Abstract :
[fr] If you asked someone on the street about magnetism, they might claim to know little about it. Yet, we all rely on magnets in our daily lives, from swiping a credit card to listening to music on speakers or hanging a picture on the fridge. Beyond these familiar solid magnets, magnetism can also be harnessed in fluids, which are used in applications such as medical devices and dynamic seals. Since melting solid magnets to create magnetic fluids is not an option, magnetic control is achieved by suspending microscopic particles in a fluid, resulting in suspensions like superparamagnetic colloids.
Superparamagnetic colloids are microscopic particles suspended in a fluid, typically water, that remain non-magnetic until an external magnetic field is applied. Under such fields, the particles exhibit magnetic behavior, causing them to form structures that depend on the characteristics of the magnetic field. In two-dimensional systems, with a rotating uniform magnetic field, particles align in chains that rotate at low speeds. As the field’s rotational speed increases, disk-like clusters form due to isotropic interactions. Focusing experimentally on the early stages of this aggregation process, we model the transition between these structures, revealing its dependence on the magnetic field’s frequency and the colloidal suspension’s volume fraction. In three-dimensional systems,
these microscopic structures impact macroscopic processes such as sedimentation. From macroscopic observations, we establish that similar aggregates likely form in these conditions, and that the rotational speed of the magnetic field significantly influences the sedimentation rate. Additionally, we identify three distinct regimes corresponding to different sedimentation behaviors.
By understanding these fundamental mechanisms, we hope to open the door for developing advanced systems for applications such as fluid mixing, pollutant capture, or targeted drug delivery.
