Aggregation in colloidal suspensions: evaluation of the role of hydrodynamic interactions by means of numerical simulations.

Numerical simulations constitute a precious tool for understanding the role of key parameters influencing the colloidal arrangement in suspensions, which is crucial for many applications. The present paper investigates numerically the role of hydrodynamic interactions on the aggregation processes in colloidal suspensions. Three simulation techniques are used: Brownian dynamics without hydrodynamic interactions, Brownian dynamics including some of the hydrodynamic interactions, using the Yamakawa-Rotne-Prager tensor, and stochastic rotation dynamics coupled with molecular dynamics. A system of monodisperse colloids strongly interacting through a generalized Lennard-Jones potential is studied for a colloid volume fraction ranging from 2.5 to 20%. Interestingly, effects of the hydrodynamic interactions are shown in the details of the aggregation processes. It is observed that the hydrodynamic interactions slow down the aggregation kinetics in the initial nucleation stage, while they speed up the next cluster coalescence stage. It is shown that the latter is due to an enhanced cluster diffusion in the simulations including hydrodynamic interactions. The higher the colloid volume fraction, the more pronounced the effects on the aggregation kinetics. It is also observed that hydrodynamic interactions slow down the reorganization kinetics. It turns out that the Brownian dynamics technique using the Yamakawa-Rotne-Prager tensor tends to overestimate the effects on cluster diffusion and cluster reorganization, even if it can be a method of choice for very dilute suspensions.

Aggregation kinetics and gel formation in modestly concentrated suspensions of oppositely charged model ceramic colloids: a numerical study.

Aggregation kinetics and gel formation in aqueous suspensions that undergo heteroaggregation are studied by means of Brownian dynamics simulations. The simulated system, described in a previous paper [M. A. Piechowiak, A. Videcoq, F. Rossignol, C. Pagnoux, C. Carrion, M. Cerbelaud, R. Ferrando, Langmuir, 2010, 26(15), 12540-12547.], is constituted of two kinds of synthesized, almost equally sized colloids: silica particles that are negatively charged and alumina-coated silica particles that are positively charged. The interactions between colloids are modeled by the DLVO potential. Several compositions are analyzed, from silica-rich to alumina-rich cases. The particle volume fraction φ is varied in the range 6-12%. The study of the aggregation kinetics allows us to clarify the effect of those variations on the clustering process. Gelation is analyzed by detection of spanning clusters in each x-, y-, z-direction of the cubic simulation box. Percolating networks start to be observed from φ = 7%, a low value of the volume fraction close to the solid volume fraction experimentally measured in sediments of those suspensions.

Oppositely charged model ceramic colloids: numerical predictions and experimental observations by confocal laser scanning microscopy.

Fluorescent silica and alumina-like spherical particles with almost equal sizes are synthesized. Dilute aqueous suspensions are prepared with various ratios of those colloidal particles that exhibit opposite surface charges. These suspensions undergo heteroaggregation for a wide range of compositions. The structure of the formed aggregates is analyzed by means of confocal microscopy. The experimental results are compared to those of Brownian dynamics simulations in which the interactions between colloids are modeled by the DLVO potential. Good agreement between experiments and simulations is obtained.

Risk factors for atrial fibrillation in adult patients in long-term observation following surgical closure of atrial septal defect type II.

BACKGROUND: The aim of the study was to find the factors predictive for paroxysmal atrial fibrillation (AF) following surgical correction of atrial septal defect type II (ASD t.II). METHODS: 93 patients, who underwent isolated surgical closure of ASD t.II between 1990 and 2001 were included. Follow-up studies were performed 2 - 11 years after surgery. Patients were divided into two groups according to the presence of AF before and after surgery. Group AF (+) consisted of 29 and group AF (-) of 64 patients. All patients underwent echocardiography, electrocardiogram (ECG) at rest, and signal-averaged P-wave duration (PWD) in signal-averaged ECG. The following parameters were assessed in echocardiography: pulmonary artery systolic pressure, left and right atrial dimensions, right ventricular dimension, tricuspid and mitral regurgitation. RESULTS: Paroxysmal AF was observed in 27 patients before surgery and in 29 after surgery. Analyzing all potential risk factors we proved that PWD may independently predict occurrence of postoperative AF. CONCLUSION: PWD may independently predict postoperative AF in long-term follow-up after surgical correction of ASD t.II.