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