The coagulation process has a dramatic impact on the properties of dispersions of colloidal particles including the change of optical, rheological, as well as texture properties. We model the behavior of a colloidal dispersion with moderate particle volume fraction, that is, 5 wt %, subjected to high shear rates employing the time-dependent Discrete Element Method (DEM) in three spatial dimensions. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to model noncontact interparticle interactions, while contact mechanics was described by the Johnson-Kendall-Roberts (JKR) theory of adhesion. The obtained results demonstrate that the steady-state size of the produced clusters is a strong function of the applied shear rate, primary particle size, and the surface energy of the particles. Furthermore, it was found that the cluster size is determined by the maximum adhesion force between the primary particles and not the adhesion energy. This observation is in agreement with several simulation studies and is valid for the case when the particle-particle contact is elastic and no plastic deformation occurs. These results are of major importance, especially for the emulsion polymerization process, during which the fouling of reactors and piping causes significant financial losses.

Large eddy simulation (LES) of a stirred tank equipped with a Rushton impeller and four cylindrical baffles was used to characterize the flow pattern and to assess the maximum turbulent kinetic energy dissipation rate e max . While the shorter baffle-impeller distance significantly affects the radial velocity profile and the trailing vortices expansion, the flow field in the impeller vicinity is comparable to that of a standard setup with rectangular baffles connected to the wall. The phase-resolved profile of e max indicates its very strong variation from 10 Á N 3 D 2 to 130 6 13 Á N 3 D 2. When using peak values of the corresponding hydrodynamic stress s max 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi lqe max p À Á , the maximum stable aggregate size measured in the same stirred tank closely correlates with breakage data obtained under laminar conditions using the same initial aggregates. This indicates that the same mechanism was involved in the aggregate breakup under both conditions, allowing us to predict aggregates breakup under various conditions.

The utilizable capacitance of Electrochemical Double Layer Capacitors (EDLCs) is a function of the frequency at which they are operated and this is strongly dependent on the construction and physical parameters of the device. We simulate the dynamic behavior of an EDLC using a spatially resolved model based on the porous electrode theory. The model of Verbrugge and Liu (J. Electrochem. Soc. 152, D79 (2005)) was extended with a dimension describing the transport into the carbon particle pores. Our results show a large influence of the electrode thickness (L e ), separator thickness (L s ) and electrolyte conductivity (κ) on the performance of EDLCs. In agreement with experimental data, the time constant was an increasing function of L e and L s and a decreasing function of κ. The main limitation was found to be on the scale of the whole cell, while transport into the particles became a limiting factor only if the particle size was unrealistically large. The results were generalized into a simplified relation allowing for a quick evaluation of performance for the design of new devices. This work provides an insight into the performance limitation of EDLCs and identifies the critical parameters to consider for both systems engineers and material scientists. Electrochemical Double Layer Capacitors (EDLCs) store energy by the adsorption of ions from an electrolyte (resulting in its capacitive deionization), storing the ions in the electrochemical double layer of a charged electrode with a very large surface area. The high charge that can be stored gives rise to the name supercapacitors or ultracapacitors. The main advantages of EDLCs when compared to batteries are their ability to quickly release the stored energy, their high efficiency and their long cycle life. The amount of energy that can be stored is, however, not unlimited. The useable capacitance is dependent on the time scale of their operation and this causes the capacitance of EDLCs to be a function of frequency. As typical application of EDLCs requires them to supply or accept pulses of energy at frequencies ranging from 10 Hz to 0.1 Hz, 1 it is important that the full capacity can be utilized in this range. In order to improve the high-frequency behavior, it is necessary to understand the factors causing the limitation and to identify the critical ones.The key to the high capacitance of EDLCs is the high specific surface area (SSA) of their electrodes. This is achieved by using highly porous activated graphitic carbon materials.2 According to their size, the pores can be classified as micropores (<2 nm), mesopores (2 − 50 nm) or macropores (>50 nm).2 Pores of different sizes are connected in a highly complex hierarchical structure depending on the shape of carbon particles. A typical supercapacitor cell consists of two carbon electrodes with the porous separator in between and current collectors closing the cell from both sides.Experimentally, the frequency-dependent capacitance is commonly measured using either Cyclic Voltammetry (CV), Electrochemical I...

The stability of colloidal dispersions is of crucial importance because the properties of dispersions are strongly affected by the degree of coagulation. Whereas the coagulation kinetics for quiescent (i.e., nonstirred) and diluted systems is well-established, the behavior of concentrated dispersions subjected to shear is still not fully understood. We employ the discrete element method (DEM) for the simulation of coagulation of concentrated colloidal dispersions. Normal forces between interacting particles are described by a combination of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) and Johnson, Kendall, and Roberts (JKR) theories. We show that, in accordance with the expectations, the coagulation behavior depends strongly on the particle volume fraction, the surface potential, and the shear rate. Moreover, we demonstrate that the doublet formation rate is insufficient for the description of the coagulation kinetics and that the detailed DEM model is able to explain the autocatalytic nature of the coagulation of stabilized dispersions subjected to shear. With no adjustable parameters we are able to provide semiquantitative predictions of the coagulation behavior in the high-shear regions for a broad range of particle volume fractions. The results obtained using the DEM model can provide valuable guidelines for the operation of industrial dispersion processes.