Liquid Mirror Telescope

The first time I heard that liquid mirrors worth studying was from a Nature’s News & Views article. Rotating liquid that creates a parabolic surface enables human to conceive about really big telescopes that have to be built on the moon. The Nature article praised the use of ionic liquids (ILs) — liquids that are hardly volatile — for liquid mirror support, because in the moon the working environment is vacuum. And it is also cold there, so the liquid mirror should also melt at as low a temperature as possible.

Then it started to seem that all researching or manufacturing efforts on reflective liquid droplets were for the future, exciting, lunar application, at least for me, who only heard of these things from the moon fantasy. Afterward I kept being informed by various science news agents of the same moon fantasy (NewScientist, NASA, the MIT based Technology Reviews, etc.), although I also knew more that liquid mirror for telescope is not new. NASA has one – a 3-meter Liquid Mirror Telescope (LMT) for tracking orbital debris:

Image from the NASA Orbital Debris Program Office

The liquid used in this ‘Earth version’ is mercury. The man in the photo was not committing suicide because the surface of mercury was covered by a layer of oxide which prevent the mercury to evaporate. But this is not likely to work on the moon where the pressure is much lower and things are much more ready to evaporate. So it can be said that ionic liquid have saved the crazy idea of lunar liquid mirror telescope, and will realize an ongoing lunar liquid mirror telescope project.

An Ancient Project

The legend of Archimedes’s weapon

The idea of “mobile” curved mirror is not new, although there have been many debates on the historical existence of the mirrors as “weapons of mass destruction” in ancient Greece implemented by Archimede – the Death Ray, in which multiple mirrors were arranged to concentrate the sun light and set the enemy in fire. Not only is it theoretically doubtful, but experimentally, an MIT group also indicated that the effect of this kind of weapon was exaggerated. The experiment was only able to set a small fire on one ship at half the distance described in the legend, and that fire was quickly extincted. Some authors even suspected the translation of the ancient Greek word πυρεια referring the very object used as weapon in the concerned war as mirrors. However, knowledge of the concentrating effect of curvature on a mirror seems to have indeed established early. No matter the legend of the Death Ray was true or not, at least there must have been someone conceiving many normal-scale plain mirrors to assemble a large-scale, ‘mobile’, curved reflective surface.

Stories at Smaller Scales

And this seems to have been realized in a much smaller scale recently. Researchers from different universities cooperated to assemble tiny hexagonal mirrors along the interface between an oil droplet and the surrounding water bath, thus created a liquid mirror droplet. The tiny mirrors are actually hydrophilic, transparent silicon platelets 8 μm in diameter and 1μm thick, fabricated by lithography. Each platelet is then covered by gold on one side (hydrophilic). When the mirror platelets was injected into the oil droplet submerged in water bath, the platelets gradually migrated to the water/oil interface with the gold side facing toward the oil phase, forming a curved reflective surface. Moreover, the researchers were able to alter the curvature of the droplet by electrowetting method, making the focus length of the liquid mirror tunable.

Tunable liquid mirror. Langmuir ASAP Article. Permission obtained from Rightslink

This work’s perspective differs quite largely from the above mentioned large scale lunar project, in that suspending droplets with enhanced boundary optical properties are also a promising step toward tunable optofluidic devices for cell, microparticle, and even nanoparticle manipulation at a really small scale world.

Robin D. Rogers (2007). Materials science: Reflections on ionic liquids Nature, 447 (7147), 917-918 DOI: 10.1038/447917a
A.S. Papadogiannis, N.S. Papadogianni, A. Carabelas, S. Tsitomeneas, P. Kyraggelos, T.G. Chondros (2008). The Mirror Weapon in Archimedes Era Proceedings of EUCOMES 08, 29-36 DOI: 10.1007/978-1-4020-8915-2_4
D. L. Simms (1991). Galen on Archimedes: Burning Mirror or Burning Pitch? Technology and Culture, 32 (1), 91-96
Michael A. Bucaro, Paul R. Kolodner, J. Ashley Taylor, Alex Sidorenko, Joanna Aizenberg, Tom N. Krupenkin (2008). Tunable Liquid Optics: Electrowetting-Controlled Liquid Mirrors Based on Self-Assembled Janus Tiles Langmuir DOI: 10.1021/la803537v