Pore size in model particle gels

Active Brownian particles in random and porous environments

The transport of active particles may occur in complex environments, in which it emerges from the interplay between the mobility of the active components and the quenched disorder of the environment. Here, we explore the structural and dynamical properties of active Brownian particles (ABPs) in random environments composed of fixed obstacles in three dimensions. We consider different arrangements of the obstacles. In particular, we consider two particular situations corresponding to experimentally realizable settings. First, we model pinning particles in (non-overlapping) random positions and, second, in a percolating gel structure and provide an extensive characterization of the structure and dynamics of ABPs in these complex environments. We find that the confinement increases the heterogeneity of the dynamics, with new populations of absorbed and localized particles appearing close to the obstacles. This heterogeneity has a profound impact on the motility induced phase separation exhibited by the particles at high activity, ranging from nucleation and growth in random disorder to a complex phase separation in porous environments.

Multi-component colloidal gels: interplay between structure and mechanical properties

We present a detailed numerical study of multi-component colloidal gels interacting sterically and obtained by arrested phase separation. Under deformation, we found that the interplay between the different intertwined networks is key. Increasing the number of components leads to softer solids that can accommodate progressively larger strains before yielding. The simulations highlight how this is the direct consequence of the purely repulsive interactions between the different components, which end up enhancing the linear response of the material. Our work provides new insight into mechanisms at play for controlling the material properties and opens a road to new design principles for soft composite solids.

Structure and rheology of colloidal particle gels: insight from computer simulation

A particle gel is a network of aggregated colloidal particles with soft solid-like mechanical properties. Its structural and rheological properties, and the kinetics of its formation, are dependent on the sizes and shapes of the constituent particles, the volume fraction of the particles, and the nature of the interactions between the particles before, during and after gelation. Particle gels may be permanent or transient depending on whether the colloidal forces between the aggregating particles lead to irreversible bonding or weak reversible interactions. With short-range reversible interactions, network formation is typically associated with phase separation or kinetic arrest due to particle crowding. Much existing knowledge has been derived from computer simulations of idealized model systems containing spherical particles interacting with well-defined pair potentials. The status of current progress is reviewed here by summarizing the underlying methodology and key findings from a range of simulation approaches: Monte Carlo, molecular dynamics, Brownian dynamics, Stokesian dynamics, dissipative particle dynamics, multiparticle collision dynamics, and fluid particle dynamics. Then it is described how the technique of Brownian dynamics simulation, in particular, has provided detailed insight into how different kinds of bonding and weak reversible interactions can affect the aggregate fractal structure, the percolation behaviour, and the small-deformation rheological properties of network-forming colloidal systems. A significant ongoing development has been the establishment and testing of efficient algorithms that are able to capture the subtle dynamic structuring effects that arise from effects of interparticle hydrodynamic interactions. This has led to an appreciation recently of the potentially important role of these particle-particle hydrodynamic effects in controlling the evolving morphology of simulated colloidal aggregates and in defining the location of the sol-gel phase boundary.

A computational study of the reconstruction of amorphous mesoporous materials from gas adsorption isotherms and structure factors via evolutionary optimization

A general method for the three-dimensional reconstruction of mesoporous materials by evolutionary optimization against target data is developed. The method is applied specifically in reconstruction of amorphous material models using gas adsorption data, structure factor data, or a combination of both. A recently introduced lattice-gas approach is used to model adsorption in these calculations, and a high-pass limited Fourier representation is used to facilitate evolution of large-scale structures during the optimization. Reconstructions are made of several material models which mimic real materials obtained either by phase separation and etching or by sol-gel processing. Analysis of the reconstructions provides considerable insight into the type and quantity of structural information probed by gas adsorption and small-angle scattering experiments. We find that reconstructions based only on structure factors tend to underestimate the mean pore size. We also find that in many cases excellent reconstructions can be obtained using only adsorption-branch data, and that in all cases reconstructions based jointly on both types of data are superior to those based only on one, suggesting that these measures contain "complementary" information. It is also found that in most cases the use of desorption data is not warranted, and that the use of adsorption data taken at many temperatures will not improve reconstructions. The reproducibility of the method is shown to be satisfactory. The method can be computationally expensive if gas adsorption data are used, but it is easily parallelized, and therefore results can still be obtained in reasonable time. Finally, the possible application of this approach to real systems, including templated porous materials, is discussed.

Heterogeneity of colloidal particle networks analyzed by means of Minkowski functionals

The heterogeneity and large scale connectivity of colloidal particle networks, which are generated by Brownian dynamics simulations, is examined. This is achieved by employing integral geometric measures in the form of the Minkowski functionals or quermassintegrals. It is found that these measures in conjunction with the parallel-body technique amount to a powerful tool to characterize the structure, going beyond the information contained in the pair-correlation function. The development of heterogeneities during network formation as well as their dependence on the volume fraction and the interaction potential is studied. In particular, it is found that slow coagulation enhances the heterogeneity of the network compared to fast coagulation.

Ultrasonic shear wave rheology of weak particle gels

Weak particle gels have attracted increasing attention in the last decade. These gels have a very short region of deformation over which their viscoelastic parameters are constant. They can be broken easily in response to external forces. Therefore the rheological measurements in these systems must be performed at very small deformations, which may frequently be below the accuracy limits of conventional rheological instruments. In the present paper we discuss the application of the thickness shear mode resonator technique for the measurement of viscoelastic parameters of weak particle gels in the MHz frequency range. The technique provides information on the viscoelasticity of weak gels in the time scale 10(-7)-10(-9) s. The length scale of the measurements, determined by the depth of penetration and the wavelength of the shear wave, falls in the submicron and micron range. The displacements in the shear deformations generated in this technique are extremely small, in the order of Angstroms, and the shear strain, approximately 10(-3), corresponds to the low limits in the classical dynamic rheology measurements. Only small volumes, down to 0.1 ml, of sample are required and this is another advantage of this technique. The measurements of the storage, G', and the loss, G'', moduli can be carried out non-invasively and continuously at various frequencies in the same sample during the whole length of the process of gelation. General and specific aspects of the measurements and interpretation of experimental results are discussed in the present paper.

Local Structure Evolution in Particle Network Formation Studied by Brownian Dynamics Simulation

The effect of solid content and colloidal interactions on the structure of forming networks of colloidal particles is studied by Brownian dynamics simulation. The different situations are compared in terms of the pair distribution function and the distribution of nearest neighbors around each particle. The results indicate that, in fast coagulation, the higher solid contents lead to a freezing-in of the liquid structure. Nevertheless, this effect can be reduced substantially by the introduction of a shallow secondary minimum and an energy barrier in the interaction potential. However, the structures resulting from such slow coagulation show a substantial degree of porosity, larger than those produced at the same solid content but by fast coagulation. It is also shown how the porosity (defined on a few particle diameters) is reflected in the distribution of nearest neighbors around the center particle, i.e., the very local conformation in the particle network. Fractal analysis shows that, at the relatively high volume fractions considered in this study, no intermediate fractal regime exists. Copyright 2000 Academic Press.

Structure and Rheology of Simulated Gels Formed from Aggregated Colloidal Particles

Soft, elastic, solvent-rich materials made from networks of aggregated colloidal particles are called particle gels. The networks may be regarded as being permanent or transient depending on whether the short-range attractive forces between the particles arise from strong irreversible bonding or weak reversible interactions. Understanding the relationships between the interparticle forces and the structure and rheology of particle gels is a challenging problem. This article shows how useful insight is being provided by Brownian dynamics simulations involving systems of spherical particles interacting with a combination of bonded and nonbonded interparticle potentials. Copyright 2000 Academic Press.