Rotating hematite cube chains

Recently a two-dimensional chiral fluid was experimentally demonstrated. It was obtained from cubic-shaped hematite colloidal particles placed in a rotating magnetic field. Here we look at building blocks of that fluid, by analyzing short hematite chain behavior in a rotating magnetic field. We find equilibrium structures of chains in static magnetic fields and observe chain dynamics in rotating magnetic fields. We find and experimentally verify that there are three planar motion regimes and one where the cube chain goes out of the plane of the rotating magnetic field. In this regime we observe interesting dynamics -- the chain rotates slower than the rotating magnetic field. In order to catch up with the magnetic field, it rolls on an edge and through rotation in the third dimension catches up with the magnetic field. The same dynamics is also observable for a single cube when gravitational effects are explicitly taken into account.Comment: 6 videos in supplementary materia

Ferromagnetic filament shapes in a rotating field reveal their magnetoelastic properties

Flexible ferromagnetic filaments can be used to control the flow on the micro-scale with external magnetic field. To accurately model them, it is crucial to know their parameters such as their magnetization and bending modulus, the latter of which is hard to determine precisely. We present a method how the ferromagnetic filament's shape in a rotating field can be used to determine the magnetoelastic number $Cm$ - the ratio of magnetic to elastic forces. Then once the magnetization of the filament is known, it is possible to determine its bending modulus. The main idea of the method is that $Cm$ is the only parameter that determines whether the filament is straight or whether its tips are bent towards the magnetic field direction. Comparing with numerical solutions, we show that the method results in an error of $15...20\%$ for the determined $Cm$, what is more precise than estimations from other methods. This method will allow to improve the comparability between theoretical filament models and experimental measurements

Gravity effects on mixing with magnetic micro-convection in microfluidics

Mixing remains an important problem for development of successful microfluidic and lab-on-a-chip devices, where simple and predictable systems are particularly interesting. One is magnetic micro-convection, an instability happening on the interface of miscible magnetic and non-magnetic fluids in a Hele-Shaw cell under applied field. Previous work proved that Brinkman model quantitatively explains the experiments. However, a gravity caused convective motion complicated the tests. Here we first improve the experimental system to exclude the gravitational convective motion. Afterwards, we observe and quantify how gravity and laminar flow play an important role in stabilizing the perturbations that create the instability. Accordingly, we improve our theoretical model and perform linear analysis. Two dimensionless quantities explain the experimental observations of change in critical field needed for instability and characteristic size of the emerging pattern. Finally, we discuss the conditions at which gravity plays an important role in microfluidic systems.Comment: Submitted to EPJE for Topical Issue (Flowing Matter, Problems and Applications),(COST Action MP1305

How gravity stabilises instability: the case of magnetic micro-convection

Finding solutions for better mixing in microfluidics remains an important challenge, including understanding fundamental aspects of these processes. Here we investigate the magnetic micro-convection on water and miscible magnetic fluid interface in a vertical microfluidic chip to understand what is the role of gravity, as fluids have different densities. Our model is reduced to two dimensionless quantities - magnetic and gravitational Rayleigh numbers. Numerical simulation results show that static magnetic field generate rich dynamics. This is confirmed quantitatively with careful experiments in initially stagnant fluids. We also show that the length of resulting mixing is limited by gravity. For this we construct a master curve, exploiting the measurements of critical field. A three-fluid layer model and linear stability analysis on its interfaces allows us to explain the limitation mechanism. Our results can help in the development of instability based micromixers.Comment: 6 supplementary movies; This draft was prepared using the LaTeX style file belonging to the Journal of Fluid Mechanic