The influence of bond-rigidity and cluster diffusion on the self-diffusion of hard spheres with square-well interaction

Hard spheres interacting through a square-well potential were simulated using two different methods: Brownian Cluster Dynamics (BCD) and Event Driven Brownian Dynamics (EDBD). The structure of the equilibrium states obtained by both methods were compared and found to be almost the identical. Self diffusion coefficients ($D$) were determined as a function of the interaction strength. The same values were found using BCD or EDBD. Contrary the EDBD, BCD allows one to study the effect of bond rigidity and hydrodynamic interaction within the clusters. When the bonds are flexible the effect of attraction on $D$ is relatively weak compared to systems with rigid bonds. $D$ increases first with increasing attraction strength, and then decreases for stronger interaction. Introducing intra-cluster hydrodynamic interaction weakly increases $D$ for a given interaction strength. Introducing bond rigidity causes a strong decrease of $D$ which no longer shows a maximum as function of the attraction strength

Tracer diffusion in colloidal gels

Computer simulations were done of the mean square displacement (MSD) of tracer particles in colloidal gels formed by diffusion or reaction limited aggregation of hard spheres. The diffusion coefficient was found to be determined by the volume fraction accessible to the spherical tracers ($\phi_a$) independent of the gel structure or the tracer size. In all cases, critical slowing down was observed at $\phi_a\approx 0.03$ and was characterized by the same scaling laws reported earlier for tracer diffusion in a Lorentz gas. Strong heterogeneity of the MSD was observed at small $\phi_a$ and was related to the size distribution of pores.Comment: 6 pages, 8 figure

Wer ist heute 'Ehrenmann' und 'Ehrenfrau'? Ethische und jugendkulturelle Aspekte des deutschen Jugendworts des Jahres

Eine Jury des deutschen Langenscheidt-Verlags wählt jedes Jahr das sogenannte 'Jugendwort des Jahres'. 2018 fiel diese Wahl auf 'Ehrenmann' bzw. 'Ehrenfrau' im Sinne von 'jemand, der etwas (Besonderes) für einen tut'. In der Folge fand jedoch weder eine öffentliche Auseinandersetzung über die Herkunft des Begriffs statt noch darüber, was Ehre für Jugendliche heute bedeutet. Der Artikel möchte daher zunächst darstellen, was Ehre ist und welche soziale Funktion sie erfüllt, und anschließend die besondere Bedeutung von Ehre für Jugendliche in Migrationssituationen reflektieren. Schließlich folgen eine pop- und jugendkulturelle Analyse der Herkunft und Verwendung des Ausdrucks 'Ehrenmann' (und 'Ehrenfrau') sowie eine abschließende kritische Reflexion der Jurywahl.Every year, a jury of the German Langenscheidt-Verlag chooses the so-called 'youth expression of the year' (Jugendwort des Jahres). In 2018, this choice fell on 'man/woman of honour' (Ehrenmann/Ehrenfrau), in a sense of ‚someone doing something (special) for somebody‘. Interestingly, no public discourse followed to examine the origin of this unusual expression or the meaning of 'honour' for today's youth. The article, therefore, aims to outline what honour is and which social functions it fulfils, and then to reflect the particular significance of honour for adolescents with a migrant background. This is followed by a pop- and youth-cultural analysis of the origin and use of the expression 'man of honour' as well as a concluding critical reflection of the jury's choice

Irreversible aggregation of patchy colloidal particles (a computer simulation study)

La variété des structures colloïdales auto assemblées est principalement due à la présence d'interactions anisotropes. Nous présentons ici une méthode numérique pour étudier l'agrégation irréversible par patches de particules sphériques. Il s'agit d'une amélioration de la dynamique d amas browniens prenant en compte les interactions par patches et les mouvements de rotation. Nous limitons notre étude aux particules à deux patches opposés de taille variable. Une interaction isotrope, faible, est également ajoutée pour modéliser une polymérisation par étape dans diverses conditions de solvant. Cet algorithme a été testé sur des chaînes isolées et nous avons retrouvé les propriétés statiques et dynamiques attendues. En jouant sur la balance entre la qualité du solvant et la taille des patches diverses morphologies hors-équilibres sont obtenues. Même dans la limite diluée, les collisions corrélées jouent un rôle important et une description de type Smoluchowski échoue.The versatility of self assembling structures is mostly due to the presence of anisotropic interactions. We present a new simulation method to study irreversible patchy aggregation of spherical particles. It is a variation of the Brownian Cluster Dynamics method taking into account patchy interactions and rotational motions. We limit our case to particles with two oppositely located patches. The size of patches can vary and an isotropic interaction is superimposed around the particle to mimic step-polymerization with various solvent qualities. This new algorithm was successfully tested on single polymer chains and expected static and dynamic properties were recovered. Depending on the balance between solvent quality and patch size various out of equilibrium morphologies could be obtained. Even in the limit dilute solutions, correlated collisions play a key role and the Smoluchowski approach fails. We propose new directions for a better understanding of anisotropy effects on kinetics.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Nanoscale texture development of C-S-H gel: A computational model for nucleation and growth

