A novel Monte Carlo - Discrete Element Method approach for the micro-mechanics of colloidal suspensions

This thesis is focused on the development of a novel computational tool capable of predicting the behaviour of dilute colloidal suspensions. The work has been focused in particular on the investigation of the aggregation and breakup phenomena occurring under the effect of a spatially uniform shear flow. Aggregation and breakup phenomena have in fact significant effects on the size of the dispersed particles, their shape, their composition, the distribution of these quantities over the entire population of particles and, consequently, on the macroscopic properties of the suspension. Frequently, the investigation of colloidal suspensions is conducted resorting to Population Balance Equations (PBE); by this method it is possible to follow the dynamics of a colloidal suspension, or more in general of a disperse system, by simply imposing the conservation of mass throughout the process. However, several factors hinder the application of PBE to colloidal suspensions such as the lack of reliable models for aggregation and breakup, their inherent inability to fully take into account the disordered structure of colloidal aggregates and their integro-differential nature which asks for complex solution techniques. Discrete Element Methods (DEM) represent a valid alternative to study colloidal suspensions; DEM simulations assume colloidal clusters to be composed by a number of distinct elements, each one subject to the colloidal forces arising from the interaction with nearby elements and to the hydrodynamic forces due to the interaction with the dispersing medium. Introducing models for such forces, DEM simulations can track the motion of each individual particle of a cluster, providing valuable insight into the suspension dynamics. However, the applicability of DEM is hindered by the high computational cost, which so far has restricted its use to the study of single aggregates or at most of very small populations. In this work, in order to circumvent the high computational cost typically associated to pure DEM simulations and to deal with the uncertainty which affects the PBE approach, a novel method was developed: it is a mixed stochastic-deterministic numerical method which couples the mean-field approach of PBE (solved stochastically with a Monte Carlo (MC) algorithm) with detailed DEM simulations; the basic idea behind such a combination is that the dynamics of a dilute suspension is determined by a sequence of binary encounter events between the suspended particles, each of which can result into an aggregation, a breakage, a restructuring of the aggregates or into any combination of these phenomena. Therefore, the MC is used to sample a sequence of such events and the DEM is used to accurately simulate them; the advantage of such a combination is that the DEM is used to track the motion of just two clusters at a time. The developed MC-DEM was proven to be a flexible and reliable tool; it was used to investigate some typical phenomena occurring in colloidal suspensions, returning valuable insights both in the aggregation/breakup dynamics and in the morphology of colloidal clusters

Dynamics of a shear-induced aggregation process by a combined Monte Carlo-Stokesian Dynamics approach

In the present work we investigated the collision efficiency of colloidal aggregates suspended in a shear flow. A Discrete Element Method (DEM), built in the framework of Stokesian Dynamics, was developed to model hydrodynamic and colloidal interactions acting on each primary particle composing the aggregates. Aggregates with complex geometries were generated by means of a combined DEM-Monte Carlo algorithm able to reproduce a shear-induced aggregation process occurring in a dilute colloidal suspension. Simulations, involving pairs of aggregates, were conducted according to a grid-based technique, in order to evaluate collision efficiencies. Size disproportion between aggregates and morphology shape anisotropy emerged as the principal causes affecting collision efficiencies. This work constitutes a first attempt to extend the traditional Von Smoluchowski’s theory of shear-induced coagulation of spherical particles to the case of randomly-structured aggregates

Aggregation and breakup of colloidal particle aggregates in shear flow: A combined Monte Carlo - Stokesian dynamics approach

A method for the simulation of aggregation and breakup processes in colloidal particle suspensions is presented. The method combines a Monte Carlo algorithm to determine, on the basis of probabilistic considerations, the sequence of aggregation and breakup events, and a Discrete Element Method, built in the framework of Stokesian dynamics and contact mechanics, to accurately reproduce them. Liquid-solid suspensions subject to a uniform shear stress are investigated. The model is seen to be able to reproduce the typical dynamic steady state which is observed in colloidal suspensions under severe shearing, in which the effects of aggregation and breakup balance each other. The structural properties of the aggregates and the dynamics of the aggregation and breakup phenomena are characterized in detail. Both fragmentation and erosion are seen to contribute to the breakup process, which is characterized by an exponent similar to the one reported in the literature for compact clusters

Dynamics of shear induced aggregation through a combined Monte Carlo-Stokesian dynamics approach

