle fracture, with running cracks Only when subjected to a constantly increasing strain did the crack in the glasses lengthen Even here, once the strain stopped increasing, the crack tip stopped moving We examined the stress-strain curve for our model materials, and found that they did not exhibit the saturated stress plateau characteristic of ductile materials We attribute the failure of crack propagation in this system to the availability of many meta-stable configurations available to the crack tip which serve to soak up the energy which would otherwise drive the crack We conclude that this class of models is inadequate to describe crack propagation in brittle glasses