arXiv subjects

Andris P. Stikuts

Publications and source records attributed to Andris P. Stikuts.

8 recordsLinked to original sources

arXiv2025

Tunable dynamics of flexible magnetic microcrosses: synchronous rotation, breathing and out-of-plane arm overtaking

We combine colloidal self-assembly and soft-lithography techniques to realize flexible magnetic microcrosses that can be manipulated via external, time dependent magnetic fields. The crosses are characterized by a central domain connected via four flexible arms. When subjected to an in-plane, rotating magnetic field, the crosses transit from a synchronous to an asynchronous spinning motion where their average rotation decreases with the driving frequency. In the asynchronous regime and at low field amplitudes, the crosses display a breathing mode, characterized by relative oscillations between the arms, while remaining localized in the two dimensional plane. In contrast, for high field amplitudes, we observe an arm overtaking regime where two opposite filaments surpass the remaining ones forcing the cross to perform a three-dimensional gyroscopic-like rotation. Using slender body theory and balancing the effect of magnetic and elastic interactions, we recover the experimental findings and show that the overtaking regime occurs due to different arm magnetizations. Our engineered microscopic colloidal rotors characterized by multiple flexible filaments may find potential applications for precise lab-on-a-chip operations or as stirrers dispersed within microfluidic or biological channels.

cond-mat.soft↗

arXiv2025

Engineering tunable fractional Shapiro steps in colloidal transport

Shapiro steps are quantized plateaus in the velocity-force or velocity-torque curve of a driven system, when its speed remains constant despite an increase in the driving force. For microscopic particles driven across a sinusoidal potential, integer Shapiro steps have been observed. By driving a single colloidal particle across a time-modulated, non-sinusoidal periodic optical landscape, we here demonstrate that fractional Shapiro steps emerge in addition to integer ones. Measuring the particle position via individual particle tracking, we reveal the underlying microscopic mechanisms that produce integer and fractional steps and demonstrate how these steps can be controlled by tuning the shape and driving protocol of the optical potential. The flexibility offered by optical engineering allows us to generate wide ranges of potential shapes and to study, at the single-particle level, synchronization behavior in driven soft condensed matter systems.

cond-mat.soft↗

arXiv2023

Macroscopic emulation of microscopic magnetic particle systems

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.

cond-mat.soft↗

arXiv2023

Local flow measurements around flexible filaments under rotating magnetic field

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.

physics.flu-dyn↗

arXiv2023

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.

cond-mat.soft↗

arXiv2022

Small deformation theory for a magnetic droplet in a rotating field

A three dimensional small deformation theory is developed to examine the motion of a magnetic droplet in a uniform rotating magnetic field. The equations describing the droplet's shape evolution are derived using two different approaches - a phenomenological equation for the tensor describing the anisotropy of the droplet, and the hydrodynamic solution using perturbation theory. We get a system of ordinary differential equations for the parameters describing the droplet's shape, which we further analyze for the particular case when the droplet's elongation is in the plane of the rotating field. The qualitative behavior of this system is governed by a single dimensionless quantity $τω$ - the product of the characteristic relaxation time of small perturbations and the angular frequency of the rotating magnetic field. Values of $τω$ determine whether the droplet's equilibrium will be closer to an oblate or a prolate shape, as well as whether it's shape will undergo oscillations as it settles to this equilibrium.We show that for small deformations, the droplet pseudo-rotates in the rotating magnetic field - its long axis follows the field, which is reminiscent of a rotation, nevertheless the torque exerted on the surrounding fluid is zero. We compare the analytic results with a boundary element simulation to determine their accuracy and the limits of the small deformation theory.

physics.flu-dyn↗

arXiv2022

A 3D BEM algorithm for simulations of magnetic fluid droplet dynamics

This paper outlines a numerical algorithm that could be used for simulating full 3D dynamics of magnetic fluid droplet shapes in external magnetic fields, by solving boundary integral equations. The algorithm works with arbitrary droplet and carrier fluid viscosity ratios. It is validated with known theoretical relationships. Thus it may be used to evaluate various approximations often used in description of ellipsoidal droplets by comparing droplet dynamics calculated from them to the results obtained numerically from first principles here. The algorithm may be used for investigations of droplet configurations in arbitrary magnetic fields, as well as for indirectly calculating its physical properties and predicting the magnetic field thresholds above which droplet shape can develop instabilities in the form of various spikes.

physics.flu-dyn↗

arXiv2019

Spontaneous order in ensembles of rotating magnetic droplets

Ensembles of elongated magnetic droplets in a rotating field are studied experimentally. In a given range of field strength and frequency the droplets form rotating structures with a triangular order - rotating crystals. A model is developed to describe ensembles of several droplets, taking into account the hydrodynamic interactions between the rotating droplets in the presence of a solid wall below the rotating ensemble. A good agreement with the experimentally observed periodic dynamics for an ensemble of four droplets is obtained. During the rotation, the tips of the elongated magnetic droplets approach close to one another. An expression is derived that gives the magnetic interaction between such droplets by taking into account the coulombian forces between magnetic charges on the droplet tips.

cond-mat.soft↗