Collective hydrodynamic transport of magnetic microrollers
We investigate the collective transport properties of microscopic magnetic rollers that propel close to a surface due to a circularly polarized, rotating magnetic field. The applied field exerts a torque to the particles, which induces a net rolling motion close to a surface. The collective dynamics of the particles result from the balance between magnetic dipolar interactions and hydrodynamic ones. We show that, when hydrodynamics dominate, i.e. for high particle spinning, the collective mean velocity linearly increases with the particle density. In this regime we analyse the clustering kinetics, and find that hydrodynamic interactions between the anisotropic, elongated particles, induce p…
Overdamped dynamics of paramagnetic ellipsoids in a precessing magnetic field
We report on the dynamical behavior of paramagnetic ellipsoidal particles dispersed in water and floating above a flat plane when subjected to an external precessing magnetic field. When the magnetic field and the long axis of the particles are on the same plane, two clear regimes are distinguished in which the particles follow the magnetic modulation synchronously or asynchronously. Both regimes are also observed when the field precesses at an angle $\ensuremath{\vartheta}l90\ifmmode^\circ\else\textdegree\fi{}$ with respect to the normal to the confining plane, while the transition frequency increases with decreasing precession angle. We combine experimental observations with a theoretical…
Orientational dynamics of colloidal ribbons self-assembled from microscopic magnetic ellipsoids
We combine experiments and theory to investigate the orientational dynamics of dipolar ellipsoids, which self-assemble into elongated ribbon-like structures due to the presence of a permanent magnetic moment, perpendicular to the long axis in each particle. Monodisperse hematite ellipsoids are synthesized via the sol-gel technique and arrange into ribbons in the presence of static or time-dependent magnetic fields. We find that under an oscillating field, the ribbons reorient perpendicular to the field direction, in contrast with the behaviour observed under a static field. This observation is explained theoretically by treating a chain of interacting ellipsoids as a single particle with or…
Dipolar Rings of Microscopic Ellipsoids: Magnetic Manipulation and Cell Entrapment
We study the formation and dynamics of dipolar rings composed by microscopic ferromagnetic ellipsoids, which self-assemble in water by switching the direction of the applied field. We show how to manipulate these fragile structures and control their shape via application of external static and oscillating magnetic fields. We introduce a theoretical framework which describes the ring deformation under an applied field, allowing to understand the underlying physical mechanism. Our microscopic rings are finally used to capture, entrap and later release a biological cell via magnetic command, i.e. performing a simple operation which can be implemented in other microfluidic devices which make us…
Orientational dynamics of fluctuating dipolar particles assembled in a mesoscopic colloidal ribbon
We combine experiments and theory to investigate the dynamics and orientational fluctuations of ferromagnetic microellipsoids that form a ribbonlike structure due to attractive dipolar forces. When assembled in the ribbon, the ellipsoids display orientational thermal fluctuations with an amplitude that can be controlled via application of an in-plane magnetic field. We use video microscopy to investigate the orientational dynamics in real time and space. Theoretical arguments are used to derive an analytical expression that describes how the distribution of the different angular configurations depends on the strength of the applied field. The experimental data are in good agreement with the…