Disertācijas (FMOF) / Doctoral Theses

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    Magnetic fluid droplets in rotating fields: theory, experiments and simulations
    (Sorbonne University, 2022-10-14) Stikuts, Andris Pāvils; Cēbers, Andrejs; PERZYNSKI, Régine
    Due to a combination of responsiveness to external magnetic fields and their deformability, magnetic fluid droplets make an interesting material that has found many applications in microfluidics. This work explores the dynamics of such droplets in a rotating magnetic field. The droplets are examined using multiple approaches – theoretically, experimentally and using simulations. When the rotating magnetic field is weak and the droplet’s deformation is small, the droplet’s motion is calculated analytically. It is found that the droplet’s shape evolution is governed by a system of three nonlinear differential equations. In the small deformation limit, the motion of the droplet is qualitatively governed by a parameter proportional to the capillary number – the ratio of viscous drag to surface tension forces. The experimental observation of magnetic droplets obtained by the separation of a ferrofluid in two liquid phases, qualitatively follows the analytic solution, however, there is a significant quantitative discrepancy. A simulation based on the boundary element methods is developed to calculate the dynamics of the droplets up to medium deformations. It is found that good mesh maintenance is required to produce accurate simulation results. A phase diagram is produced, which shows the droplet dynamics depending on the rotating field strength and frequency. Finally, the collective dynamics of the droplets is examined experimentally. For a certain magnetic field strength and frequency, the droplets form rotating ensembles with a triangular order – two dimensional rotating crystals. The dynamics of small ensembles is reproduced by treating the droplets as point torques.
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    Разработка и применение специальных численных методов для решения задач математической физики, гидродинамики и магнитной гидродинамики
    (Латвийский университет, 1993) Калис, Харий; Латвийский университет, Кафедра дифференциальных уравнений и приближенных методов
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    Numerische Simulation der Phasengrenzen und Schmelzenströmung bei der Züchtung von Siliziumeinkristallen mit dem Floating-Zone Verfahren
    (1997) Virbulis, Jānis; Universität Lettland. Fakultät für Physik und Mathematik, Lehrstuhl für Elektrodynamik und Kontinuumsmechanik
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    Hydrodynamic instabilities in microfluidic magnetic fluid flows
    (2015-07-16) Kitenbergs, Guntars; Cēbers, Andrejs; Perzynski , Régine
    Abstract in English: Magnetic field induced instabilities of magnetic fluids in microfluidic environment are investigated experimentally using electrically stabilized water-based magnetic nanocolloids. Magnetic micro-convection, observed at a miscible interface in a Hele-Shaw cell in homogeneous field, is studied quantitatively and is compared with theoretical predictions and numerical simulations. An application towards mixing enhancement in microfluidics is considered. The micro-convection study reveals a large effective nanoparticle diffusion coefficient, which mostly arises from a flow induced by the density difference between the miscible fluids. Experiments with magnetic drops, formed by phase separation, show that in a precessing field at magic angle, the drops undergo two shape instabilities as in a rotating field except the initial shape deformation. - - - ANOTĀCIJA LATVIEŠU VALODĀ: Eksperimentāli pētītas magnētiskā lauka izraisītas magnētisko šķidrumu nestabilitātes mikrofluidikā, lietojot elektriski stabilizētus magnētiskus nanokoloīdus uz ūdens bāzes. Magnētiskā mikro-konvekcija, ko novēro homogēnā laukā uz sajaukties spējīgas robežas Hele-Šou šūnā, tiek pētīta kvantitatīvi un tiek salīdzināta ar teorētiskajām sakarībām un skaitliskajiem aprēķiniem. Ir apskatīts pielietojums maisīšanas uzlabošanai mikrofluidikā. Mikro-konvekcijas pētījums atklāj lielu nanodaļiņu efektīvo difūzijas koeficientu, kas galvenokārt rodas no šķidrumu blīvumu atšķirību radītās plūsmas. Eksperimenti ar magnētiskiem pilieniem, kas rodas fāžu atdalīšanās procesā, rāda, ka pilieniem precesējošā laukā pie maģiskā leņķa, gluži kā rotējoša lauka gadījumā, novērojamas divas formas nestabilitātes, izņemot sākotnējo formas deformāciju.
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