adaptive optics using ferro-fluids mathieu de goer-de herve and raphael-david lasseri (ens cachan)
TRANSCRIPT
Resolution in optics
• Two limitant factors :→ Diffraction
→ Atmosphere turbulences : speckle patterns.
Characteristic length DAiry disc : Ø=λxf/D
Characteristic length dc (correlation)dc~10cmDisc : Ø=λxf/dc
D
Ø
How?● Mechanics deformation of a “usual” mirror using piezoelectric devices
● Shack-Hartmann wavefront sensor
Ferrofluids
• General definition:
Colloidal suspensions of magnetic nanoparticles conferring super-paramagnetic properties to the fluid.
When a magnetic field is applied to the system their magnetic moments tend to align along the applied field, leading to a net magnetization.
Rosenweig instability • Domain of Stability
“Smooth surface”
• Domain of the RW Instability
“Hedgehog surface”
B>Bc B<Bc
Approximations : We will neglect the influence of the capillary forces in this stable domain,
3 Major Factors:- Gravity-Magnetostrictive Pressure - Laplace Forces
When the Equilibrium is reached -> Bernouilli Generalised Equation (1)
MHD Fundamentals Equations
Numerical Resolution• Finite Element Method applied to model the interaction of the field and
the fluid.
Field of a cylindric shaped magnet
Height of the ferrofluid sample (From Top)
Experiment VS Theory
Numerical approach • Deformation of the fluid above the
magnet
Physical Experiment • Ferrofluid sample subjected to a strong
magnet