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Macroscopic Dynamics of Ferromagnetic Nematics
Author: Tilen Potisk Advisor: Doc. Dr. Daniel SvenΕ‘ek
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OUTLINE Motivation Theory of Macroscopic Dynamics Experiments
Numerical results Conclusion
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MOTIVATION Hypothesized in 1970 (Brochard and de Gennes).
Nematic liquid crystals + magnetic nanoparticles Magneto optic devices, magnetic field visualization, LCDs, ... Ferromagnetic liquid crystal Discovered in 2013 (Mertelj et al)
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MACROSCOPIC DYNAMICS Our wish - description of a macroscopic system out of equilibrium. Only few variables are slow in homogeneous limit: π πβπ βπ Three types: Consevation laws (global symmetries) β mass, energy, momentum Can not be created or destroyed localy (only transported). Spontaneous symmetry breaking β director Homogenous rotation of director does not cost energy. Slowly relaxing variables β magnetization Slow relaxation
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MACROSCOPIC DYNAMICS Relevant variables: π π + π π =0 π π + π π =0
Mass π Momentum π Energy π Concentration π Order parameter π Director π Magnetization π΄ A. Mertelj et al: Magneto-optic and converse magnetoelectric effects in a ferromagnetic liquid crystal π π + π π =0 π π + π π =0 A. Mertelj et al: Ferromagnetism in suspensions of magnetic platelets in liquid crystal
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MACROSCOPIC DYNAMICS 1. step: Describe statics
π=β π π π΅ π π΄ 1 πβ
π π΄ 2 π π 2 + πΎ ππππ π» π π π π» π π π Coupling: Interaction of nematic with the surface of nanoplatelets (nanometre sized ferromagnetic platelets). Elastic term: πΎ 1 π»β
π πΎ 2 πβ
π»Γπ πΎ 3 nΓ π»Γπ nΓ π»Γπ 2 Coupling Elastic term Splay Twist Bend
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MACROSCOPIC DYNAMICS 2. Step: Derive thermodynamic forces
Forces are thermodynamic conjugates (variational derivatives): β π π = ππ π π π β π» π ππ π π» π π π β πΏπ πΏ π π In equilibrium: β π π =0. Forces: nematic molecular field, magnetic molecular field, temperature gradient π»π, concentration gradient π»π, density gradient π»π.
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MACROSCOPIC DYNAMICS π π + π π =0 π π + π π =0
π π + π π =0 π π + π π =0 3. Step: Expand (quasi) currents into forces. Symmetry of reversible and irreversible (dissipative) currents. Example: π‘ββπ‘, time reversal symmetry, π½= π½ π· + π½ π
. Entropy production π π β π π½ π π π In the lowest order π π is a quadratic form in forces (dissipation function): π π = π,π πΏ ππ π π π π =2π
. Obtain dissipative currents by partial derivation: ππ
π π π = π½ π π· π½ π = π πΏ ππ π π
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MACROSCOPIC DYNAMICS π π + π π =0 π π + π π =0
π π + π π =0 π π + π π =0 Dissipative part of the currents: π π π· = 1 πΎ 1 πΏ ππ β₯ β π π + π ππ π· β π π π π π· = π ππ π· β π π + π ππ π· β π π π ππ π· = π 1 π· πΏ ππ β₯ π π π π + π 2 π· πΏ ππ β₯ π π π π π ππ π· = π β₯ πΏ ππ β₯ + π β₯ π π π π πΏ ππ β₯ = πΏ ππ β π π π π Lowest order in allowed symmetries: odd in π and π΄. Uniaxial symmetry. Transverse Kroenecker delta: Projection on plane perpendicular to π.
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MACROSCOPIC DYNAMICS For reversible currents, require entropy production to be zero π π =0. Entropy is here a conserved quantity. π π π
= πΎ β1 ππ β π π + π π
(πΓ β π ) π π π
= π ππ π
β π π + π π
(πΓ β π ) Structure of π ππ π
and πΎ β1 ππ : π ππ π
= π 1 π
π πππ π π + π 2 π
π πππ π π π π π π + π 3 π
( π πππ π π π π π π β π πππ π π π π π π ) Not present in ordinary nematics or isotropic ferrofluids. Even in director π: 0th order, 2nd order Odd in magnetization π: 1st order
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EXPERIMENTS A cell filled with ferromagnetic liquid crystal.
Director parallel to the surface plates. Sample is quenched to obtain a stable phase. A. Mertelj et al: Magneto-optic and converse magnetoelectric effects in a ferromagnetic liquid crystal
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EXPERIMENTS Magnetic field perpendicular to glass plates was turned on. Examined with polarizing microscopy. The sample is put between the polarizer (P) and the analyzer (A). A. Mertelj et al: Ferromagnetism in suspensions of magnetic platelets in liquid crystal
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EXPERIMENTS Optical axis defined by director π.
Extraordinary and ordinary rays. Normalized phase difference was measured: r B =1β π(π΅) π(0) π= π 0 π 0 π ( π π (π§)β π 0 )dz . π π π§ extraordinary refractive index, π 0 ordinary refractive index, π 0 wavenumber, π cell thickness, π β azimuthal angle of magnetization
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NUMERICAL RESULTS: STATICS
Equilibrium normalized phase difference. Equilibrium normalized z-component of magnetization: π π§ π 0 = 0 π cos π , π β azimuthal angle of magnetization
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NUMERICAL RESULTS: DYNAMICS
Director reorientation in magnetic field: π=( cos π sin π , sin π sin π ,cosβ‘(π))
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NUMERICAL RESULTS: DYNAMICS
Response is faster at higher magnetic fields. A high value of crosscoupling β overshoot in phase difference Numerical results Experiment A. Mertelj, N. Osterman: experimental results, unpublished A. Mertelj, N. Osterman: experimental results, unpublished
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NUMERICAL RESULTS: DYNAMICS
Relaxation time: model function π π‘ = π 0 [1β π β π‘ π π ] Experimental observation: 1/π linear in magnetic field. Numerical results: Weak dissipative crosscoupling β saturation Strong dissipative crosscoupling β 1/π linear in magnetic field Numerical results Experiment A. Mertelj, N. Osterman: experimental results, unpublished
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CONCLUSION Ferromagnetic nematic liquid crystals (hypothesized 1970, discovered 2013) Coupling of π and π΄ βΉ nontrivial dynamics. Relaxation rate (1/π) grows linearly with π©. Further investigations: linear momentum, fluctuation modes. Magnetooptic devices, smart fluids
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