Assembly and Propulsion of Magnetic Janus Particles

Janus particles are widely considered as a fundamental building block for fabricating intricate machines able to perform complex functions at sub-micron scale. Due to their dual surface functionality, these particles have dissimilar surface interactions with surrounding particles and medium. The inherent asymmetry of Janus particles can drive their assembly into complex phases and/or generate net force on the particles inducing a motion. In this talk I will present the use external magnetic field as to tool to direct the assembly and propulsion of Janus particles suspended in ferrofluids. First, I will show how the intrinsic anisotropy in magnetic interactions combined with the asymmetry of the Janus particles can induce unusual magnetic interaction energy landscape around the particle. By combining patchy and non-patchy particles, we modulate these interactions to construct clusters with morphologies resembling colloidal molecules. I will discuss the assembly of such clusters, their programmability by manipulation of patch size, their in-situ tunability using the magnetic field, and I will show a technique to permanently bind them once assembled. Secondly, I will demonstrate how to use time-periodic magnetic fields to drive the coherent dynamic surface rolling of Janus particles in the ferrofluids. I will show how external field allows controlling characteristics of the active motion such as rolling, rotation, speed, and direction of motion of the Janus particles. Our experimental results display the potential in microstructuring at the single cluster level as well as controlling active motion and collective behaviors of the Janus colloids, using a single external magnetic field. The simplicity and versatility of the approach stems from exploiting the anisotropy of magnetic interactions combined with the asymmetry of the Janus particles enabling a precise control over both the equilibrium assembly and non-equilibrium motion of the particles.