Ferrofluids

1. Ferrofluids Joshua Tan Christian Spenninger Sun Yudong

2. Agenda • Ferrofluids • Intro • How is clumping inhibited? • Surfactants • Effects of magnetic fields on ferrofluids

3. What are Ferrofluids • In short, it is a liquid that responds to magnets and magnetic fields. • The longer answer is ferrofluid is an extremely fine powder, coated with a soap-like material called a surfactant, suspended in a mineral oil liquid base. • The resulting magnetic suspension is called a ferrofluid.

4. What are Ferrofluids • When no magnetic field is present, ferrofluid behaves and flows like a normal liquid. • However, when a magnet or magnetic field is introduced, the ferrofluid is attracted to the field. • Spikes then form along the magnetic field lines when the magnetic surface force exceeds the stabilizing effects of fluid weight and surface tension.

5. Introduction • Ferrofluids are, basically, liquids which have been magnetised by a magnetic field. • They are colloidal liquids, having microscopic particles evenly distributed throughout it. • These particles are ferromagnetic: • They exhibit a magnetic force strong enough to be felt in the physical world, as opposed to other kinds of magnetism recognised in physics which can only be detected by sensitive equipment.

6. How is clumping inhibited? • The magnetic particles mentioned in the previous slide are coated with a surfactant that prevents the particles from clumping together. • The magnetic attraction of nanoparticles is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping.

7. How is clumping inhibited? • True ferrofluids are stable. This means that the solid particles do not clump together even in extremely strong magnetic fields. • However, the surfactant tends to break down over time , and eventually the nanoscale particles will clump together, and they will separate out and no longer contribute to the fluid's magnetic response.

8. Surfactants • These are some examples of surfactants: • oleic acid (found in various animal and vegetable fats) • citric acid • soy lecithin oleic acid • soy lecithin

9. Effects of magnetic fields on ferrofluids • When a ferrofluid is subjected to a strong vertical magnetic field, the surface forms a regular pattern of peaks and valleys. This effect is known as the normal-field instability. The instability is driven by the magnetic field; it can be explained by considering which shape of the fluid minimizes the total energy of the system.

10. Conditions • Ferrofluids usually do not retain magnetization in the absence of an externally applied field and thus are more accurately described as strong liquid electromagnets rather than ferromagnets.

11. Ferrofluids • The particles in a ferrofluid primarily consist of nanoparticles which are suspended by Brownian motion and generally will not settle under normal conditions.

12. Resistance • Ferrofluids also change their resistance according to the following equation: • With ρ as the resistance in MΩ, • V as the Vollema Value, different for each ferrofluid, • B as the strength of the magnetic field in mT, • and p as the Pietrow constant, currently measured at 0.09912.