Simulations on dynamics and structure formation of active and passive anisotropic colloids in quasi-two-dimensional systems

Abstract

Colloidal systems, which generally consist of small particles in a dispersion medium, are not only inherent parts of our daily routine but also indispensable model systems of interdisciplinary research. Because of their mesoscopic size of the order of nano- and micrometers, they can, for example, act as stand-ins for atoms and molecules on an easy-to-resolve length scale to study phenomena that would not be possible to observe otherwise. As their interactions and dynamics can efficiently be adjusted in experiments, they are suited for the analysis of structure formation and emerging non-equilibrium effects. For this, it is instrumental that they can be resolved by electron or optical microscopes, and that their interaction strengths are comparable to the thermal energy. In recent years, colloids proved to be essential tools in studying and modeling collective motion of active matter such as animal groups or artificial micromachines. Both, the mesoscopic as well as the macroscopic properties of colloidal systems strongly depend on the shape of the corresponding particle content. Hence, investigating the influence of the particle shape on different colloidal phenomena opens up the possibility of tuning and designing novel materials such as coatings based on monolayers of ordered anisotropic particles. In the present work, we discuss our results of Brownian dynamics simulations studying various quasi-two-dimensional colloid systems containing anisotropic dumbbells, linear trimers, and spherocylinders in and far from the thermodynamic equilibrium. While the simulations are treated as two-dimensional systems during the computations, the particular colloids are interpreted as three-dimensional objects that are confined to a single plane. In detail, the particle centers and the symmetry axes of the analyzed rod-like particles are bound to this plane. Additionally, hydrodynamic interactions between the colloids are neglected. The studies presented in the following intend to investigate the influence of an anisotropic particle shape on different example systems. To perform Brownian dynamics simulations, diffusion constants are needed. However, these quantities also depend on the shape of the colloids. As simple analytic expressions for the diffusion coefficients of dumbbells, linear trimers, and short spherocylinders are unknown, the first research task presented in this work is to obtain fitting relations that capture the diffusive behavior for the analyzed particle types. For this, we utilize an empirical approach, where we obtain relations via fits through the results for the diffusion coefficients that we obtain using hydrodynamic bead-shell calculations. In these hydrodynamic computations, the surface of a colloidal particle is modeled by shells consisting of a progressively larger number of spherical friction elements, and it is assumed that the limit of infinite friction elements with a vanishing diameter retrieves the correct diffusive behavior. The computed formulas reproduce known results (for instance, obtained by measurements) remarkably well. We then utilize the microscopic diffusion coefficients to study the long-time dynamics of rod-like colloids in crowded monolayers and corroborate experiments studying the structure formation in dumbbell monolayers. The experimental data are obtained via the Langmuir-Blodgett method, which uses movable barriers to compress particles at the air/water interface to assemblies with large area fractions above 65% that are extracted and dried on a silicon wafer for analysis. Using an approach where we compare Voronoi tessellations and the distribution of the shape factors of the corresponding polygons, we find density-dependent features that are present in the measurements and the numerical results alike. With simulations consisting of active spherocylinders of varying aspect ratio, we expand our discussion of shape-dependent phenomena to active matter systems. Here, we try to answer how the anisotropy of the colloids influences collective phenomena previously only studied for spherical model colloids. In particular, we investigate group formation through a visual perception model and colloidal cargo transport via dense colloidal swarms. In both cases, we find that the aspect ratio of the spherocylinders influences the observed behavior and a high particle anisotropy can suppress the collective motion found for systems of spheres. In the last results presented within this work, we utilize anisotropic particles as building blocks, which we can use to mimic complex experimental setups. In particular, we use spherocylinders to form a funnel geometry to corroborate experimental studies analyzing the influence of cohesion on the flow of active particles through obstacles. Here, we find that cohesion is not always detrimental to the flow rates corresponding to cohesive groups trying to pass bottlenecks. This thesis demonstrates the strong influence of shape and anisotropy on the dynamics, structure formation, and emerging collective phenomena in quasi-two-dimensional colloid systems. The computed empirical formulas for the microscopic diffusion coefficients of dumbbells, linear trimers, and spherocylinders open up the possibility for future many-body simulations and other studies containing the corresponding particle types. The excellent agreement between the results obtained via experimental Langmuir-Blodgett assemblies and the Brownian dynamics simulations suggests that our approach is a suitable and efficient tool for analyzing the structure formation of colloidal particles at the air/water interface and we expect similar accordance for different particle shapes. Shape-dependent collective motion has significance for the design of micromachines, which often have complex geometries. Here, our results indicate that steering policies must be tailored to the specific shape of the microrobots.