A ferrofluid mirror is a type of

deformable mirror Deformable mirrors (DM) are mirrors whose surface can be deformed, in order to achieve wavefront control and correction of optical Optical aberration, aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time con ...

with a reflective liquid surface, commonly used in

adaptive optics Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...

. It is made of

ferrofluid Ferrofluid is a dark liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended inside a carrier fluid (usually an organic solvent or water). Each magnetic ...

and magnetic iron particles in

ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes: as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...

, the basis of automotive antifreeze. The ferrofluid mirror changes shape instantly when a

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

is applied. As the ferromagnetic particles align with the magnetic field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Since any shapes can be produced by changing the magnetic field geometries, wavefront control and correction can be achieved. Ferrofluid Deformable mirror

A ferrofluid mirror is controlled by a number of

actuator An actuator is a machine element, component of a machine that produces force, torque, or Displacement (geometry), displacement, when an electrical, Pneumatics, pneumatic or Hydraulic fluid, hydraulic input is supplied to it in a system (called an ...

s, often arranged in a hexagonal array. Pure ferrofluids have low reflectivity, so they must be coated with a reflective layer. Water-based ferrofluids hold the reflective layer effectively, but water evaporates so quickly that the mirror could disappear within hours. Depositing a thin silver

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...

known as a metal liquid-like film (MELLF) on the ferrofluid surface solves the problem of fast evaporation and low reflectivity of pure ferrofluids. The combination of fluid and metal results in a liquid optical surface that can be precisely shaped in a

Applications

Astronomy

Ferrofluid mirror telescopes have been built to obtain

astronomical Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...

data and used to take images of deep space; subjects for research include

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

s. The main challenge astronomers and scientists face is image distortions due to

wavefront In physics, the wavefront of a time-varying ''wave field (physics), field'' is the set (locus (mathematics), locus) of all point (geometry), points having the same ''phase (waves), phase''. The term is generally meaningful only for fields that, a ...

errors caused by the atmosphere. The solution to this problem is to create mirrors with controllable surface shapes, known as deformable mirrors. Ferrofluid mirrors are used as deformable mirrors because when ferrofluids are exposed to a magnetic field, the liquid forms a shape to minimize the energy of the system which involves magnetic, gravitational, and surface tension forces of the liquid.

Ophthalmology

While ferrofluid mirrors are widely used in telescopes, ferrofluid mirrors might also be applied in the field of visual science. Human eyes suffer from many optical imperfections.

Ophthalmologists Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...

look into the eyes to detect and diagnose diseases by examining the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

. Ferrofluid mirrors could be rapidly adjusted to compensate for the large

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

s in diseased eyes during eye exams or treatments. They can generate surfaces having complex shapes, and thus can be used to determine the shape of the lens of the human eye, the

crystalline lens The lens, or crystalline lens, is a Transparency and translucency, transparent Biconvex lens, biconvex structure in most land vertebrate eyes. Relatively long, thin fiber cells make up the majority of the lens. These cells vary in architecture and ...

. This may allow the measurement of high-order aberrations of the crystalline lens so that they can be corrected with appropriate medical procedures. The magnetically shaped reference surface would further be used to verify the correction made to the lens of the eye before, during or after the procedures.

Developments

Deformable solid mirror

Before ferrofluid mirrors were invented, solid

s were used to correct atmospheric distortion and keep a steady focus. Deformable solid mirrors use flexible mirrors with complex actuators underneath to correct for atmospheric distortion and keep a steady focus. Shortcomings of traditional deformable solid mirrors include cost, fragility, and the need for continuous power. Images from these mirrors have an undesirable quilt pattern due to the discrete actuators beneath the mirror surface.

Mercury mirror

Mercury was used as the main material of early deformable

liquid-mirror telescope Liquid-mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well (for example, low-melting point alloys of gallium). The liquid and its container are ...

s because of its high reflectivity and low melting temperature. However, as a magnetic liquid, it had problems. It is difficult to obtain a stable metallic based magnetic liquid, and the high density of mercury necessitates a larger deforming force and thus a strong magnetic field.

Ferrofluid mirror

The limitations associated with the use of mercury for deformable liquid mirrors can be solved through the use of ferrofluids. Ferrofluid mirrors use cheaper materials, and are able to make more corrections due to its larger range of motion. Stable ferrofluids have wide ranges of physical properties thus can be produced to suit many practical application needs.

References

{{Reflist Mirrors