arXiv subjects

Andrejs Cebers

Publications and source records attributed to Andrejs Cebers.

15 recordsLinked to original sources

arXiv2025

Cryogenic pressure sensing with an ultrafast Meissner-levitated microrotor

Magnetically levitated spinning rotors are key elements in important technologies such as navigation by gyroscopes, energy storage by flywheels, ultra-high vacuum generation by turbomolecular pumps, and pressure sensing for process control. However, mechanical rotors are typically macroscopic and limited to room temperature and low rotation frequencies. In particular, sensing pressure at low temperatures remains a technological challenge, while emerging quantum technologies demand a precise evaluation of pressure conditions at low temperatures to cope with quantum-spoiling decoherence. To close this gap, we demonstrate wide range pressure sensing by a spinning rotor based on a micromagnet levitated by the Meissner effect at 4.2 Kelvin. We achieve rotational speeds of up to 138 million rotations per minute, resulting in very high effective quality factors, outperforming current platforms. Beside sensing applications, we envision the use of levitated rotors for probing fundamental science including quantum mechanics and gravity, enabled by ultralow torque noise.

quant-ph↗

arXiv2025

Driven shear flow in biological magneto-active fluids

Active fluids made of powered suspended particles have unique abilities to self-generate flow and density structures. How such dynamics can be triggered and leveraged by external cues is a key question of both biological and applied relevance. Here we use magnetotactic bacteria to explore how chemotaxis and magnetotaxis -- leading, respectively, to positional and orientational responses -- combine to generate global scale flows. Such steady regime can be quantitatively captured by a magneto-active hydrodynamic model, while time-dependent magnetic driving unveils additional patterning complexity. Overall, our findings shed light on how active fluids respond to the ubiquitous situation of multiple external information, also suggesting routes for their manipulation.

cond-mat.soft↗

arXiv2025

Image-based physical characterization of magnetotactic bacteria from an environmental sample

Magnetotactic bacteria (MTB) are a diverse group of microorganisms that are able to biomineralize magnetic nanoparticles. Most MTB remain uncultured, making population-level characterization from natural environments difficult. We report the discovery of a new and diverse MTB-rich site in the Ogre River, Latvia, and present an integrated approach combining 16S rRNA sequencing, transmission electron microscopy, and novel open-source, automated image-based physical methods to characterize bacteria populations within environmental samples. We introduce a pipeline for cell velocimetry using a static magnetic field and a method to classify cell populations based on their magnetic moment using a modified U-turn method where cell behavior is studied in an alternating magnetic field. This study demonstrates that our physical analysis methods provide a powerful, fast, and robust toolset for MTB population analysis in complex environmental samples.

physics.bio-ph↗

arXiv2025

Explicit and fully automatic analysis of magnetotactic bacteria motion reveals the magnitude and length scaling of magnetic moments

Magnetotactic bacteria (MTB) are a diverse group of microorganisms whose movement can be directed via a magnetic field, which makes them attractive for applications in medicine and microfluidics. One of their key properties is the magnetic moment $m$, which is challenging to measure experimentally. We perform optical imaging experiments with MSR-1 MTB, and derive both the $m$ statistics and the scaling of $m$ with the MTB size using an explicit and fully automated method to determine $m$ from the MTB trajectories via the U-turn protocol, which measures $m$ based on the U-shaped trajectories exhibited by the MTB in an applied alternating magnetic field. The proposed method is an alternative to the standard U-turn time-based moment calculation and uses the theoretical U-turn shape function we have derived. This directly accounts for the U-turn geometry and determines the moment from the U-turn branch width. We couple this approach with a robust U-turn decomposition algorithm that detects U-turns from MTB tracks regardless of their orientations. We report a linear dependence of $m$ on the size of the bacteria, accounting for the bacteria velocity variations during the U-turns. We also demonstrate that the new U-turn shape-based and the conventional time-based methods produce significantly different results. The proposed method can be used to differentiate between various types of MTB within the same population based on their velocity and magnetic moments, and to precisely characterize the magnetic properties of a culture.

physics.bio-ph↗

arXiv2024

Magnetic control of magnetotactic bacteria swarms

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.

physics.bio-ph↗

arXiv2021

Collective hydrodynamic transport of magnetic microrollers

We investigate the collective transport properties of microscopic magnetic rollers that propel close to a surface due to a circularly polarized, rotating magnetic field. The applied field exerts a torque to the particles, which induces a net rolling motion close to a surface. The collective dynamics of the particles result from the balance between magnetic dipolar interactions and hydrodynamic ones. We show that, when hydrodynamics dominate, i.e. for high particle spinning, the collective mean velocity linearly increases with the particle density. In this regime we analyse the clustering kinetics, and find that hydrodynamic interactions between the anisotropic, elongated particles, induce preferential cluster growth along a direction perpendicular to the driving one, leading to dynamic clusters that easily break and reform during propulsion.

