Microrollers in tight spaces: confinement-induced structuring and hydrodynamic trapping

Abstract:  Driven suspensions, where energy is input at a particle scale, are both models for understanding general principles of out-of-equilibrium self-organization, and also materials with enormous near-term applications potential. My work is focused on magnetically-actuated suspensions; I study how these materials assemble into dynamical structures, as well as how they interact with obstacles and boundaries. While the model system we employ is very simple (spinning particles in water), the strong hydrodynamic interactions between individual particles and with nearby boundaries lead to a rich array of emergent and dynamically-assembled structures.   Recently, we found that even simple modulations to nearby boundaries can have dramatic consequences: a single post-shaped obstacle can act as a hydrodynamic trap, and capture a passing particle.  Moreover, the strength of this trapping can be easily be tuned by adjusting either the obstacle curvature or the particle-obstacle repulsive potential. This work demonstrates the complexity of this dynamical system: microrollers can become trapped by an obstacle, this trapping is stochastic, and most surprising, it is enabled by and not destroyed by thermal fluctuations. We are currently exploring interactions with more complex structure, for example how highly confined structures (channels, tunnels, etc) modify the mobility of these driven suspensions.  We find that this strong confinement induces unexpected density fluctuations, which are the result of large-scale flow recirculation. This work provides fundamental insights to help us understand suspension transport in more complex structured environments, for example as found in living systems, as well as how we can use these particles to reconfigure their local environment.