# Ferrofluid mirror
A ferrofluid mirror is an optical mirror whose reflective surface is a magnetic liquid (ferrofluid) shaped by applied magnetic fields, used most often as a deformable mirror that corrects wavefront (surface of equal-phase light; its distortion blurs images) aberrations in adaptive optics systems. Because the reflecting surface is a liquid, it can be deformed continuously and cheaply by an array of small electromagnets, offering strokes far larger than those of conventional solid deformable mirrors.
| Key fact | Detail |
|---|---|
| Working principle | A ferrofluid surface is deformed by an array of small current-carrying coils beneath it; a thin reflective film makes the surface usable as a mirror[1](https://arxiv.org/abs/0807.2397) |
| Reflective coating | A silver colloid called a metal liquid-like film (MELLF) deposited on the ferrofluid solves the low reflectivity of pure ferrofluids[2](https://arxiv.org/abs/astro-ph/0212189) |
| Demonstrated correction | A prototype corrected an 11 µm low-order aberrated wavefront to a residual RMS error of 0.05 µm[1](https://arxiv.org/abs/0807.2397) |
| Speed | Ferrofluids with viscosities up to 494 cP allowed adaptive optics correction frequencies as high as 900 Hz[3](https://opg.optica.org/ao/abstract.cfm?uri=ao-48-1-1) |
| Stroke | Strokes range from a fraction of a wave to several hundred micrometers, at a cost of a few dollars per actuator[4](https://opg.optica.org/ao/abstract.cfm?origin=search&uri=ao-45-15-3495) |
| Main limitation | Time response of the order of a few milliseconds, unlikely to match the speed of solid mirrors[4](https://opg.optica.org/ao/abstract.cfm?origin=search&uri=ao-45-15-3495) |
| Large-aberration use | In a liquid mirror telescope test system, a magnetic fluid deformable mirror reduced wavefront RMS error from 53.9 µm to 0.18 µm[5](https://wulixb.iphy.ac.cn/article/doi/10.7498/aps.75.20251610) |
## How it works
A ferrofluid is a stable colloidal suspension of nano-sized, single-domain ferri- or ferromagnetic particles, about 10 nm in diameter, dispersed in a liquid carrier. In an external magnetic field the particles align with the field and the liquid becomes magnetized, so a locally applied magnetic field deforms the free surface[6](https://rcastoragev2.blob.core.windows.net/a8c66b919fb9ce92b12507c873d5e619/PMC6695980.pdf). A ferrofluid deformable mirror places miniature electromagnetic coils under a layer of the fluid; the cumulative field of the coil array sculpts the surface into the shape needed to correct an incoming wavefront[6](https://rcastoragev2.blob.core.windows.net/a8c66b919fb9ce92b12507c873d5e619/PMC6695980.pdf).
Pure ferrofluids reflect little light, so the surface must be given a reflective skin. The Laval University group of Ermanno F. Borra, professor of physics at Université Laval who pioneered liquid mirror telescopes, deposited a thin silver colloid known as a metal liquid-like film (MELLF) on the ferrofluid, producing a liquid optical surface that can be precisely shaped in a magnetic field[2](https://arxiv.org/abs/astro-ph/0212189). A later variant lays an elastomer membrane coated with an aluminum film on the fluid, which both improves reflectivity and speeds the response[7](https://arxiv.org/abs/1810.09987).
The surface deformation depends on the square of the magnetic field, which makes the mirror's response nonlinear. Superposing a strong, uniform magnetic field, produced for example by a Maxwell coil, on the actuator fields linearizes the response[6](https://rcastoragev2.blob.core.windows.net/a8c66b919fb9ce92b12507c873d5e619/PMC6695980.pdf). A 91-actuator mirror using this technique produced the first 36 [Zernike polynomials](https://www.edgepedia.org/zernike-polynomials), with predicted peak-to-valley wavefront amplitudes up to 30 µm at RMS residuals of λ/10 at 632.8 nm[8](https://arxiv.org/abs/1006.2843).
