Investigations into Quasi-Two-Dimensional Active Matter Systems using Colloidal Probes

Active matter research is a relatively new and exciting field of study that often merges concepts from non-equilibrium soft matter physics and biology. Active matter systems are composed of individual units that consume energy locally to produce collective phenomena that can lead to rich and complex behavior. Examples of experimentally studied active matter systems include bacteria suspensions, artificial swimmers, and collections of biopolymers driven by molecular motors. Investigations of these systems have the potential to provide important insights into the workings of these biologically relevant systems, as well as to serve as model systems for studies of non-equilibrium physics. In particular, many active systems of biological importance exist in viscoelastic environments, yet little work so far has studied the interplay of activity and viscoelasticity in these systems. The work in this thesis focuses on investigating the dynamical and material properties of two quasi-two-dimensional interfacial viscoelastic active matter systems, a biofilm-forming bacterial suspension and a microtubule-based "active nematic'' film, which is an out-of-equilibrium liquid crystal. In chapters 3 and 4, results are presented in which magnetic micro-sized colloidal rods and disks are employed that couple hydrodynamically to the active nematic film. This coupling enabled the investigation of the dynamical and material properties of the film, and also the local manipulation and ordering of the otherwise chaotic flows. In addition, sufficiently large applied hydrodynamic stresses produced by rotating the disks drove the formation of +1 charge topological vortices through the merger of two +1/2 defects. Chapter 5 presents a comprehensive experimental study of the dynamics and rheology of an oil-water interface during biofilm formation by the bacteria strain Pseudomonas Aeruginosa, PA14. The evolution of the system is studied during this transition using both passive and active microrheology in parallel with differential-dynamic-microscopy to characterize the motion of the bacteria at the interface. The passive microrheology demonstrates a dependence on size of the colloidal probes at late ages that is significantly greater than that expected in a homogeneous system in equilibrium, indicating either length-scale-dependent rheology of the biofilm or widely different effects of the bacteria activity on the motion of the spheres