Orientational dynamics of colloidal ribbons self-assembled from microscopic magnetic ellipsoids

Abstract

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 orientational and demagnetizing field energy. The model allows us to describe the orientational behaviour of the chain and captures well its dynamics at different strengths of the actuating field. The understanding of the complex dynamics and assembly of anisotropic magnetic colloids is a necessary step for controlling the structure formation, which has direct applications in different fluid-based microscale technologies.

Propulsion and hydrodynamic particle transport of magnetically twisted colloidal ribbons

“…In absence of any external field (H 0 = ), the hematite ellipsoids spontaneously assemble into chains or rings due to attractive dipolar interactions arising from their permanent magnetic moments [30]. In a previous work [31], we investigated the orientational dynamics of individual ellipsoids under a static external field and measured an average magnetic moment of m 2.3 10 A m 16 2 =´-. Since this moment is perpendicular to the long axis of the ellipsoid, when chaining the particles arrange side by side, forming a ribbon.…”

“…This field has different effects on the chain of ellipsoids. First, the component H x orients the ribbon along the x axis, minimizing the magnetostatic and the effective demagnetizing energy densities [31]. This effect makes the chain stiffer, as it further aligns the particle moments along the ribbon axis.…”

“…The magnetic interactions are important to assemble the chain of particles and to generate the net torque, but do not have any direct influence on the produced hydrodynamic flow. As a matter of fact, such interactions have been already addressed in a different context [30,31], and here we assume that the ellipsoids rotates at an average angular velocity áWñ . Using equation (1) it also possible to determine the average speed for a chain of spherical rotors in a similar way as done in [44], where paramagnetic colloids were magnetically assembled in chains that translated parallel to the surface and along their main axis.…”

Leveraging collective effects in externally driven colloidal suspensions: experiments and simulations

“…In the intermediate range, when magnetic interactions are comparable to hydrodynamic interactions, chain-like structures assemble, but are much more flexible. Due to the finite relaxation time of particle magnetization, these chains orient perpendicular rather than parallel to the applied field [50,51,52,53]. The hydrodynamic flow adjacent to these assembled chains can be used to shuttle cargo [53].…”

Multidirectional colloidal assembly in concurrent electric and magnetic fields

“…[1][2][3][4][5] The physical properties of materials formed on the basis of such structures are governed by the spatial organization of the particles, which can further be altered by changing the interparticle interactions. [6][7][8][9][10] One of the most efficient, robust and widely used methods for directing the colloidal assembly and controlling the interparticle interactions is the application of external electric or magnetic fields. [11][12][13][14][15] Depending upon the physiochemical characteristics of the particles and the applied field(s), the colloids can be assembled into linear chains, 2D or 3D bundles or crystals of desired symmetry.…”