## Performance and demonstrated results
**Wavefront correction.** The 2008 prototype, with its surface actuated by a triangular array of small coils, corrected an 11 µm low-order aberrated wavefront to a residual RMS wavefront error of 0.05 µm[1](https://arxiv.org/abs/0807.2397). Early mirrors of this class were built for high stroke and low-order, axially symmetrical aberrations, driven by five concentric copper coils[9](https://arxiv.org/abs/physics/0611160).
**Speed.** [Ferrofluid](https://www.edgepedia.org/ferrofluid) mirrors were long thought to be restricted to correction frequencies below 10 Hz. Experiments showed, counterintuitively, that increasing the viscosity of the ferrofluid raises the limiting operational frequency: fluids with viscosities up to 494 cP allowed correction frequencies as high as 900 Hz, and overdriving the actuators, combining high-current short pulses with ordinary step functions, counters the amplitude loss that high viscosity causes[3](https://opg.optica.org/ao/abstract.cfm?uri=ao-48-1-1). A closed-loop adaptive optics system running at about 500 Hz was described as a realistic near-term goal[3](https://opg.optica.org/ao/abstract.cfm?uri=ao-48-1-1).
**Response time.** With a step input through a single actuator, a membrane-free mirror settled in 100 ms; combinations of overdrive inputs reduced this to 25 ms, and adding the elastomer membrane halved the response time again. Membrane and overdrive techniques combined improved the time response by a factor of 20, and simulations suggested that mirrors using membranes and improved electronics could reach settling times of the order of a millisecond[7](https://arxiv.org/abs/1810.09987).
**Large aberrations.** Because the free surface of a liquid moves without the mechanical limits of a solid face sheet, magnetic fluid deformable mirrors can produce strokes exceeding 100 µm, both for a single actuator and between actuators[6](https://rcastoragev2.blob.core.windows.net/a8c66b919fb9ce92b12507c873d5e619/PMC6695980.pdf). Solid deformable mirrors are normally limited to deflections of tens of micrometers, and spatial light modulators to a few micrometers, so magnetic fluid mirrors target applications with aberrations too large for those correctors[6](https://rcastoragev2.blob.core.windows.net/a8c66b919fb9ce92b12507c873d5e619/PMC6695980.pdf).
## Advantages and limitations
The 2006 demonstration of a ferrofluid adaptive mirror summarized the trade-offs. Key strengths include a stroke ranging from less than one wave to several hundred micrometers, an actuator price of only a few dollars, and the ability to scale up—arrays of hundreds of thousands of actuators appear practical, as do mirrors roughly a meter across.[4](https://opg.optica.org/ao/abstract.cfm?origin=search&uri=ao-45-15-3495) The most important disadvantage is the time response, of the order of a few milliseconds, which is unlikely to match the speed of solid mirrors[4](https://opg.optica.org/ao/abstract.cfm?origin=search&uri=ao-45-15-3495). Liquid mirrors must also conserve volume, so influence functions cannot simply be superposed, and the fluids can evaporate[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC3574679/).
## Applications
**Astronomy.** The technology was developed largely for adaptive optics in large telescopes, where the need for many low-cost actuators is acute[2](https://arxiv.org/abs/astro-ph/0212189). A proposed astronomical use is a ferrofluidic adaptive secondary mirror consisting of a permanent magnet with added wires and coils to shape the wavefront[9](https://arxiv.org/abs/physics/0611160). Magnetic liquid mirrors can also correct the aberrations of large, lower optical quality primary mirrors held by simple support systems[11](https://arxiv.org/abs/1206.1014). Liquid mirror telescopes, which observe only near the zenith and suffer large off-axis aberrations when tilted, are a natural fit: in one experimental system a magnetic fluid deformable mirror with a stroke exceeding 100 µm reduced the wavefront RMS error from 53.9 µm to 0.18 µm[5](https://wulixb.iphy.ac.cn/article/doi/10.7498/aps.75.20251610).
**Vision science.** A ferrofluid mirror design for ophthalmic imaging was presented as an inexpensive adaptive optics element with large stroke for retinal imaging[12](https://opg.optica.org/abstract.cfm?uri=AOPT-2009-JWF2).
