Threshold magnetic fields and Fréedericksz transition in a ferronematic

Abstract. Using continuum and statistical mechanical theories, we study the switching properties of a ferronematic in a nematic liquid crystal cell subject to homeotropic boundary conditions at the cell and particle walls. An external magnetic field normal to the cell plane is also imposed. At low fields we find thresholdless switching of the nematic director, consistent with experimental data. At higher fields, there are three regimes, depending on the strength of the anchoring interaction between the director and the ferroparticle orientation. For low anchoring strengths, there is an inverse Frederiks effect, and the nematic reorientation reduces and then disappears continuously at a critical magnetic field. At intermediate fields, the degree of reorientation reduces at high fields but remains finite. For high fields, however, the director switching saturates. The dimensionless temperature scale in the problem involves the temperature, the mean nematic elastic constant, the colloidal density, and the cell dimension. If this quantity is sufficiently low, then high magnetic fields can cause magnetic segregation. The segregation order parameter is coupled to the director distortion, and this can change the inverse Frederiks transition into a first order transition, leading to bistability in an intermediate field regime. These features are perturbed but not changed structurally by the effect of a small bias magnetic field (< 10 Oe) normal to the unperturbed director. Subject to suitable choice of parameters, the theory is also quantitatively consistent with the results of the classic experiment of Chen and Amer in 1983.

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“…To begin with we put t → ∞ [18]. The colloidal concentration responds in a Boltzmann-like fashion to its local potential energy.…”

Section: Unsegregated Limitmentioning

confidence: 99%

The Frederiks Effect and Related Phenomena in Ferronematic Materials

Zadorozhnii¹,

Sluckin²,

Reshetnyak³

et al. 2008

SIAM J. Appl. Math.

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“…The stationary case in the absence of flow ðx ¼ 0Þ has been analyzed earlier by Zakhlevnykh [19]. The author has obtained solutions corresponding to the different orientational phases of FN with homeotropic, angular, and planar conditions of coupling between the director and magnetization (see Fig.…”

Section: The Magnetic Field Is Normal To a Flow Alignment Directionmentioning

confidence: 99%

“…This orientational phase corresponds to the solutions j ¼ c ¼ j L of Eqs. (19). The director and the magnetization vectors are turned on the magnetic field direction at the increasing of its strength.…”

Section: The Magnetic Field Is Normal To a Flow Alignment Directionmentioning

confidence: 99%

“…Different groups of scientists successfully synthesized this new class of complex ferromagnetic fluids [2][3][4][5]. Now, there is plenty of theoretical and experimental works devoted to the collective phenomena in FNs [6][7][8][9] and external fields influence on its structure [10][11][12][13][14][15][16][17][18][19][20][21]. However, rheological FN properties are extremely poorly investigated.…”

Section: Introductionmentioning

confidence: 99%

“…These mechanisms lead to different orientational ordering of a FN in a magnetic field. Magnetic field-induced FN phases with homeotropic, angular and planar types of the magnetic particles coupling with a NLC-matrix were analyzed by Zakhlevnykh [19]. Besides, viscous anisotropic properties of the NLC carrier allow to control FN structure.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Magnetic field-induced orientational phases of ferronematics in shear flow

Makarov

Zakhlevnykh

2008

Journal of Magnetism and Magnetic Materials

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Ferronematic Co(II) Complex: An Active Filler for Magnetically Actuated Soft Materials

Damoc,

Tiron,

Tugui

et al. 2023

Small

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Ferronematics that are generally based on nematic liquid crystals (LCs) doped with magnetic nanoparticles, synergistically taking advantage of the anisotropic and flow characteristics of the nematic host and the magnetic susceptibility of the dopant, have powerful applications as magnetically actuated soft materials. In this work, a Co(II) complex, which alone presents both characteristics, is built with a salen‐type ligand 3,5‐dichlorosubstituted at the aromatic nuclei and has a tetramethyldisiloxane spacer, which makes it one of the few metallomesogens containing this structural motif. Paramagnetic crystals, through heat treatment above 110 °C, change into magnetic nematic LCs. Applying a perpendicular magnetic field of 50 mT, the nematic droplets align two by two through dipole–dipole interactions. By incorporating it into a silicone matrix consisting mainly of polydimethylsiloxane, a 3D printable ink is formulated and crosslinked under various shapes. In this environment, the cobalt complex is stabilized in an LC state at room temperature and, due to its anisotropy, facilitates the mechanical response to magnetic stimuli. The resulting objects can be easily manipulated on fluid or rough surfaces using external magnetic fields, behave like magnets by themselves, and show reversible locomotion.

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Threshold magnetic fields and Fréedericksz transition in a ferronematic

Cited by 40 publications

References 8 publications

The Frederiks Effect and Related Phenomena in Ferronematic Materials

The Frederiks Effect and Related Phenomena in Ferronematic Materials

Magnetic field-induced orientational phases of ferronematics in shear flow

Ferronematic Co(II) Complex: An Active Filler for Magnetically Actuated Soft Materials

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