Biomedical subjects

Brian B Laird

Publications and source records attributed to Brian B Laird.

7 recordsLinked to original sources

PubMed2006

Kinetic coefficient for hard-sphere crystal growth from the melt.

Using molecular-dynamics simulation, we determine the magnitude and anisotropy of the kinetic coefficient (mu) for the crystal growth from the melt for the hard-sphere system through an analysis of equilibrium capillary fluctuations in interfacial height. We find mu100 = 1.44(7), mu110 = 1.10(5), and mu111 = 0.64(3) in units of square root (kB/(mTm)), where kB is Boltzmann's constant, m is the particle mass, and Tm is the melting temperature. These values are shown to be consistent, with some exceptions, with those obtained in recent simulation results a variety of fcc metals, when expressed in hard-sphere units. This suggests that the kinetic coefficient for fcc metals can be roughly estimated from C square root (R/(MTm)), where R is the gas constant, M is the molar mass, and C is a constant that varies with interfacial orientation.

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PubMed2006

The anisotropic hard-sphere crystal-melt interfacial free energy from fluctuations.

We have calculated the interfacial free energy for the hard-sphere system, as a function of crystal interface orientation, using a method that examines the fluctuations in the height of the interface during molecular dynamics simulations. The approach is particularly sensitive for the anisotropy of the interfacial free energy. We find an average interfacial free energy of gamma=0.56+/-0.02k(B)Tsigma(-2). This value is lower than earlier results based upon direct calculations of the free energy [R. L. Davidchack and B. B. Laird, Phys. Rev. Lett. 85, 4751 (2000)]. However, both the average value and the anisotropy agree with the recent values obtained by extrapolation from direct calculations for a series of the inverse-power potentials [R. L. Davidchack and B. B. Laird, Phys. Rev. Lett. 94, 086102 (2005)].

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PubMed2006

Calculation of the crystal-melt interfacial free energy of succinonitrile from molecular simulation.

The crystal-metal interfacial free energy for a six-site model of succinonitrile [N triple bond C-(CH(2))(2)-C triple bond N] has been calculated using molecular-dynamics simulation from the power spectrum of capillary fluctuations in interface position. The orientationally averaged magnitude of the interfacial free energy is determined to be (7.0+/-0.4)x10(-3) J m(-2). This value is in agreement (within the error bars) with the experimental value [(7.9+/-0.8)x10(-3) J m(-2)] of Marasli et al. [J. Cryst. Growth 247, 613 (2003)], but is about 20% lower than the earlier experimental value [(8.9+/-0.5)x10(-3) J m(-2)] obtained by Schaefer et al. [Philos. Mag. 32, 725 (1975)]. In agreement with the experiment, the calculated anisotropy of the interfacial free energy of this body-centered-cubic material is small. In addition, the Turnbull coefficient from our simulation is also in agreement with the experiment. This work demonstrates that the capillary fluctuation method of Hoyt et al. [Phys. Rev. Lett. 86, 5530 (2001)] can be successfully applied to determine the crystal-melt interfacial free energy of molecular materials.

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PubMed2005

Direct calculation of the crystal-melt interfacial free energy via molecular dynamics computer simulation.

We review our recent work on the direct calculation of the interfacial free energy, gamma, of the crystal-melt interface via molecular dynamics computer simulation for a number of model systems. The value of gamma as a function of crystal orientation is determined using a thermodynamic integration technique employing moving cleaving walls [Phys. Rev. Lett. 2000, 85, 4751]. The calculation is sufficiently precise to resolve the small anisotropy in gamma, which is crucial in determining the kinetics and morphology of dendritic growth. We report values of gamma for the hard-sphere and Lennard-Jones systems, as well as recent results on the series of inverse-power potentials. For the inverse sixth-, seventh-, and eighth-power systems, we determine gamma for both fcc and bcc crystal structures. For these systems, the bcc-melt gamma is lower than that for fcc crystals by about 25%, consistent with recent experiments and computer simulations on fcc-forming systems that show preferential formation of bcc nuclei in the initial stages of crystallization. In addition, we show that our results give a molecular interpretation of Turnbull's rule, which is the empirical relationship between gamma and the enthalpy of fusion.

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PubMed2005

Crystal structure and interaction dependence of the crystal-melt interfacial free energy.

We examine via molecular simulation the dependence of the crystal-melt interfacial free energy gamma on molecular interaction and crystal structure (fcc vs bcc) for systems interacting with inverse-power repulsive potentials, u(r)=epsilon(sigma/r)(n), 6< or =n< or =100. Both the magnitude and anisotropy of gamma are found to increase as the range of the potential increases. Also we find that gamma(bcc)<gamma(fcc), consistent with recent observations that some fcc forming fluids nucleate via formation of metastable bcc nuclei. The anisotropy in gamma is also seen to be smaller in the bcc systems. By extrapolation, we also obtain an improved estimate of gamma for hard spheres.

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PubMed2004

Structure of a soft-sphere fluid at a soft repulsive wall: a comparison of weighted density-functional theories.

Density-functional theory is used to investigate the structure of a soft-sphere fluid at a soft wall. The fluid is modeled by an inverse sixth-power repulsive pair potential and the fluid particles interact with a flat stationary wall defined by an inverse-twelth power repulsive external potential. For comparison we examine results using three weighted density approximations (WDA), namely those due to Phys. Rev. A 32, 2909 (1985)]], Phys. Rev. A 39, 426 (1989)]], and the partitioned WDA of Mol. Phys. 90, 951 (1997)]]. The degree to which each of these approximations can accurately predict the structure of the fluid at the wall is evaluated for several densities through comparison with Monte Carlo simulation data.

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PubMed2003

Generalized dynamical thermostating technique.

We demonstrate that the Nosé method for constant-temperature molecular-dynamics simulation [Mol. Phys. 52, 255 (1984)] can be substantially generalized by the addition of auxiliary variables to encompass an infinite variety of Hamiltonian thermostats. Such thermostats can be used to enhance ergodicity in systems, such as the one-dimensional harmonic oscillator or certain molecular systems, for which the standard Nosé-Hoover methods fail to reproduce converged canonical distributions. In this respect the method is similar in spirit to the method of Nosé-Hoover chains, but is both more general and Hamiltonian in structure (which allows for the use of efficient symplectic integration schemes). In particular, we show that, within the generalized Nosé formalism outlined herein, any Hamiltonian system can be thermostated with any other, including a copy of itself. This gives one an enormous flexibility in choosing the form of the thermostating bath. Numerical experiments are included in which a harmonic oscillator is thermostated with a collection of noninteracting harmonic oscillators as well as by a soft billiard system.

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