**Liquid optics in space.** The broader idea of liquid mirrors in orbit is being pursued by NASA's Fluidic Telescope (FLUTE) project, jointly led with Technion, which envisions a space telescope primary mirror about 164 feet (50 meters) in diameter formed in orbit from liquid materials and shaped by surface tension. Such a mirror would self-repair after damage such as a micrometeorite impact, and small-scale liquid shaping experiments have been run in ground analogs, parabolic flights, and aboard the [International Space Station](https://www.edgepedia.org/international-space-station)[13](https://www.nasa.gov/science-research/astrophysics/what-is-the-fluidic-telescope/).
## References
1. [Brousseau, D., Borra, E. F., et al. (2008). Wavefront correction with a ferrofluid deformable mirror: experimental results and recent developments. arXiv:0807.2397.](https://arxiv.org/abs/0807.2397)
2. [Laird, P., Bergamasco, R., Berube, V., Borra, E. F., et al. (2002). Ferrofluid Based Deformable Mirrors – a New Approach to Adaptive Optics Using Liquid Mirrors. arXiv:astro-ph/0212189.](https://arxiv.org/abs/astro-ph/0212189)
3. [Parent, J., Borra, E. F., Brousseau, D., Ritcey, A. M., Déry, J.-P., Thibault, S. (2009). Dynamic response of ferrofluidic deformable mirrors. Applied Optics 48(1), 1–6.](https://opg.optica.org/ao/abstract.cfm?uri=ao-48-1-1)
4. [Laird, P., Caron, N., Rioux, M., Borra, E. F., Ritcey, A. (2006). Ferrofluidic adaptive mirrors. Applied Optics 45(15), 3495–3500.](https://opg.optica.org/ao/abstract.cfm?origin=search&uri=ao-45-15-3495)
5. [Xu, M., Ding, H., Wang, H., Qian, F., Niu, S., Wu, Z. (2026). Off-axis aberration correction system for liquid mirror telescope based on magnetic fluid deformable mirror. Acta Physica Sinica 75(11): 110403.](https://wulixb.iphy.ac.cn/article/doi/10.7498/aps.75.20251610)
6. [Wei, X., Wang, Y., Cao, Z., Mbemba, D., Iqbal, A., Wu, Z. (2019). Large Aberration Correction by Magnetic Fluid Deformable Mirror with Model-Based Wavefront Sensorless Control Algorithm. International Journal of Molecular Sciences 20(15): 3697.](https://rcastoragev2.blob.core.windows.net/a8c66b919fb9ce92b12507c873d5e619/PMC6695980.pdf)
7. [Rochette, M., Borra, E. F., Déry, J.-P., Ritcey, A. M. (2018). Dynamic response of ferrofluidic deformable mirrors using elastomer membrane and overdrive techniques. arXiv:1810.09987.](https://arxiv.org/abs/1810.09987)
8. [Brousseau, D., Borra, E. F., Rochette, M., Bouffard Landry, D. (2010). Linearization of the response of a 91-actuator magnetic liquid deformable mirror. arXiv:1006.2843.](https://arxiv.org/abs/1006.2843)
9. [Brousseau, D., Borra, E. F., Jean-Ruel, H., Parent, J., Ritcey, A. (2006). A magnetic liquid deformable mirror for high stroke and low order axially symmetrical aberrations. arXiv:physics/0611160.](https://arxiv.org/abs/physics/0611160)
10. [Adaptive Optics Technology for High-Resolution Retinal Imaging. PMC3574679.](https://pmc.ncbi.nlm.nih.gov/articles/PMC3574679/)
11. [Borra, E. F. (2012). Magnetic liquid deformable mirrors for astronomical applications: Active correction of optical aberrations from lower-grade optics and support system. arXiv:1206.1014.](https://arxiv.org/abs/1206.1014)
12. [Brousseau, D., Borra, E. F., Ritcey, A. M., Campbell, M., Thibault, S., Drapeau, J., Naderian, A. (2009). A New Ferrofluid Mirror for Vision Science Applications. OSA Technical Digest, paper JWF2.](https://opg.optica.org/abstract.cfm?uri=AOPT-2009-JWF2)
13. [Figliozzi, G. (2023). What is the Fluidic Telescope? NASA.](https://www.nasa.gov/science-research/astrophysics/what-is-the-fluidic-telescope/)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Waves and optics › Optical technologies and instruments › Adaptive and active optics › Wavefront correction devices*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license