cond-mat.soft↗

arXiv2017

Orientational Dynamics of Fluctuating Dipolar Particles Assembled in a Mesoscopic Colloidal Ribbon

We combine experiments and theory to investigate the dynamics and orientational fluctuations of ferromagnetic microellipsoids that form a ribbon-like structure due to attractive dipolar forces. When assembled in the ribbon, the ellipsoids displays orientational thermal uctuations 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/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 uctuations 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 micro uidics and lab-on-a-chip applications.

cond-mat.soft↗

arXiv2017

Magnetic micro-droplet in rotating field: numerical simulation and comparison with experiment

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.

physics.flu-dyn↗

arXiv2016

Dipolar Rings of Microscopic Ellipsoids: Magnetic Manipulation and Cell Entrapment

We study the formation and dynamics of dipolar rings composed by microscopic ferromagnetic ellipsoids, which self-assemble in water by switching the direction of the applied field. We show how to manipulate these fragile structures and control their shape via application of external static and oscillating magnetic fields. We introduce a theoretical framework which describes the ring deformation under an applied field, allowing to understand the underlying physical mechanism. Our microscopic rings are finally used to capture, entrap and later release a biological cell via magnetic command, i.e. performing a simple operation which can be implemented in other microfluidic devices which make use of ferromagnetic particles.

cond-mat.soft↗

arXiv2016

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

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 in each particle of a permanent magnetic moment perpendicular to the long axis. Monodisperse hematite ellipsoids are synthesized via sol-gel technique, and arrange into ribbons in 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 an orientational and demagnetizing field energy. The model allows describing 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 towards controlling the structure formation which has direct applications in different fluid-based microscale technologies.

cond-mat.soft↗

arXiv2016

Hydrodynamics with spin in bacterial suspensions

We describe a new kind of self-propelling motion of bacteria based on the cooperative action of rotating flagella on the surface of bacteria. Describing the ensemble of rotating flagella in the framework of the hydrodynamics with spin the reciprocal theorem of Stokesian hydrodynamics is generalized accordingly. The velocity of the self-propulsion is expressed in terms of the characteristics of the vector field of flagella orientation and it is shown that unusually high velocities of \textit{Thiovulum majus} bacteria may be explained by the cooperative action of the rotating flagella. The expressions obtained enable us to estimate the torque created by the rotary motors of the bacterium and show quantitative agreement with the existing experimental data.

cond-mat.soft↗

arXiv2015

Current Induced Fingering Instability in Magnetic Domain Walls

The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce finger-like patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the non-adiabatic contribution of the spin transfer torque mechanism.

cond-mat.mtrl-sci↗

arXiv2013

Diffusion in active magnetic colloids

Properties of active colloids of circle swimmers are reviewed. As an particular example of active magnetic colloids the magnetotactic bacteria under the action of a rotating magnetic field is considered. The relation for a diffusion coefficient due to the random switching of the direction of rotation of their rotary motors is derived on the basis of the master equation. The obtained relation is confirmed by the direct numerical simulation of random trajectory of a magnetotactic bacterium under the action of the Poisson type internal noise due to the random switching of rotary motors. The results obtained are in qualitative and quantitative agreement with the available experimental results and allows one to determine the characteristic time between the switching events of a rotary motor of the bacterium.

cond-mat.soft↗

arXiv2011

Twisting and buckling: a new undulation mechanism for artificial swimmers

We present an artificial swimmer consisting in a long cylinder of ferrogel which is polarized transversely and in opposite directions at each extremity. When it is placed on a water film and submitted to a transverse oscillating magnetic field, this artificial worm undulates and swims. Whereas symmetry breaking is due to the field gradient, the undulations of the worm result from a torsional buckling instability as the polarized ends tend to align with the applied magnetic field. The critical magnetic field above which buckling and subsequent swimming is observed may be predicted using elasticity equations including the effect of a magnetic torque. As the length of the worm is varied, several undulation modes are observed which are in good agreement with the bending modes of an elastic rod with free ends.

cond-mat.soft↗

arXiv2005

Nucleation and Collapse of the Superconducting Phase in Type-I Superconducting Films

The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis. Connection with similar effects in systems with long-range magnetic interactions is emphasized.

cond-mat.supr-con↗