Degree
Doctor of Philosophy (PhD)
Department
Cain Department of Chemical Engineering
Document Type
Dissertation
Abstract
Active colloids are microscopic particles that convert energy at the single-particle level into sustained motion and dynamically evolving assemblies. Their ability to transform continuous energy input into controlled motion opens new frontiers in microrobotics, adaptive materials, and targeted transport. Yet, achieving efficient propulsion at the microscale remains fundamentally challenging because viscous forces dominate while inertia becomes negligible. Unlike macroscopic systems that sustain propulsion through momentum, active colloids must harness non-reciprocal interactions with the surrounding fluid to generate persistent, time-irreversible motion. Studying minimal active systems—the simplest realizations that still capture the essential physics—provides a powerful route to uncover the non-equilibrium principles governing collective behavior and to inform the design of intelligent active materials.
First, we investigate the boundary-driven dynamics of an active ferromagnetic Janus particle near a stationary substrate when actuated by an oscillating magnetic field. By systematically varying the particle–boundary separation, we identify distinct regimes of motion—rolling at the boundary, pure rotation far from the wall, and, at intermediate elevations, tumbling and trochoidal trajectories that depend on the field frequency. These dynamic states arise from the interplay between magnetic and gravitational torques and hydrodynamic resistance. Particle Image Velocimetry reveals elevation-dependent flow fields and fluctuating vortical structures, demonstrating how boundaries reshape particle–medium coupling and sustain non-reciprocal motion even far from contact.
Second, we explore the organizing principles of pairwise interactions between active Janus particles in confinement. Within a specific range of interparticle distances and orientations, the particles exhibit predator–prey–like chases, where one persistently follows the other without capture. These interactions amplify the system’s non-reciprocity through coupled hydrodynamic and magnetic fields, reducing drag and enhancing translational speeds relative to isolated motion. The identification of an effective interaction region and velocity–direction correlations provides a quantitative description of when such collective pursuit emerges.
Together, these studies establish how boundaries act as decisive levers of non-reciprocal interaction, linking microscopic asymmetries to macroscopic organization. The insights developed here lay the foundation for designing next-generation microrobots and reconfigurable soft materials capable of navigating and adapting within complex, confined environments.
Date
11-3-2025
Recommended Citation
Raghu, Amrutha, "Non-Reciprocal Coupling of Minimal Active Colloids under Confinement: Rotating Ferromagnetic Janus Particles near Boundaries" (2025). LSU Doctoral Dissertations. 6936.
https://repository.lsu.edu/gradschool_dissertations/6936
Committee Chair
Bhuvnesh Bharti
Included in
Condensed Matter Physics Commons, Engineering Physics Commons, Other Chemical Engineering Commons, Thermodynamics Commons, Transport Phenomena Commons