63 research outputs found
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 - 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 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 for the
determined , 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
- …