Shape and scale dependent diffusivity of colloidal nanoclusters and aggregates

© 2016, EDP Sciences and Springer.The diffusion of colloidal nanoparticles and nanomolecular aggregates, which plays an important role in various biophysical and physicochemical phenomena, is currently under intense study. Here, we examine the shape and size dependent diffusion of colloidal nano- particles, fused nanoclusters and nanoaggregates using a hybrid fluctuating lattice Boltzmann-Molecular Dynamics method. We use physically realistic parameters characteristic of an aqueous solution, with explicitly implemented microscopic no-slip and full-slip boundary conditions. Results from nanocolloids below 10 nm in radii demonstrate how the volume fraction of the hydrodynamic boundary layer influences diffusivities. Full-slip colloids are found to diffuse faster than no-slip particles. We also characterize the shape dependent anisotropy of the diffusion coefficients of nanoclusters through the Green-Kubo relation. Finally, we study the size dependence of the diffusion of nanoaggregates comprising N ≤ 108 monomers and demonstrate that the diffusion coefficient approaches the continuum scaling limit of N−1/3

Simulations mésoscopiques de fluides complexes (suspensions colloïdales)

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Auto-organisation de surfaces cristallines pendant la croissance épitaxiale (une étude théorique)

CLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Kinetically driven ordered phase formation in binary colloidal crystals

International audienceimulations for dilute suspensions. It is shown that, under appropriate conditions, the formation of colloidal crystals is dominated by kinetic effects leading to the growth of well-ordered crystallites of the sodium-chloride (NaCl) bulk phase. These crystallites form with very high probability even when the cesium-chloride (CsCl) phase is more stable thermodynamically. Global optimization searches show that this result is not related to the most favorable structures of small clusters, which are either amorphous or of the CsCl structure. The formation of the NaCl phase is related to the specific kinetics of the crystallization process, which takes place by a two-step mechanism. In this mechanism, dense fluid aggregates form at first and then crystallization follows. It is shown that the type of short-range order in these dense fluid aggregates determines which phase is finally formed in the crystallites. The role of hydrodynamic effects in the aggregation process is analyzed by stochastic rotation dynamics - molecular dynamics simulations, and we find that these effects do not play a major role in the formation of the crystallite

Study of the B1-B2 transition in colloidal clusters

International audienceThe possible mechanisms for the B1 (NaCl-type) to B2 (CsCl-type) transition in crystalline colloidal clusters of equally sized particles are studied by means of two computational techniques: metadynamics and nudged elastic band calculations. The system is modelled by a screened Coulomb potential. Different interaction ranges are considered. The transition from a perfect NaCl cubic cluster to a full CsCl cluster is forced by metadynamics, revealing a transition path with intermediate metastable configurations in which planes are shifted one by one. The presence of metastable configurations in the transition path, corresponding to a certain number of NaCl planes turned into CsCl, has clear analogies with the known Hyde and O'Keeffe mechanism for ionic crystals, with some important differences due to finite-size effects. These comprise the fact that the transition starts by shifting a surface plane by means of a row-by-row mechanism that has no analog in bulk crystals. The energy barriers between the local minima in the transition path are calculated, showing that the barriers strongly depend on the screening length, in such a way that the B1 metastable phase can have very long lifetimes when the interaction is sufficiently long-range

Hydrodynamics in simulations of colloidal suspensions

Cette thèse est consacrée à l'étude numérique des suspensions colloïdales diluées et concentrées, où les interactions hydrodynamiques (IH) jouent un rôle important. Pour cela, des simulations numériques basées sur la technique hybride "stochastic rotation dynamics - molecular dynamics" (SRD-MD) ont été effectuées. Cette technique permet d'inclure efficacement les IH. L'étude du coefficient de diffusion des colloïdes en fonction de la fraction volumique colloïdes fait l'objet de la première partie de cette thèse. Les choix des paramètres de la simulation SRD-MD et des différentes échelles (longueur, masse et temps) sont adaptés à le simulation de supensions colloïdales sur conditions réalistes. Les résultats obtenus ont été comparés à certaines prédictions théoriques, à des résultats obtenus par d'autres techniques de simulation (accelerated Stokesian dynamics, lattice-Boltzmann et dissipative particle dynamics), ainsi qu'à des résultats expérimentaux. L'étude a montré que notre implémentation de la SRD-MD permet à plusieurs égards de correctement décrire l'hydrodynamique dans les suspensions colloïdales simulées : l'effet de taille finie de la boîte de simulation et le comportement diffusif aux temps courts sont bien reproduits. Dans la deuxième partie de cette thèse, le rôle de l'hydrodynamique dans les processus d'agrégation de particules colloïdales monodisperses, soumises à des interactions fortement attractives, est étudié pour une fraction volumique en colloïdes allant de 2,5 à 20%. Une attention particulière est accordée à l'étude de la cinétique d'agrégation (évolution temporelle de la taille des agrégats, du nombre de particules libres, etc...) et du coefficient de diffusion de petits agrégats. Pour cela, trois techniques de simulation sont comparées : la dynamique brownienne (avec et sans IH) et la SRD-MD. Il est démontré que l'inclusion des IH n'a pas une grande influence sur le processus d'agrégation dans son ensemble : le temps auquel un seul cluster est obtenu dans la simulation est essentiellement le même avec les trois techniques. Toutefois, les IH ralentissent la formation de dimères dans la première étape de nucléation et accélèrent la coalescence des agrégats dans la seconde phase du processus, parce qu'elles augmentent le coefficient de diffusion des agrégats. Il est aussi montré que les IH ralentissent la cinétique de réorganisation de trimères.This thesis is dedicated to the numeral study of colloidal suspensions under dilute and crowded conditions, where hydrodynamics interactions play an important role. For this purpose, computer simulations based on the hydrid stochastic rotation dynamics-molecular dynamics (SRD-MD) technique, which solves efficiently the hydrodynamics of colloids embedded in a liquid, are carried out. A study of the colloidal tracer diffusion coefficient as a function of the colloidal volume fraction is a subject of invertigation in the first part of this thesis. This choices for the SRD-MD simulation parameters and for the different scales (length, mass and time) are carefully adapted to simulate colloidal suspensions under realistic conditions. The results are compared with some theoretical predictions, and also with the results obtained by other simulation techniques (accelerated Stokesian dynamics, lattice-Boltzmann and dissipative particle dynamics), as well as this experimental results. The study shows, that our implementation of the SRD-MD allows describing carefully the hydrodynamics in simulated colloidal suspensions : the finite size effect of the simulation box and diffusive behavior in the short time are correctly reproduced. In the second part of this thesis the role of hydrodynamics interactions (HIs) in the aggregation process with strongly attractive monodisperse colloidal particles is studied for a colloid volume fraction ranging from 2.5 to 20%. Special attention is paid to the study of aggregation kinetics (time evolution of the cluster size, the number of free particles, etc.) and the diffusion coefficient of small aggregates. To this end, the comparison of the aggregation kinetics in between three simulation techniques is carried out : Brownian dynamics (with and without HIs) and SRD-MD. It is shown that the inclusion of HIs has no signifiant influence on the aggregation kinetics for the whole aggregation process, and the time at which a single cluster is obtained in the simulation is essentially the same process, and the time at which a single cluster is obtained int the simulation is essentially the same in the different techniques. However, HIs slow down the dimer formation in the nucleation stage and accelerate the aggregate coalescence in the second stage of the process, because of an increase of the cluster diffusion coefficients. It is also shown that HIs slow down the reorganization kinetics of trimers.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Colloidal suspension by SRD-MD simulation on GPU

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