Magnetic Quincke Rollers with Tunable Single Particle Dynamics and Collective States
Electrohydrodynamically driven active particles based on Quincke rotation have quickly become an important model system for emergent collective behavior in non-equilibrium colloidal systems. Like most active particles, Quincke rollers are intrinsically non-magnetic, preventing the use of magnetic fields to control their complex dynamics on-the-fly. Here, we report on magnetic Quincke rollers based on silica particles doped with superparamagnetic iron oxide nanoparticles. We show that their magnetic nature enables the application of both externally controllable forces and torques at high spatial and temporal precision, leading to several new control mechanisms for their single particle dynamics and collective states. These include tunable interparticle interactions, potential energy landscapes, and advanced programmable and teleoperated behaviors - allowing us to discover and probe active chaining, anisotropic active sedimentation-diffusion equilibria and collective states in various geometries and dimensionalities.
Electrically Controllable Ferrofluids
Ferrofluids are well known for their strong magnetic responsivity. However, their response to electric fields is often not strong or studied at all. Here we show that it is possible to prepare strongly electrically responsive ferrofluids (“electroferrofluids”) by inducing a small but non-vanishing electric charge on iron oxide nanoparticles dispersed in a nonpolar solvent by using charge-carrying reverse micelles. Application of electric field leads to electrophoretic motion of the charged nanoparticles and formation of steady-state dissipative nanoparticle concentration gradients. Because the nanoparticles are superparamagnetic, this translates to voltage-controlled local magnetic susceptibility and saturation magnetization. The magnetic nanoparticle gradients respond to magnetic fields analogously to the well- studied ferrofluid free surfaces. As a demonstration of this, we show that the gradients exhibit instabilities and pattern formation in magnetic field that resemble the classic labyrinthine instability and the normal-field instability
Jaakko Timonen is an assistant professor of experimental condensed matter physics. He is broadly interested in physics of soft and biological materials. Some examples include pattern formation in ferrofluids, lotus-mimicking superhydrophobic surfaces, magnetic tweezing of 'non-magnetic' matter, coloration of a certain spider from Madagascar and fundamentals of slippery lubricated surfaces.