The development of C-S-H (Calcium-Silicate-Hydrate) gels during cement hydration is often investigated by nucleation and growth models which fit reasonably well with the calorimetric measurements but predict hydration degrees which grossly exceed the experimental values. Here, a computational model is presented which explicitly considers the intrinsic nanoparticulate nature of C-S-H gel. Based on a nucleation and growth algorithm the model reproduces the experimental calorimetric and hydration degree measurements without invoking to any diffusion mechanism. The model also suggests that the peak in the calorimetric curves can be ascribed to the percolation point of the hydrates themselves within the interstitial pore volume. © 2013 AIP Publishing LLC.This study was carried out under the umbrella of the BASKRETE initiative and supported by funding from the Basque Government through the Nanoiker project (Grant No. E11-304) under the ETORTEK Program, the FP7 program of the European Commission (CODICE project—Contract NMP3-SL-2008-214030), the Spanish Ministerio de Ciencia y Tecnologia of Spain (Grant Nos. TEC2007- 68065-C03-03 and FIS2010-19609-C02-02), and the University of the Basque Country (Grant No. IT-366-07).Peer Reviewe

Propriétés statiques et dynamiques de sphères dures attractives (étude par simulation numérique)

Ce travail de thèse porte sur l'étude des propriétés statiques et dynamiques de solutions de sphères dures attractives à courte portée. Pour ce faire, nous avons développé au laboratoire un algorithme original appelé Browian Cluster Dynamics (BCD). Avec cet algorithme, une assemblée de sphères dures attractives et browniennes peut être relaxée depuis une température infinie jusqu'à son état d'équilibre à une température donnée en suivant une trajectoire réaliste dans l'espace des phases. Cette technique est basée sur le mouvement coopératif diffusionnel des amas transitoires formés au cours de la réaction d'agrégation. Cela nous permet d'accéder au propriétés statiques, cinétiques et dynamiques du système, à tout instant, même loin de l'équilibre thermodynamique. De plus, la technique BCD permet de prendre en compte la rigidité des liens formés entre les sphères [P05]. En limitant la portée de l'interaction à une adhésion de contact, on a pu empêcher la séparation de phase et étudier ainsi les phénomènes de percolation [P0l] et de transition vitreuse en diminuant la température. Ainsi nous avons pu montrer que les verres attractifs n'existent qu'à température nulle [P03]. En rendant les liens formés rigides, nous avons pu nous affranchir des phénomènes de cristallisation naturellement présents dans ce type de système et étudier la cinétique de séparation de phase de type liquide-liquide ainsi que les phénomènes de percolation dans la solution [P02]. L'introduction de la flexibilité des liens nous a permis d'étudier son influence sur le diagramme de phase (cristallisation) mais aussi sur les phénomènes d'agrégation irréversible [P06]. Cela nous a permis de comprendre la structure locale des amas formés au cours de certains processus d'agrégation irréversible au cours desquels les amas se restructurent par glissement des particules les constituant. Enfin, nous nous sommes intéressés aux propriétés de transport de matière dans les gels formés par agrégation irréversible. Pour cela nous avons étudié le mouvement brownien d'un traceur de taille variable dans des gels fabriqués à différentes concentrations et conditions d'agrégation [P04]. Nous avons pu établir une loi phénoménologique reliant le coefficient de diffusion du traceur au volume accessible à son centre de masse. La loi d'échelle gouvernant la transition entre diffusion et localisation du traceur est bien expliquée par la théorie de la percolation.This thesis deals with the study of the structure and the dynamics of attractive spheres. For this purpose a new simulation technique called the Brownian cluster dynamics (BCD) was introduced. With BCD we can relax the system by cooperative cluster relaxation and can also study the effect of bond rigidity. The method was compared with Event Driven Brownian Dynamic simulations which gave the same static and dynamic properties. Using zero interaction range we were able to suppress phase separation and thereby to study the slow dynamics of strongly attracting spheres. The results disproves the existence of so-called attractive glasses proposed in the literature. By introducing rigid bonds we suppressed crystallization for short range interaction which allowed us to study the kinetics of phase separation and more specifically the inter play between phase separation and gelation. As we distribute bonds between nearest neighbors we were able to trace out two kinds of percolation lines as a function of volume fraction and interaction strength : the bond percolation and the contact percolation. We also studied the effect of flexibility of bonds on irreversible aggregation. Even though the structures created by flexibly bonded diffusion limited cluster aggregation were locally quite dense they showed fractal behavior at large length scales. Tracer diffusion was studied in gels formed by irreversible aggregation of hard spheres. It was found that the diffusion coefficient is determined by the accessible volume, i.e. the volume available to the centre of mass of the tracer. We put forward a phenomenological equation connecting the diffusion coefficient and the accessible valid for different gel structures and tracer sizes.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Static structure factor of dilute solutions of polydisperse fractal aggregates

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Analysis of Relaxation Functions Characterized by a Broad Monomodal Relaxation Time Distribution

We demonstrate the advantage of using the so-called generalized exponential (GEX) function for the analysis of relaxation functions characterized by a monomodal broad relaxation time distribution. We give a number of characteristics of this function and compare it to two functions that are currently widely for this type of analysis: the Kohlraush-Williams-Watt and the Havriliak-Negami functions. The main advantages of the GEX-function are that it can be used easily both in the time and the frequency domain, and that it has a relatively simple expression for the corresponding relaxation time distribution. Three important applications are discussed: the glass transition dynamics, and the relaxation of concentration fluctuations in dilute and concentrated solutions of broad distributions of selfsimilar particles. In the latter two cases the relation between the parameters of the GEX-function and the molecular characteristics of the solutions is made explicit

Structure and distribution of aggregates formed after heat-induced denaturation of globular proteins

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