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Magnetotactic bacteria (MTB) are of significant fundamental and practical interest, especially for applications such as drug delivery and general-purpose object manipulators and payload carriers. While magnetic and other modes of control for individual MTB have been demonstrated, formation, motion and control of MTB swarms are much less studied and understood. Here, we present a torque dipole-based theoretical model for magnetic control of MTB swarms and two methods for swarm formation, and provide experimental validation of the proposed motion model. Model predictions are in good qualitative and quantitative agreement with experiments and literature. Additionally, we were able to determine the torque generated by Magnetospirillium gryphiswaldense (MSR-1) MTB, and the value corresponds to the reported estimates reasonably well.

Phase-separated magnetic fluids provide a very strong magnetic response ($\mu>25$) in a liquid state material. Even small fields can cause a notable material response, but this depends on its properties, which are often difficult to control. Here, we investigate how temperature affects the properties of the system, where phase separation is induced by an increase in ionic strength. Following the deformations of individual microscopic droplets, we can extract surface tension, magnetic permeability, and viscosity. We find that the temperature increase does not affect the surface tension, while the magnetic permeability and viscosity increase. Applying this knowledge to induce a shape instability for a drop only using temperature shows the potential of applicability of our findings., Comment: Submitted to JMMM special issue of the 16th International Conference on Magnetic Fluids

In this work we show that macroscopic experiments can be used to investigate microscopic systems. Such macroscopic experiments enable testing the assumptions and results obtained by theoretical considerations or simulations that can not be obtained under microscope (e.g., the orientation of a spherical particle). To emulate the dynamics of a single hematite cube immersed in water and subjected to a rotating magnetic field, a cubic magnet is firmly positioned in a 3D printed superball shell. The dimensionless parameters of the microscopic and macroscopic systems can be equalized by using a glycerol-water mixture instead of water as well as by the material and infill of the 3D printed shell. The pose of the superball is tracked using ArUco stickers which act as fiducial markers. It was found that there is a qualitative agreement between the macroscopic experiments and theoretical predictions. However, the reduced thermal effects in the macroscopic experiments lead to an increase in friction between the superball and the surface, thus shifting the critical frequency of the system. Since the shell is 3D printed, the given method can be extended to any shape and magnetization orientation., Comment: 21 page, 6 figures, 1 table. Submitted to JMMM special issue ICMF 2023. Presented as poster in ICMF 2023

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 Mechanics

Effective mixing of fluids at the microfluidic scale is important for future applications in biology, medicine, and chemistry. A promising type of micromixers are magnetic filaments, which can be activated by an external magnetic field. However, there is a lack of research that combines experiments and numerical modelling of hydrodynamics around such filaments. Here we use micro-particle image velocimetry to measure flow fields around rotating flexible ferromagnetic filaments and compare them to numerical data from elastic rod model. We measure that rotating filaments hover above the surface, whereas the resulting fluid velocities are highly dependent on the hovering distance. We also find that the rotating filament causes a 3D flow coming from the rotational plane and being extracted along the axis of rotation. These findings will help develop better micromixers.

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.

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: 8 videos in supplementary material

Hematite at room temperature is a weak ferromagnetic material. Its permanent magnetization is three orders smaller than for magnetite. Thus, hematite colloids allow us to explore a different physical range of particle interaction parameters compared to ordinary ferromagnetic particle colloids. In this paper we investigate a colloid consisting of hematite particles with cubic shape. We search for energetically favorable structures in an external magnetic field with analytical and numerical methods and molecular dynamics simulations and analyze whether it is possible to observe them in experiments. We find that energetically favorable configurations are observable only for short chains. Longer chains usually contain kinks which are formed in the process of chain formation due to the interplay of energy and thermal fluctuations as an individual cube can be in one of two alignments with an equal probability., Comment: 14 pages, 22 figures

