Biomedical subjects

Jerome Delhommelle

Publications and source records attributed to Jerome Delhommelle.

10 recordsLinked to original sources

PubMed2006

Insights into the molecular mechanism underlying polymorph selection.

We use molecular simulations to study polymorph selection during the crystallization of charge-stabilized colloidal suspension. By modifying the conditions of crystallization, we invert the stability of two polymorphs and induce the formation of crystallites whose structure is predominantly that of the stable polymorph. However, our simulations reveal that kinetics play a major role not only during the nucleation step but also in the growth mechanism. The growth of postcritical crystallites of the stable polymorph proceeds through a complex mechanism involving the cross-nucleation of a third metastable polymorph followed by the conversion of this third polymorph into the stable structure.

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PubMed2006

Molecular mechanism for the cross-nucleation between polymorphs.

We use molecular simulations to study the early stages of crystallization in a supercooled liquid of Lennard-Jones particles. We observe the onset of concomitant polymorphism and demonstrate that this phenomenon results from the cross-nucleation of a metastable polymorph on the stable polymorph. We also show that cross-nucleation is selective as it only takes place between polymorphs of almost equivalent free energy. Our simulations provide detailed insights into the molecular mechanism underlying concomitant polymorphism and cross-nucleation between polymorphs.

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PubMed2005

Conductivity of molten sodium chloride in an arbitrarily weak dc electric field.

We use nonequilibrium molecular-dynamics (NEMD) simulations to characterize the response of a fluid subjected to an electric field. We focus on the response for very weak fields. Fields accessible by conventional NEMD methods are typically of the order of 10(9) V m(-1), i.e., several orders of magnitude larger than those typically used in experiments. Using the transient time-correlation function, we show how NEMD simulations can be extended to study systems subjected to a realistic dc electric field. We then apply this approach to study the response of molten sodium chloride for a wide range of dc electric fields.

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PubMed2005

Shear thickening in a model colloidal suspension.

We study the rheology of model colloidal suspensions using molecular-dynamics simulations. We relate the onset of shear thickening to the transition from a low-viscosity regime, in which the solvent facilitates the flow of colloids, to a high-viscosity regime associated with jamming of the colloids and the formation of chains of colloids. In the low-viscosity regime, the colloidal particles are, on average, surrounded by two layers of solvent particles. On the contrary, in the high-viscosity regime, the solvent is expelled from the interstice between the jammed colloids. The thickening in suspensions is shown to obey the same criterion as in simple fluids. This demonstrates that jamming, even without the divergence of lubrication interactions, is sufficient to observe shear thickening.

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PubMed2005

Hydrogen bonding in ethanol under shear.

We study the dependence of viscosity of ethanol on shear rate using constant volume and constant pressure nonequilibrium molecular dynamics simulations, with the emphasis of the interrelationship between breaking, stability, and alignment of hydrogen bonds and shear thinning at high shear rates. We find that although the majority of hydrogen bond breakings occur at low shear rates, we do not observe shear thinning until there is some shear-induced alignment of the hydrogen bonds with the direction of shear.

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PubMed2005

Atomistic simulation of the homogeneous nucleation and of the growth of N2 crystallites.

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N(2) for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N(2). We show that, in accord with Ostwald's rule of stages, the structure of the critical nucleus is predominantly that of a metastable polymorph (alpha-N(2) for the state point investigated). We then monitor the evolution of several critical nuclei through a series of unbiased molecular dynamics trajectories. The growth of N(2) crystallites is accompanied by a structural evolution toward the stable polymorph beta-N(2). The microscopic mechanism underlying this evolution qualitatively differs from that reported previously. We do not observe any dissolution or reorganization of the alpha-like core of the nucleus. On the contrary, we show that alpha-like and beta-like blocks coexist in postcritical nuclei. We relate the structural evolution to a greater adsorption rate of beta-like molecules on the surface and show that this transition actually starts well within the precritical regime. We also carefully investigate the effect of the system size on the height of the free energy barrier of nucleation and on the structure and size of the critical nucleus.

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PubMed2005

Should "lane formation" occur systematically in driven liquids and colloids?

We report on nonequilibrium molecular dynamics simulations of binary mixtures of particles in a color field. Both nonequilibrium molecular dynamics and Brownian dynamics generally assume that the mechanical noise is of thermal origin only and that, at a given temperature, its amplitude remains constant however strong the applied field is. We show that this postulate systematically results in the strong ordering of particles into lanes. By applying a nonequilibrium molecular dynamics method which does not exert any constraint on the noise amplitude, we show that releasing this constraint prevents the systematic "lane formation" from occurring. We observe the onset of density inhomogeneities and jamming instead. This behavior is reminiscent of the shear-thickening regime observed experimentally on colloidal suspensions and in simulations taking into account hydrodynamic interactions.

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PubMed2004

Reorganization and growth of metastable alpha-N2 critical nuclei into stable beta-N2 crystals.

We report on results on the crystal nucleation and growth of nitrogen. Using molecular dynamics simulations, we show that while nucleation proceeds into the metastable alpha-phase (i.e., the crystalline phase associated with the lowest free energy barrier of formation), growth of the crystallite proceeds through a reorganization of the nucleus into the thermodynamically stable beta-phase.

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PubMed2004

Non-Newtonian behavior in simple fluids.

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheology of a microscopic sample of simple fluid. The calculations were performed using a configurational thermostat which unlike previous nonequilibrium molecular dynamics or nonequilibrium Brownian dynamics methods does not exert any additional constraint on the flow profile. Our findings are in agreement with experimental results on concentrated "hard sphere"-like colloidal suspensions. We observe: (i) a shear thickening regime under steady shear; (ii) a strain thickening regime under oscillatory shear at low frequencies; and (iii) shear-induced ordering under oscillatory shear at higher frequencies. These results significantly differ from previous simulation results which showed systematically a strong ordering for all frequencies. They also indicate that shear thickening can occur even in the absence of a solvent.

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PubMed2003

Reexamination of string phase and shear thickening in simple fluids.

In 1984, Erpenbeck observed a shear-induced alignment of particles into strings in nonequilibrium molecular dynamics simulations of hard spheres. Since then, it has remained unclear if this effect was genuine or if it arose from the use of a thermostat which assumed an incorrect form for the velocity profile. All studies performed up to now have focused on improving the accuracy with which the velocity profile is determined. We propose here a radically different approach: we apply a recently developed configurational expression for the temperature. This expression does not require any knowledge of the streaming velocity profile. Using a configurational thermostat, we show that the string phase is an artifact and we observe a shear-thickening regime, as seen in experiments on concentrated "hard-sphere"-like colloidal dispersions.

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