Biomedical subjects

Surajit Sengupta

Publications and source records attributed to Surajit Sengupta.

5 recordsLinked to original sources

PubMed2006

Direct test of defect-mediated laser-induced melting theory for two-dimensional solids.

We investigate by direct numerical solution of appropriate renormalization flow equations the validity of a recent dislocation unbinding theory for laser-induced freezing and melting in two dimensions. The bare elastic moduli and dislocation fugacities are obtained for three different two-dimensional systems namely, the hard disk, inverse 12th power, and Derjaguin-Landau-Verwey-Overbeek potentials. A restricted Monte Carlo simulation sampling only configurations without dislocations is used to obtain these quantities. These are then used as inputs to the flow equations. Numerical solution of the flow equations then yields the phase diagrams. We conclude that (a) the flow equations need to be correct at least up to third order in defect fugacity to reproduce meaningful results, (b) there is excellent quantitative agreement between our results and earlier conventional Monte Carlo simulations for the hard disk system, and (c) while the qualitative form of the phase diagram is reproduced for systems with soft potentials there is some quantitative discrepancy which we explain.

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PubMed2005

Stress relaxation in a perfect nanocrystal by coherent ejection of lattice layers.

We show that a small crystal trapped within a potential well and in contact with its own fluid responds to large compressive stresses by a novel mechanism--the transfer of complete lattice layers across the solid-fluid interface. Further, when the solid is impacted by a momentum impulse set up in the fluid, a coherently ejected lattice layer carries away a definite quantity of energy and momentum, resulting in a sharp peak in the calculated phonon absorption spectrum. Apart from its relevance to studies of stability and failure of small sized solids, such coherent nanospallation may be used to make atomic wires or monolayer films.

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PubMed2004

Constrained deformation of a confined solid: anomalous failure by nucleation of smectic bands.

We report results of computer simulations of the deformation and failure behavior of a thin crystalline strip of "hard disks" in two dimensions confined within a quasi-one-dimensional "hard-wall" channel of fixed width corresponding to a few disk diameters. Starting from a commensurate triangular solid, stretching the strip along its length introduces a rectangular distortion. This, beyond a critical strain, leads to failure of the solid by "phase separation" into alternating bands of solid and smectic-like phases. The critical strain is inversely proportional to the channel width, i.e., thinner strips are stronger. The large plastic deformation which precedes failure is observed to be reversible.

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PubMed2003

Nucleation of solids in solids: ferrites and martensites.

When a solid such as iron is cooled across a structural transition, its final microstructure depends sensitively on the cooling rate and the depth of quench. For instance, an infinitesimally slow cooling or a shallow quench results in an equilibrium "ferrite," while a rapid cooling or a deep quench gives rise to a metastable twinned "martensite." In this paper, we arrive at a single formalism which qualitatively describes the transformation to both a ferrite and a martensite. Fundamental to this understanding is our identification of the crucial dynamical role played by nonelastic degrees of freedom in determining the final microstructure of the product solid.

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PubMed2002

Growing smooth interfaces with inhomogeneous moving external fields: dynamical transitions, devil's staircases, and self-assembled ripples.

We study the steady state structure and dynamics of an interface in a pure Ising system on a square lattice placed in an inhomogeneous external field with a profile designed to stabilize a flat interface and translated with velocity v(e). For small v(e), the interface is stuck to the profile, is macroscopically smooth, and is rippled with a periodicity in general incommensurate with the lattice parameter. For arbitrary orientations of the profile, the local slope of the interface locks in to one of infinitely many rational values (devil's staircase) which most closely approximates the profile. These "lock-in" structures and ripples disappear as v(e) increases. For still larger v(e) the profile detaches from the interface.

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