Magnetic microrollers maneuvering in a structured fluid

Micro swimmers in a fluid can either self-propel or be driven by external forces. The interactions between micro swimmers and their surroundings lead to a wide range of phenomena: self-assembly, phase separation, transportation of cargo, etc. Likewise, studying such interactions helps us to probe local rheology and material parameters such as the effective viscosity of a colloidal solution or the viscoelasticity of a non-Newtonian fluid. In this talk, I will demonstrate how magnetically driven microrollers maneuver in structured environments. When the rollers are spun by an external magnetic field, they pump fluid around themselves, driving them to move in the desired direction. This also generates strong flows that can reconfigure the nearby environment. I will discuss two hydrodynamic phenomena we study in this microroller system. I will show how the microrollers restructure passive colloids to form a new, steady configuration, and argue that the balance between Brownian motion and the flow velocity determines the size of the structure, which is an order of magnitude larger than the roller. We will additionally explore how these microrollers interact with a structured polymeric fluid.