Chapter 15: Special Topics

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1 Chapter 15: Special Topics 1. Magnetic Liquids 2. Magnetoelectrochemistry 3. Magnetic Levitation 4. Magnetism in Biology and Medecine 5. Planetary and Cosmic Magnetism Comments and corrections please: Dublin April

2 Further reading R. E. Rosencwaig, Ferrohydrodynamics, Dover, New York 1990 Everything you need to understand ferrofluids P. A. Davidson An introduction to Magnetohydrodynamics, CUP, A lucid and readable introduction to MHD. M Yamaguchi and Y Tanamori (editor), Magneto-science, Kodansha, Tokyo 2006, A compendium of unusual applications of magnetism. Dublin April

3 Myths and Dreams - Perpetual Motion Dublin April

4 Myths and Dreams - Levitation Gulliver waving at Laputa, the flying magnetic island Dublin April

5 15.1 Magnetic Liquids Dublin April

6 Ferrofluids. Ferrofluids are colloidal suspensions of ferromagnetic nanoparticles, nm in size. They are coated with surfactant or embedded in a polymer bead (dynabead). Thermal energy k B T must prevent sedimentation. Particles must not agglomerate due to dipole-dipole forces, nor move appreciably in a field gradient. Typical ferrofluid has 10 % by volume magnetite. M 50 ka m -1. (J 0.05 T). m = Bohr magnetons M = M 0 (coth x - 1/x) where x = mb/kt Can approach saturation in sub-tesla fields. = Note that magnetic self-energy -(1/2)µ 0 MH d = -(1/2) µ 0 NM J/m 3 NB g 50,000 J/m 3 Dublin April

7 Peaked instability in a vertical field Dublin April

8 Ferrofluid Applications Seals. Rotary seals for high vacuum Magnetic separation Magnetorheological fluids. Magnetic Bernoulli equation: ( v/ t) + v. v = - P* + µ 0 M H + g Dublin April

9 Magnetic confinement 1 mm iron track in perspex disc Dublin April

10 Paramagnetic liquid tubes H CoCl 2 liquid tube stabilised in water in a vertical magnetic field 1.5 M CoCl 2 red = M NiSO 4 green = M ErCl 3 pink = Dublin April

11 Dublin April

12 Horizontal field, antitube Dublin April

13 Iron track immersed in water Dublin April

14 15.2 Magnetoelectrochemistry There are two magnetic body forces which an produce measurable effects in electrochemical cells: Lorentz force F l = j x B Field gradient force F = c mol B B/µ 0 Dublin April

15 14.3 Magnetic Levitation A sumo wrestler standing on a magnetic plate levitated above a large disc of cuprate superconductor Dublin April

16 Levitation of diamagnetic holes Graphite Silicon Buoyancy condition B = -gµ 0 ( 0 - sol ) /( m - sol ) sol Titanium Water CoCl 2 (1M) DyNO 3 (1M) Dublin April

17 Earnshaw s theorem (1842) It is impossible to levitate a fixed magnetic dipole with any configuration of static magnetic field. This is true for any object whose energy satisfies Laplace s Equation 2 U = 0 (charges, masses) Dublin April

18 Magnetic bearings V.Fd 3 r = S F.dS = 0. The potential has a saddle point. Bearing stiffness K is defined as a vector with components - F x / x, - F y / y, - F z / z. surface K x + K y + K z = 0 Axial bearing Radial bearing Linear bearing Dublin April

19 Maglev Pudong airport (Shanghai) Transrapid Maglev train Dublin April

20 Any diamagnet can be levitated by an appropriate combination of magnetic field and magnetic field gradient. U = -µ 0 V H 2 /2 The force - U must balance the weight - gv B B = -gµ 0 / m Levitation condition for graphite B B = 250 T 2 m -1 Levitation condition for water B B = 1400 T 2 m -1 ( m = m 3 kg -1 ) Dublin April

(Magnetospirillum magnetotacticum) Thin section of MS-1.

21 15.4 Magnetism in Biology and Medecine Magnetotactic bacteriae Row of biogenic magnetite particles Strain MS-1 (Magnetospirillum magnetotacticum) Thin section of MS-1. Magnetsomes are approx. 45 nm in diameter. Dublin April

22 Magnetic biochip Another example of a silicon back-end process. Dublin April

23 Dublin April

Interplanetary fields are 1-10 nt. Field at the surface of earth or sun is 0.1 mt.

24 15.5 Planetary and Cosmic Magnetism Magnetic fields spanning 21 orders of magnitude are found in the cosmos. The ambient field in interstellar space is 0.1 nt. Interplanetary fields are 1-10 nt. Field at the surface of earth or sun is 0.1 mt. Fields of T exist in neutron stars and magnetars. Self-sustaining dynamo Dublin April

25 Dublin April

26 Magnetic moment of planets and moons Dublin April

27 The Sun Dublin April

28 From Japan s Hinode Solar Optical Telescope A magnetic map of the flare zone on the southern flanks of sunspot 930 Dublin April

29 Dublin April

Magnetic Materials. From Akita, Japan RWF Chemistry H2A

Magnetic Materials. From Akita, Japan RWF Chemistry H2A Magnetic Materials Fe3O4 From Akita, Japan RWF Chemistry H2A We will learn how to predict the paramagnetic properties of a molecule from MO theory, based on whether it possesses unpaired electrons: For