Self-organisation of Colloids with Temperature-Dependent Tuneable Interactions

Active matter are materials that can transform energy into movement. It is of great interest in the study of the collective movement of animals (schooling, flocking and swarming), crowd behaviour and functional materials. Its concepts are being explored to create a wide range of applications, such as biosensing and scaffolding. Active colloids are particles in the microscopic range that can take energy from their environment to move, interact and perform tasks. They can self-assemble into different structures, like living chains and clusters, due to their far-from-equilibrium interactions. Tuning these interactions can introduce dynamic changes in the final self-assembled structures, which are novel and demand further research for applications like displays and model systems. This thesis focuses on tuning the interactions in two different experimental systems of light-responsive active colloids. The first one describes a simple experimental approach where the intensity of light controls the interaction between active colloids in water. At lower light intensities the colloids are attracted to each other and form a cluster with hexagonal packing, whereas higher intensities give rise to a repulsive interaction between the particles that leads to a more dynamic structure. While the attractive interaction between the particles is due to thermo-osmotic flows, the repulsive interaction is due to positive thermophoresis. The second experimental system studies the group dynamics of Janus particles in complex non-uniform light patterns. The Janus particles self-assemble into groups, with dynamic properties that change with the topography of the light pattern. The groups in a complex light pattern are smaller and more stable than in a homogenous light profile. The critical mechanism is that the Janus particles orient themselves perpendicular to the local light gradient due to the torque. We envision that similar tuneable interactions in colloidal particles can be utilised to dynamically control the self-assembly of functional materials.