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David T Wu

Publications and source records attributed to David T Wu.

7 recordsLinked to original sources
PubMed2006

A new procedure for analyzing the nucleation kinetics of freezing in computer simulation.

A new method for deriving the size of the critical nucleus and the Zeldovich factor directly from kinetic data is presented. Moreover, in principle, the form of G(n), the free energy of formation of nuclei consisting of n molecules, can be inferred. The method involves measuring times of first appearance of nuclei of size n in the transient regime and applying the Becker-Doring theory. Times of first appearance exhibit the same characteristics as the conventional times associated with N(n,t), the number of nuclei of at least size n per unit volume that have materialized at time t. That is, they are well represented by three nucleation parameters, the reduced moment, the time lag, and the steady state nucleation rate. But unlike the conventional steady state rate which is independent of n, the steady state times of first appearance vary with n. In order to characterize the three nucleation parameters with precision, however, thousands of independent stochastic events with known n are required. Such sets of data are readily generated in molecular dynamic simulations but, so far, not in laboratory experiments. Results are illustrated by an analysis of simulations of the spontaneous freezing of large clusters of SeF6.

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PubMed2006

Binary to unary transition problem in nucleation theory.

The limiting behavior in binary nucleation theory when one component vanishes is examined. Failure of the traditional continuum theory to predict the transition from binary to unary nucleation is due to breakdown of the continuum approximation. A semidiscrete model is constructed for the transition regime. For binary systems of arbitrary composition, a general formula that combines the semi-discrete and continuum nucleation rates is proposed. Comparison with the exact solution calculated using a fully discrete matrix method shows good agreement.

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PubMed2006

Direct measurement of colloidal particle rotation and field dependence in alternating current electrohydrodynamic flows.

We have measured the influence of both applied alternating current (AC) field strength and frequency on the electrohydrodynamic (EH) flows present in colloidal systems near an electrode surface. The effect of the flows is visualized by the rotation of the colloids, fluorescently labeled by a novel technique involving EH-driven aggregation of much smaller tracer colloids to the surface of the larger colloids. Our results show an E2 dependence of these flows, consistent with an induced charge mechanism for effective colloidal interactions. We have also observed a crossover in frequency that suggests a change in the origin of the induced charge, consistent with predictions from available theory. The EH flows appear to be hydrodynamically screened inside clusters, as evidenced by the lack of rotation of interior colloids and the cluster-size independent rotation rate of colloids on the boundary.

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PubMed2004

Invariance of density correlations with charge density in polyelectrolyte solutions.

We present a theory for the equilibrium structure of polyelectrolyte solutions. A simple and general optimization method is introduced that enables theories such as the random phase approximation to handle the strong repulsive forces present in such systems. Quantitative comparison is made with data from recent neutron-scattering experiments of randomly charged, hydrophilic polymers in salt-free, semidilute solution at various charge densities. We show that the invariance observed at high charge fraction may be the result of polymer-polymer correlations, rather than the conventional explanations involving counterion condensation.

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PubMed2004

In vitro papillomavirus capsid assembly analyzed by light scattering.

Pentamers of the L1 major capsid protein of human papillomavirus (HPV type 11) were purified after expression in E. coli and analyzed for the kinetics of in vitro capsid self-assembly using multi-angle light scattering (MALS). Pentamers self-assembled into capsid-like structures at a rate that was a function of protein concentration. The kinetics of capsid formation were sigmoidal with a concentration-dependent lag phase, followed by a rapid increase in polymerization. Nucleation size and the rate order of subsequent subunit addition were calculated from the concentration dependence of the extent of capsid formation and the rate of the fast phase, respectively. Assembly was second order with a nucleation size of two pentamers. Thus, we suggest that dimers of pentamers are the nucleus for L1 assembly into capsid-like structures, with rapid sequential addition of single pentamers to the growing shell. Although studied in vitro without accessory factors that may be present in vivo, these data are in contrast with the "five-around-one" assembly nucleus previously proposed for polyomaviruses.

Capsid↗
PubMed2004

Anomalous mixing behavior of polyisobutylene/polypropylene blends: molecular dynamics simulation study.

The unusual mixing behavior of polyisobutylene (PIB) with head-to-head (hhPP) and head-to-tail polypropylene (PP) is studied using large-scale molecular dynamics (MD). The heats of mixing and Flory chi parameters were computed from MD simulations of both blends using a united atom model. The chi parameters from the simulations were estimated from the structure factors using the random phase approximation in analogy with neutron scattering (SANS) experiments. MD simulations for syndiotactic hhPP/PIB predicted a lower critical solution temperature with a chi parameter in very good agreement with SANS experiments on the atactic hhPP/PIB blend. MD simulations also predicted that the isotactic PP/PIB blend was immiscible at high molecular weight in qualitative agreement with cloud point measurements on atactic PP/PIB.

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