(61g) Janus Particles on a Roll

Active colloids can potentially exhibit intelligence by performing complex functions at micron scale under the influence of an external electric or magnetic field. This enables us to leverage them as smart materials and machines that can be building blocks of future technology. Manipulating active colloids precisely requires understanding the fundamental physics of the phenomena at display and how the behavior varies as a function of external field, medium and nature of the particle. We aim to study the phenomena displayed by iron coated Janus particles in magnetic medium (Ferrofluid) and non-magnetic medium (water) driven by out-of-plane magnetic field. Janus particles with magnetic patch when suspended in ferrofluid exhibits five different modes of motion, which are governed by the frequency and strength of the applied magnetic field. Each of these dynamic state arises due to change in the magnetic anisotropy, magnetic field, hydrodynamic forces, and the interference of the iron nanoparticles present in the ferrofluid. We performed control measurements in water to decouple the effects magnetic field and hydrodynamic forces by tracking how the Janus particles propel at different heights above the substrate in water and at the surface. The further the particles are away from the substrate, the slower is their propulsion. We will demonstrate the role of hydrodynamic forces impeding the individual particle motion, particle inertia and many body interactions due to attractive and repulsive forces through theoretical models and Particle Image Velocimetry (PIV).