Several methods have been proposed to investigate the dynamics of processes including aggregation and breakup of colloidal particles. Most approaches resort to Population Balance Equations, often solved in a stochastic way (Monte Carlo methods). This method has a relatively low computational cost, but is not completely predictive, in that it needs models for the rates of aggregation and breakup and the morphology of the aggregates. On the contrary, highly accurate and fully predictive description of single aggregation or breakup events can be obtained by Discrete Element Methods (DEMs), where the motion of each primary particle of an aggregate is tracked by solving its equation of motion. However, so far the high computational cost of DEMs has restricted their use to the simulation of short sequences of events, thus preventing their application to representative samples of a population of particles. The present work aims to investigate the mechanism of flow-induced coagulation of a large population of particles suspended in an aqueous medium and subject to uniform shear flow. The developed method combines a Monte Carlo approach to determine, on the basis of probabilistic considerations, the sequence of aggregation and breakup events and the clusters involved, and a Discrete Element Method, built in the framework of Stokesian Dynamics, to accurately reproduce the event; the DEM model is able, in fact, to evaluate the fluid-dynamic stresses acting on each monomer and to model properly the inter-particle interactions: besides Van der Waals attraction and contact forces between monomers, an elastic spring-like model, to give tangential and torsional resistance to the bonded monomers, has been implemented; this model has been proven to reproduce accurately the resistance to relative motion that monomers exhibit at the contact area. Simulations were performed to predict the dynamic behavior of the suspension with particular regard to the determination of size distribution, morphologies of aggregates and their temporal evolution, as determined by the simultaneous aggregation and breakage processes. Results highlight that collision efficiencies, breakage phenomena and, consequently, size distributions are significantly influenced by the flow field intensity. Results are encouraging and demonstrate that a Monte Carlo approach combined with a DEM model can be a powerful tool for the study of the dynamics of a colloidal particle population

Alterations in oxidative, inflammatory and apoptotic events in short-lived and long-lived mice testes

Aged testes undergo profound histological and morphological alterations leading to a reduced functionality. Here, we investigated whether variations in longevity affect the development of local inflammatory processes, the oxidative state and the occurrence of apoptotic events in the testis. To this aim, well-established mouse models with delayed (growth hormone releasing hormone-knockout and Ames dwarf mice) or accelerated (growth hormone-transgenic mice) aging were used. We hereby show that the testes of short-lived mice show a significant increase in cyclooxygenase 2 expression, PGD2 production, lipid peroxidation, antioxidant enzymes expression, local macrophages and TUNEL-positive germ cells numbers, and the levels of both pro-caspase-3 and cleaved caspase-3. In contrast, although the expression of antioxidant enzymes remained unchanged in testes of long-lived mice, the remainder of the parameters assessed showed a significant reduction. This study provides novel evidence that longevity confers anti-inflammatory, anti-oxidant and anti-apoptotic capacities to the adult testis. Oppositely, short-lived mice suffer testicular inflammatory, oxidative and apoptotic processes.Fil: Matzkin, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Bioquímica Humana; ArgentinaFil: Miquet, Johanna Gabriela. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica; ArgentinaFil: Fang, Yimin. Southern Illinois University. School Of Medicine; Estados UnidosFil: Hill, Cristal Monique. Southern Illinois University; Estados UnidosFil: Turyn, Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Química y Físico-Química Biológicas "Prof. Alejandro C. Paladini". Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Instituto de Química y Físico-Química Biológicas; ArgentinaFil: Calandra, Ricardo Saul. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; ArgentinaFil: Bartke, Andrzej. Southern Illinois University; Estados UnidosFil: Frungieri, Monica Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Bioquímica Humana; Argentin

Evidence for a GABAergic system in rodent and human testis: Local GABA production and GABA receptors

The major neurotransmitter of the central nervous system, gamma-aminobutyric acid (GABA), exerts its actions through GABA(A), GABA(B) and GABA(C) receptors. GABA and GABA receptors are, however, also present in several non-neural tissues, including the endocrine organs pituitary, pancreas and testis. In the case of the rat testis, GABA appears to be linked to the regulation of steroid synthesis by Leydig cells via GABA(A) receptors, but neither testicular sources of GABA, nor the precise nature of testicular GABA receptors are fully known. We examined these points in rat, mouse, hamster and human testicular samples. RT-PCR followed by sequencing showed that the GABA-synthesizing enzymes glutamate decarboxylase (GAD) 65 and/or GAD67, as well as the vesicular GABA transporter vesicular inhibitory amino acid transporter (VIAAT/VGAT) are expressed. Testicular GAD in the rat was shown to be functionally active by using a GAD assay, and Western blot analysis confirmed the presence of GAD65 and GAD67. Interstitial cells, most of which are Leydig cells according to their location and morphological characteristics, showed positive immunoreaction for GAD and VIAAT/VGAT proteins. In addition, several GABA(A) receptor subunits (alpha1-3, beta1-3, gamma1-3), as well as GABAB receptor subunits R1 and R2, were detected by RT-PCR. Western blot analysis confirmed the results for GABA(A) receptor subunits beta2/3 in the rat, and immunohistochemistry identified interstitial Leydig cells to possess immunoreactive GABA(A) receptor subunits beta2/3 and alpha1. The presence of GABA(A) receptor subunit alpha1 mRNA in interstitial cells of the rat testis was further shown after laser microdissection followed by RT-PCR analysis. In summary, these results describe molecular details of the components of an intratesticular GABAergic system expressed in the endocrine compartment of rodent and human testes. While the physiological significance of this peripheral neuroendocrine system conserved throughout species remains to be elucidated, its mere presence in humans suggests the possibility that clinically used drugs might be able to interfere with testicular function. Copyright (C) 2003 S. Karger AG, Basel