Computational studies of the rheological properties of anisotropic colloids

Understanding the rheology of colloidal suspensions is of significant importance for both fundamental interest and practical applications. Many industrial processes and products rely on either the stability of colloidal suspensions, such as in the foods, adhesives and paints, or the aggregation of colloids out of suspension, such as for optical crystals. These processes and products will inevitably experience external forces and understanding how the colloidal suspensions will react to these external forces is very important to industry. It is well established that the microstructure plays a key role in the rheological behaviour of colloidal systems. The focus of colloid rheology has largely been on suspensions of spherical colloidal particles, where the inter-particle interactions, which govern the microstructure, are isotropic. It would be of fundamental interest to study systems that are able to produce more complex microstructure, and therefore potentially interesting rheological behaviours. Since the turn of the 21st century, advances in particle synthesis have made available a variety of colloidal particles that involve anisotropic inter-particle interactions due to shape and/or surface chemistry. Colloidal dumbbells offer an attractive model system to advance our understanding of how the microstructure governs colloidal rheology beyond the well-studied isotropic limit. In this context, we have investigated systems of symmetric as well as asymmetric colloidal dumbbells, interacting via depletion interaction, using Brownian Dynamics simulations. We find that for dense suspensions of symmetric colloidal dumbbells interacting via depletion, the aspect ratio, i.e. the length-to-breadth ratio, critically influences the shear viscosity of the suspensions, which exhibit a non-monotonic variation with the aspect ratio showing a minimum at an intermediate value. This non-monotonic behaviour becomes more pronounced as the effective packing fraction increases. We argue that the non-monotonic variation of the shear viscosity with the aspect ratio is a manifestation of translation-rotation decoupling. The suspension exhibits shear-thinning behaviour, which we attribute to a microstructure arising from an alignment of the axis of the cylindrical symmetry with the flow direction. For asymmetric colloidal dumbbells, we exploit the size ratio and a hierarchy of interactions between the lobes to promote self-limiting cluster formation upon gradual cooling. The cluster fluid becomes more viscous as the fraction of colloidal particles forming tetramers grows upon cooling. Under an external shear, we observe a strong shear thinning behaviour, which is attributed to the disassembly of the clusters with increasing shear rate. This study found that the shape and surface chemistry can be exploited to form clusters in a controlled way to manipulate colloid rheology. When the system of colloidal symmetric dumbbells is subject to a temperature quench, the arrested structure is found to show local ordering that is sensitive to the aspect ratio. At a lower aspect ratio, the system undergoes shear-induced crystallisation into a close-packed structure, whereas at a high aspect ratio the system arrests into a gel-like structure