Orientational dynamics of fluctuating dipolar particles assembled in a mesoscopic colloidal ribbon

Abstract

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 developed model for all the range of field parameters explored. Understanding the role of fluctuations in chains composed of dipolar particles is important not only from a fundamental point of view, but it may also help understanding the stability of such structures against thermal noise, which is relevant in microfluidics and laboratory-on-a-chip applications.

The most energetically favorable configurations of hematite cube chains

“…In search for the missing effect we performed finite temperature MD simulations as in experiments at room temperature cubes and ellipsoids [7,9] significantly fluctuate. We initialized our calculations with energetically favorable configurations depending on the chain length and the moment orientation.…”

“…They can be synthesized in different shapes: cubes, disks, ellipsoids, peanuts, and others [1][2][3][4] and maintain a permanent dipole moment even at large sizes (up to 15 µm) [5,6]. Apart from a new physical regime where steric forces compete with magnetic forces, compared to ordinary ferromagnetic colloids, they provide also an opportunity to directly observe the different structures in colloids with an optical microscope [1,5,7,8]. As hematite is a weak ferromagnetic at room temperature, thermal fluctuations play an important role.…”

“…As hematite is a weak ferromagnetic at room temperature, thermal fluctuations play an important role. Hematite particles can form chains and rings, in which fluctuations are clearly visible [7][8][9].…”

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].…”

“…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]. Additionally, these chains can easily be made into rings, which can be assembled around a cargo particle, and then used to transport it, see Figure 1c [50,54].…”