Ferromagnetic filaments in a rotating magnetic field are studied both numerically and experimentally. The filaments are made from micron-sized ferromagnetic particles linked with DNA strands. It is found that at low frequencies of the rotating field a filament rotates synchronously with the field and beyond a critical frequency it undergoes a transition to a three dimensional regime. In this regime the tips of the filament rotate synchronously with the field on circular trajectories in the plane parallel to the plane of the rotating field. The characteristics of this motion found numerically match the experimental data and allow us to obtain the physical properties of such filaments. We also discuss the differences in behaviour between magnetic rods and filaments and the applicability of filaments in mixing., Comment: Submitted to RSC Soft Matter Journal , uses rsc template

Energy input by under-field rotation of particles drives the systems to emergent non-equilibrium states. Here we investigate the suspension of rotating magnetic cubes. Micron-sized hematite cubes are synthesized and observed microscopically. When exposed to a rotating magnetic field, they form rotating swarms that interact with each other like liquid droplets. We describe the swarming behaviour and its limits and characterize swarm size and angular velocity dependence on magnetic field strength and frequency. A quantitative agreement with a theoretical model is found for the angular velocity of swarms as a function of field frequency. It is interesting to note that hematite particles with peanut or ellipsoidal shapes do not form swarms., Comment: submitted to JMMM for ICMF2019 special issue. Uses Elsevier template

Magnetic fields and magnetic materials have promising microfluidic applications. For example, magnetic micro-convection can enhance mixing considerably. However, previous studies have not explained increased effective diffusion during this phenomenon. Here we show that enhanced interface smearing comes from a gravity induced convective motion within a thin microfluidic channel, caused by a small density difference between miscible magnetic and non-magnetic fluids. This motion resembles diffusive behavior and can be described with an effective diffusion coefficient. We explain this with a theoretical model, based on a dimensionless gravitational Rayleigh number, and verify it by numerical simulations and experiments with different cell thicknesses. Results indicate the applicability and limitations for microfluidic applications of other colloidal systems. Residual magnetic micro-convection follows earlier predictions., Comment: Published in JMMM, in ICMF2019 special issue. Uses Elsevier template

Research on magnetic particles dispersed in a fluid medium, actuated by a rotating magnetic field, is becoming increasingly active for both lab-on-chip and bio-sensing applications. In this study, we experimentally investigate the behaviour of ferromagnetic filaments in a rotating field. Filaments are synthesized by linking micron-sized ferromagnetic particles with DNA strands. The experiments were conducted under different magnetic field strengths, frequencies and filament sizes, and deformation of the filaments was registered via microscope and camera. The results obtained showed that the body deformation is larger for longer filaments and higher frequencies and lower for larger magnetic field. The angle between the filament tangent at the centre and the magnetic field direction increases linearly with frequency at low-frequency regime. A further increase in the frequency will result in filament movement out of plane when the angle approaches 90 degrees. The experimental results were used to estimate magnetic moment and the bending elasticity of the filament., Comment: JMMM for ICMF2019 special issue, 5 pages, 7 figures

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)

Magnetic droplets obtained by induced phase separation in a magnetic colloid show a large variety of shapes when exposed to an external field. However, the description of shapes is often limited. Here we formulate an algorithm based on three dimensional boundary-integral equations for strongly magnetic droplets in a high-frequency rotating magnetic field, allowing us to find their figures of equilibrium in three dimensions. The algorithm is justified by a series of comparisons with known analytical results. We compare the calculated equilibrium shapes with experimental observations and find a good agreement. The main features of these observations are the oblate-prolate transition, the flattening of prolate shapes with the increase of magnetic field strength and the formation of star-fish like equilibrium shapes. We show both numerically and in experiments that the magnetic droplet behaviour may be described with a tri-axial ellipsoid approximation. Directions for further research are mentioned, including the dipolar interaction contribution to the surface tension of the magnetic droplets, account for the large viscosity contrast between the magnetic droplet and the surrounding fluid., Comment: This is a first revision of the article, submitted to the Journal of Fluid Mechanics (cambridge.org/core/journals/journal-of-fluid-mechanics)